[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Verus You're actually comparing two different boxes. no6b@ wrote: Nope - comparing GLB vs. GaAsFET/pass cavity combo Still not the equivalent box... you're still missing the post active-device stages. I don't need them. But I suppose one could add a 2nd pass cavity AFTER the preamp. I've never had to do that in ~30 years of repeater building. I have used post preamplifier filters in the interest of managing extremely high signal levels. Care to publish your results here? Why don't you tell us what you found? I've asked more than once for some reason you're reluctant to publish your results. I can't help but be even more suspicious of the GLB. Because of time... I can post things only when I have the available time and quickly replying in detail requires that I should probably review my notes. If those notes are not readily at hand you will obviously just have to wait. Lacking the hard data, I'm going to do a little guesswork here: a typical GaAsFET preamp has 17 dB of gain @ 440 MHz. The GLB Preamplifier I repaired has a dual gate Mosfet. The version I received had a blown device so I replaced it. With different amounts of bias I could actually get up to nearly 25dB. The NF of the device alone depending on the bias was anywhere from about .7 to 1.2 dB again depending on the bias. The Simrex preselector has a spec'd overall gain of 8 dB. Which is similar to what I ended up with after replacing the bad Mosfet and selecting a bias point resulting in a gain of about 9.2dB through the box. All other things being equal, the combined loss of the resonators in the preselector would then be 9 dB. Kevin says the distribution is 2 stages before 2 after. The 224 MHz version I have here is 1 before and 3 trailing stages. If all the stages are equivalent, then the pre-active device loss is 4.5 dB. Assume 0.5 dB NF of the actual GaAsFET device, I come up with 5 dB NF. Am I close? Using your above figures with one pre-device stage... figure about 2.3dB and .7 for 3dB NF at 224 MHz. Yeah, but the numbers are off. To properly compare the two you'd need to use more than one pass-cavity. At least one additional cavity (min) following the active device and to really be honest, more than one trailing BP Cavity. See above, Kevin's post. In many cases, the trailing cavity isn't needed. But not in every case... If you're trying to manage extremely high signal levels the trailing filters can greatly help filter unwanted RF. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. The post-preamp filtering can and does contribute in the management (not necessarily the prevention) of high signal levels issues. ..only for RXs that need it. I guess I'm a bit biased because I use real RXs (GEs), so the only protection needed is for the preamp going in front of it. Depends a lot on the specific situation... the last real GE receiver front end I swept had a modestly wide front- end. Although it might have survived only better than some other brand and model receivers there are situations where post preamplifier filtering before the receiver would be a real benefit. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. There are 2 resonators in front of the Active Device. The higher Q of a 1/4 wave cavity is obviously better. The honest to thyself person should determine the NF difference, which is probably not a huge amount. Once again, I'm still waiting for the NF numbers. See the above... IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B Sure, it's a compromise that works well for what they are. I'd probably (and do) park a Simrex or GLB Pre-selectors in front of less than bullet-proof receivers. Something else to consider: if your less than bullet-proof RX has good sensitivity, a preamp isn't even needed - just throw a pass cavity in front of it. Simple cheap, you'll probably still end up with better sensitivity than if you used the Simrex preselector. Bob NO6B Because a number of less than bullet proof receivers don't have great sensitivity. And the response (shape) of a pass cavity is different than a multi-stage pre-selector. s.
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
The better skirts are mucho desired. n...@... wrote: As you so often like to state, it all depends on the application - in many cases it simply isn't necessary. And in many cases it's beneficial to have better skirts... But there are other solutions: if you want a brute-force window filter they're out there too. I have a few 5 MHz wide 1 dB loss UHF filters sitting on the shelf here that I acquired at the Dayton Ft. Tuthill hamfests. ~$40 each. Seems like a good deal... but the 5MHz band-width is only (for me) usable in some system applications. Some of those applications where the 5MHz band-width would be excessive but the expected Simrex band-width would not be... Again, the loss is low enough that in most cases a leading preamp simply isn't needed. Leading or Post Preamp? Kind of makes it look like helicals are a sin doesn't it... Ever wonder why the MVP/MastrII Micors are so deaf compared to more modern RXs? Nope... I haven't measured the loss of the UHF helical assembly, but the VHFHB front-end helicals have ~6 dB of loss. In all those radios, their own helicals effectively are all the post-preamp filtering you'll ever need. No it isn't... if you sweep the front end of Micor you'll find it's actually fairly wide. I seem to remember sweeping some GE Receivers and their front ends were relatively wide on the order of at least a few MHz. From Memory I seem to remember the Micor being at least 4MHz wide. The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. Depends on the Pre-selector Model and age of the box ... I have GLB units here with two pre-device stages and three post-device stages. And I have versions with a more traditional helical design and others with more of a lumped parts layout. A 2-Meter version I have lots of pictures of has 1 stage of pre-selection and four trailing stages. The active device is an MRF-901. OK. MRF-901 NF @ 2M is ~1 dB, so maybe 2-3 dB NF for the unit. And that jives with my informal recorded notes for the 224 MHz GLB Pre-selector with a dual gate Mosfet. Not bad for VHFHB, but having only 1 little resonator in front of the active device doesn't offer it much OOB protection. Better put a (gasp) PASS CAVITY in front of it! ;) To quote someone who recently wrote: As you so often like to state, it all depends on the application - in many cases it simply isn't necessary. However, In more than a few real world situations you might really need the filter pre-selection a lot more than the most optimum NF. A practical trade of pre-selection for a slightly higher noise figure can and does sometimes make the difference in a usable radio system. OK fine. But again, we DON'T KNOW the noise figure for the device. It's not mandatory to know the NF for every situation, only helpful for those specific situations where making a logical assumption is not allowed. Furthermore, since the filtering distribution varies with the model, it's very difficult to predict the dynamic range characteristics of the unit. True along with the different active devices. Fun to play with? Yes. Can solve some IMD/overload problems? Certainly. But not a tool for any seriously engineered RF system. Bob NO6B Really depends a lot on whose money you're spending. I've seen a lot of seriously engineered RF systems that don't work very well out there in the real world. s.
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 8/1/2010 01:15, you wrote: But there are other solutions: if you want a brute-force window filter they're out there too. I have a few 5 MHz wide 1 dB loss UHF filters sitting on the shelf here that I acquired at the Dayton Ft. Tuthill hamfests. ~$40 each. Seems like a good deal... but the 5MHz band-width is only (for me) usable in some system applications. Some of those applications where the 5MHz band-width would be excessive but the expected Simrex band-width would not be... Then we're back to the pass cavity solution. Just saying there are other inexpensive, well-engineered options out there. Again, the loss is low enough that in most cases a leading preamp simply isn't needed. Leading or Post Preamp? Leading meaning pre-preamp. Kind of makes it look like helicals are a sin doesn't it... Ever wonder why the MVP/MastrII Micors are so deaf compared to more modern RXs? Nope... I haven't measured the loss of the UHF helical assembly, but the VHFHB front-end helicals have ~6 dB of loss. In all those radios, their own helicals effectively are all the post-preamp filtering you'll ever need. No it isn't... if you sweep the front end of Micor you'll find it's actually fairly wide. I seem to remember sweeping some GE Receivers and their front ends were relatively wide on the order of at least a few MHz. From Memory I seem to remember the Micor being at least 4MHz wide. The VHF HB MVP front-end helical assembly has a 3 dB BW of 1.8 MHz. At 40 dB down the BW is 5.7 MHz. Granted the selectivity curves of the Simrex preselector curves are narrower, but keep in mind that they are in fact misleading, since most of that selectivity is AFTER the preamp stage. Add to that the fact that the mixer in the GE radios has very high dynamic range (remember, the stock GEs don't have a gain stage ahead of the mixer), you likely end up INCREASING the GE's susceptibility to IMD by using one. The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. Depends on the Pre-selector Model and age of the box ... I have GLB units here with two pre-device stages and three post-device stages. And I have versions with a more traditional helical design and others with more of a lumped parts layout. A 2-Meter version I have lots of pictures of has 1 stage of pre-selection and four trailing stages. The active device is an MRF-901. OK. MRF-901 NF @ 2M is ~1 dB, so maybe 2-3 dB NF for the unit. And that jives with my informal recorded notes for the 224 MHz GLB Pre-selector with a dual gate Mosfet. Not bad for VHFHB, but having only 1 little resonator in front of the active device doesn't offer it much OOB protection. Better put a (gasp) PASS CAVITY in front of it! ;) To quote someone who recently wrote: As you so often like to state, it all depends on the application - in many cases it simply isn't necessary. Agreed: in the above example the Simrex preselector isn't necessary: simply omit it use just a pass cavity. However, In more than a few real world situations you might really need the filter pre-selection a lot more than the most optimum NF. A practical trade of pre-selection for a slightly higher noise figure can and does sometimes make the difference in a usable radio system. OK fine. But again, we DON'T KNOW the noise figure for the device. It's not mandatory to know the NF for every situation, only helpful for those specific situations where making a logical assumption is not allowed. Kind of like saying you don't need to know how much output power your TX is running, so long as your users can hear it. Fun to play with? Yes. Can solve some IMD/overload problems? Certainly. But not a tool for any seriously engineered RF system. Bob NO6B Really depends a lot on whose money you're spending. I've seen a lot of seriously engineered RF systems that don't work very well out there in the real world. ...and in almost every case I've seen this, it's due to the engineering failing to take into account all of the real-world parameters. If your models are flawed, everything falls apart. Bob NO6B
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier n...@... wrote: Then we're back to the pass cavity solution. Just saying there are other inexpensive, well-engineered options out there. Sure, the pass cavity is one of many possible options. The VHF HB MVP front-end helical assembly has a 3 dB BW of 1.8 MHz. At 40 dB down the BW is 5.7 MHz. Granted the selectivity curves of the Simrex pre-selector curves are narrower, but keep in mind that they are in fact misleading, since most of that selectivity is AFTER the preamp stage. My question to you is... what function would you think the multiple section/stage post active device selectivity serves? Add to that the fact that the mixer in the GE radios has very high dynamic range (remember, the stock GEs don't have a gain stage ahead of the mixer), you likely end up INCREASING the GE's susceptibility to IMD by using one. Depends on what you park in front of the receiver. in the above example the Simrex preselector isn't necessary: simply omit it use just a pass cavity. Once again a pass cavity is totally different compared to the Simrex GLB Preselector tuned circuits. To equate the two layouts you would need to add trailing resonant filters, which are in many examples tighter/sharper than the front end selectivity of the following receiver. If I tried to provide some type of speculative explanation regarding the Simrex GLB Preselector Management and/or control of extremely high level inputs, IMD and unwanted signals through the trailing helicals... you'll probably jump on the not a well-engineered label again. So I'm not even going to try and I'm pretty much outta this subject thread after this reply post. It's not mandatory to know the NF for every situation, only helpful for those specific situations where making a logical assumption is not allowed. Kind of like saying you don't need to know how much output power your TX is running, so long as your users can hear it. Sure... kind of If I assume the Tx Power of a 100 watt Power Amplifier is within 15% of its nominal rated value based on indicated current draw, supply voltage, a spectral view and knowing the output path to the antenna is working properly... I should be able to make a logical assumption users within a normal expected coverage area should be able to hear the machine... even though I've never measured the output with an accurate watt meter. Fun to play with? Yes. Can solve some IMD/overload problems? Certainly. But not a tool for any seriously engineered RF system. Bob NO6B Really depends a lot on whose money you're spending. I've seen a lot of seriously engineered RF systems that don't work very well out there in the real world. ... and in almost every case I've seen this, it's due to the engineering failing to take into account all of the real-world parameters. If your models are flawed, everything falls apart. Bob NO6B In the most (unfortunately to many) recent examples of poorly preforming RF Systems I've seen up close were due to the lack of the Engineers, interest, experience knowledge not including the mention of the bureaucracy or incompetence placing that person on the project. s. That's it for me... cheers
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
It looks like Skipp and I have found that the GLB preselector has been successful at helping less then ideal repeaters work better. I can think of two examples that I have had. I had a Midland 13-509 repeater back in the late 80's on 223.72MHz. It used a set of homebrew copper pipe duplexers. The duplexers would drift with temperature swings, but by adding a GLB preselector I was able to add enough additional rejection to make the repeater play quite nicely. I also had a UHF Maggiore repeater that used an antenna at 450 feet on the top of a tower. I was being plagued by occasional desense that I was never able to track down. It never seemed to happen when I was at the site. By adding the GLB preselector I was able to reject whatever it was that was getting past the duplexers and solve the problem. My point is that the GLB is not just something to add to a repeater to make it have a hotter receiver. The BIG advantage is that you can get a bandpass characteristic with very tight skirts that will help a receiver that is passing too much crap through it's front end. You can also get this in a very small package that will fit in an area where you don't have room to hang a big cavity filter on the wall. The GLB is not the answer for everybody, but it has it's place. 73, Joe, K1ike
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/30/2010 08:31, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? Once again, the NF depends on what device arrives in your specific Simrex (aka GLB) Pre-Selector. It wouldn't hurt or be impossible to ask. Nothing new here - same goes for different versions of preamps, i.e. ARR bipolar vs. GaAsFET. A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. You're actually comparing two different boxes. Nope - comparing GLB vs. GaAsFET/pass cavity combo. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). Care to publish your results here? I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... Perfectly fair. Used pass cavities in good condition are plentiful. Can't remember the last time I saw a used GLB or equivalent unit for sale, so I'm simply comparing what's readily available. One of the really nice (and mostly overlooked) items about the Simrex (GLB) unit construction (and operation) is the Post (active) Amplifier Filtering (tuned circuits). For more than one real reason they can be one of the most under reported bacon saver in your fry pan. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Hello Bob, I agree that the GLB is a space saving device, but don't you think that the multistage helical coil stages in the preselector with beat a single cavity in skirts and out-of-band rejection? Joe On 7/31/2010 9:30 AM, n...@no6b.com wrote: A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Bob, Some of us are geographically disadvantaged when it comes to availability of equipment. Here in rural Australia, anything secondhand for Amateur Repeater construction is rare, shipping of anything is expensive. From what I read on the group, the U.S. Is overloaded with surplus equipment, sometimes at bargain prices. For me to buy and ship that cheap 1/4 wave can and pre-amp would be more expensive than buying a new Simrex Pre-Selector :-) Since this is a hobby for me...not a paid commercial day job I will take the chance on the NF aspect and have my fun experimenting and testing :-) IF the Simrex-GLB product did not perform..the word would spread faster than the Black Plague and no one would buy them 73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Jul 31, 2010, at 11:30 PM, n...@no6b.com wrote: At 7/30/2010 08:31, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? Once again, the NF depends on what device arrives in your specific Simrex (aka GLB) Pre-Selector. It wouldn't hurt or be impossible to ask. Nothing new here - same goes for different versions of preamps, i.e. ARR bipolar vs. GaAsFET. A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. You're actually comparing two different boxes. Nope - comparing GLB vs. GaAsFET/pass cavity combo. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). Care to publish your results here? I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... Perfectly fair. Used pass cavities in good condition are plentiful. Can't remember the last time I saw a used GLB or equivalent unit for sale, so I'm simply comparing what's readily available. One of the really nice (and mostly overlooked) items about the Simrex (GLB) unit construction (and operation) is the Post (active) Amplifier Filtering (tuned circuits). For more than one real reason they can be one of the most under reported bacon saver in your fry pan. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Joe wrote: Hello Bob, I agree that the GLB is a space saving device, but don't you think that the multistage helical coil stages in the preselector with beat a single cavity in skirts and out-of-band rejection? Curves for the GLB are available here: http://www.repeater-builder.com/glb/glb-preselector-order-form.pdf While the skirts and OBR of the GLB might beat a single cavity, many times it isn't necessary - especially if the receiver that follows is able to cope with high out of band signals and the preamp that follows the cavity has a high overload point. The problem is, like any receiver that has several helicals in cascade before the first active stage, the loss that precedes the active stage has a majority role in the overall NF of the system that follows. It matters little what the quality of the active stage is, because the loss has already determined (for the most part) the system Noise Figure. The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. If they are over-coupled (and I believe they are not looking at the response curves) only a few dB of loss will occur before the active stage. If they are lightly coupled (which I believe is true) then several dB of loss occurs ahead of the active stage. Every dB of loss ahead of the first active stage ADDS to the system NF - period. This loss can NEVER be recovered no matter how good the preamp is that follows. Kevin
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier Jack Chomley ra...@... wrote: Well, I have decided to buy and try a Simrex Pre-Selector,experiment with it, test it ...whatever. Something to never say in an airport... Hi Jack, Hopefully you will find the Simrex / GLB Pre-selector to be a very interesting and useful box. There are multiple stages of filtering in front and behind the active device. During the earlier posts I tried to highlight the difference between the additional filtering although I didn't (want to) go into much detail about at least one under-played advantage of having post filtering stages after the active device. If we were to actually compare the pre-filtering against a high Q band pass cavity, you will see a different shape band pass due to a single cavity versus a multi-stage layout. You could and most likely would see a lower loss similar shape if you had two or more series high-Q band cavities in front of an external RF-Amplifier (not even addressing the post filtering). A real world question ... is the size, cost and hardware complexity of the High-Q Cavities with a separate RF Pre-Amplifier configured in the same layout as a Simrex / GLB circuit going to yield a major advantage in your specific application? Will the Simrex / GLB Pre-selector provide a benefit to your system? In many of my personal experience examples using a GLB Pre-selector has clearly improved the receiving system performance. At the very least I will learn something from my experiences and it may end up being useful to me, anyway. It's nice to read that you're willing to make the effort and spend money for the education. I expect you won't be disappointed with your Simrex unit. I certainly appreciate the different points of view here and have learned of alternative methods and the reasoning behind them. The jackpot of knowledge... and you weren't even in a Casino. For me, all of it is good information :-) 73, Jack. VK4JRC Cheers Jack, s. Sent from my Apple iPad Tablet PC Response sent from a painfully slow dial-up internet connection.
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
That is the system on my uhf repeater. It's sensitivity is unmatched with this setup. I'm using one moto t1500 can before my arr gasfet preamp in a nema enclosure at 100 feet on the tower. The 9db rx only antenna sits 15 feet above that at 110 with the TX antenna 50 feet below at about 60 feet. Ldf5 from the enclosure to one wacom bandpass before receiver, ldf4 from the the 2 bandpass cans to the TX antenna. Hamtronics rx on this one... Ross Kc7rjk Jack Chomley ra...@irock.com.au wrote: Bob, Some of us are geographically disadvantaged when it comes to availability of equipment. Here in rural Australia, anything secondhand for Amateur Repeater construction is rare, shipping of anything is expensive. From what I read on the group, the U.S. Is overloaded with surplus equipment, sometimes at bargain prices. For me to buy and ship that cheap 1/4 wave can and pre-amp would be more expensive than buying a new Simrex Pre-Selector :-) Since this is a hobby for me...not a paid commercial day job I will take the chance on the NF aspect and have my fun experimenting and testing :-) IF the Simrex-GLB product did not perform..the word would spread faster than the Black Plague and no one would buy them 73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Jul 31, 2010, at 11:30 PM, n...@no6b.com wrote: At 7/30/2010 08:31, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? Once again, the NF depends on what device arrives in your specific Simrex (aka GLB) Pre-Selector. It wouldn't hurt or be impossible to ask. Nothing new here - same goes for different versions of preamps, i.e. ARR bipolar vs. GaAsFET. A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. You're actually comparing two different boxes. Nope - comparing GLB vs. GaAsFET/pass cavity combo. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). Care to publish your results here? I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... Perfectly fair. Used pass cavities in good condition are plentiful. Can't remember the last time I saw a used GLB or equivalent unit for sale, so I'm simply comparing what's readily available. One of the really nice (and mostly overlooked) items about the Simrex (GLB) unit construction (and operation) is the Post (active) Amplifier Filtering (tuned circuits). For more than one real reason they can be one of the most under reported bacon saver in your fry pan. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Verus You're actually comparing two different boxes. n...@... wrote: Nope - comparing GLB vs. GaAsFET/pass cavity combo Still not the equivalent box... you're still missing the post active-device stages. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). Care to publish your results here? Never thought about it much... those who were honestly interested in the subject the last time we rehashed the GLB active device topic here on the RB Group Emailed me direct and we exchanged various notes, diagrams and information. Much of what I have is based on the Pre-selector with the dual-gate mosfet, which I believe is similar to the BF-998 device. GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... Perfectly fair. Used pass cavities in good condition are plentiful. Can't remember the last time I saw a used GLB or equivalent unit for sale, so I'm simply comparing what's readily available. Yeah, but the numbers are off. To properly compare the two you'd need to use more than one pass-cavity. At least one additional cavity (min) following the active device and to really be honest, more than one trailing BP Cavity. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. The post-preamp filtering can and does contribute in the management (not necessarily the prevention) of high signal levels issues. I don't really care to detail out the above text theory but I will say I feel it's a critical often over-looked (at least in this RB Group - threads example) subject. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. There are 2 resonators in front of the Active Device. The higher Q of a 1/4 wave cavity is obviously better. The honest to thyself person should determine the NF difference, which is probably not a huge amount. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B Sure, it's a compromise that works well for what they are. I'd probably (and do) park a Simrex or GLB Pre-selectors in front of less than bullet-proof receivers. If the active device in your Simrex / GLB Pre-selector is a decent GasFet layout, the only major difference is the pre and post resonant circuits and how helpful/useful they are (or are not) in your application. s.
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier Kevin Custer kug...@... wrote: While the skirts and OBR of the GLB might beat a single cavity, many times it isn't necessary - The better skirts are mucho desired. especially if the receiver that follows is able to cope with high out of band signals and the preamp that follows the cavity has a high overload point. The question in each example/application is how well and is it enough... The problem is, like any receiver that has several helicals in cascade before the first active stage, the loss that precedes the active stage has a majority role in the overall NF of the system that follows. It matters little what the quality of the active stage is, because the loss has already determined (for the most part) the system Noise Figure. Kind of makes it look like helicals are a sin doesn't it... The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. Depends on the Pre-selector Model and age of the box ... I have GLB units here with two pre-device stages and three post-device stages. And I have versions with a more traditional helical design and others with more of a lumped parts layout. A 2-Meter version I have lots of pictures of has 1 stage of pre-selection and four trailing stages. The active device is an MRF-901. And the coupling for this model is a slightly different method than some (not all) of the other models I have looked at. Kind of interesting how the various models differ... I do have a number internal view pictures of GLB Models if anyone really burns to see them. If they are over-coupled (and I believe they are not looking at the response curves) only a few dB of loss will occur before the active stage. If they are lightly coupled (which I believe is true) then several dB of loss occurs ahead of the active stage. Both the above situations occur depending on the band/model and production version. The coupling of the one model I have nearby is with/a (relatively) high-Q capacitor probe. Another vhf model uses a more traditional wire probe layout. Every dB of loss ahead of the first active stage ADDS to the system NF - period. This loss can NEVER be recovered no matter how good the preamp is that follows. Kevin Yep. However, In more than a few real world situations you might really need the filter pre-selection a lot more than the most optimum NF. A practical trade of pre-selection for a slightly higher noise figure can and does sometimes make the difference in a usable radio system. The lunch my not be free, but it doesn't have to cost a lot. now go do the right thing cheers, s.
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/31/2010 06:58, you wrote: Hello Bob, I agree that the GLB is a space saving device, but don't you think that the multistage helical coil stages in the preselector with beat a single cavity in skirts and out-of-band rejection? No. There is a compromise in that although the out of band (OOB) rejection looks really good, there is a caveat in that the active device in the preselector is NOT behind that selectivity curve. As Skipp points out, the filtering is distributed before after the preamp. Since we have no idea what that distribution is, the actual dynamic range of the preselector as a function of frequency is unknown. It does provide a lot of OOB rejection to the RX to connect it to, but you have to hope that the active device in the preselector isn't getting clobbered. Add to that the unknown NF, which is going to set your system NF, I conclude there are just too many unknowns in this beast to recommend it. Sure it works for many, but it's by chance, not by engineering. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Kevin wrote: The problem is, like any receiver that has several helicals in cascade before the first active stage, the loss that precedes the active stage has a majority role in the overall NF of the system that follows. It matters little what the quality of the active stage is, because the loss has already determined (for the most part) the system Noise Figure. Skipp wrote: Kind of makes it look like helicals are a sin doesn't it... Hamtronics thinks it's a sin - that's why their receivers have some of the best sensitivity numbers of all of the stuff we commonly build repeater from. However, their receivers get hammered in high RF density locations, and the result is the need of some added filtering ahead of it. This is where the GLB can and does provide an improvement. I speak with experience here. I have used many Hamtronics receivers and GLB preselector/preamps (that I bought new) over the years. If you are lucky enough to have a really clean repeater site, the Hamtronics receiver line will work fine with no added preselection. I bought the GLB's primarily for protection to the repeater receiver when I was using a frequency agile in-band remote base. Many times it was possible to link to repeaters on the adjacent channel when I had the GLB in place. The addition of the GLB wasn't without its drawbacks though. My Hamtronics receiver would hear at -125 dBm for 12 dB SINAD stock with no preamp or preselection (the first GaAs device is basically unprotected). Running an in-band remote base anywhere within a MHz or so of the repeater input would desensitize the Hamtronics repeater receiver. After installing the GLB, I could link to repeaters 15 kHz away from my input with very little desense from the remote base transmitter. Anything over 60 kHz had no affect on repeater receiver sensitivity. The link beams were 50 to 100 feet below the repeater antenna and I normally run 5 watts on the Icom 900 stack. A 2-Meter version I have lots of pictures of has 1 stage of pre-selection and four trailing stages. The active device is an MRF-901. And the coupling for this model is a slightly different method than some (not all) of the other models I have looked at. Are you sure that's not 4 section preceding the device and 1 following? In more than a few real world situations you might really need the filter pre-selection a lot more than the most optimum NF. A practical trade of pre-selection for a slightly higher noise figure can and does sometimes make the difference in a usable radio system. No argument here If the site noise figure is worse than the NF of the GLB, adding it won't hurt your actual effective receiver sensitivity, and the added filtering can be a real boost. However, I'm blessed with sites that allow me to realize most of the sensitivity from a good preamp - with not much ahead of it (now that I don't do much RF linking). Kevin
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/31/2010 07:04, you wrote: Bob, Some of us are geographically disadvantaged when it comes to availability of equipment. Here in rural Australia, anything secondhand for Amateur Repeater construction is rare, shipping of anything is expensive. From what I read on the group, the U.S. Is overloaded with surplus equipment, sometimes at bargain prices. For me to buy and ship that cheap 1/4 wave can and pre-amp would be more expensive than buying a new Simrex Pre-Selector :-) I don't believe that. But if you want NEW coaxial resonator filters: http://anglelinear.com/filters/coax_filters.html' These are actually a bit smaller than your typical cavity resonator, they're very low loss. Apparently Chip decided to tradeoff a bit of Q to keep the loss low. But with 2 resonators you end up with a very low system NF selectivity only a bit less than the GLB (which is misleading, as I explained in my previous post because some of that selectivity is after the preamp, so it's still partially susceptible to OOB overload). If you factor in the ham discounts, the total for the dual coaxial resonator/preamp combo is a bit more than the Simrex preselector. But we KNOW the NF will be ~1.1 dB, we KNOW that ALL of the preselection will be ahead of the preamp, so with that info we can properly design a RX system. The Simrex units are made in the US, so you still have to pay to have it shipped down unduh. IF the Simrex-GLB product did not perform..the word would spread faster than the Black Plague and no one would buy them Not saying it doesn't work, but I am saying as an RF engineer that it's impossible with the data in hand to properly apply it's use in a repeater system. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/31/2010 10:02, you wrote: Re: Simrex - GLB Pre-Selector Verus You're actually comparing two different boxes. n...@... wrote: Nope - comparing GLB vs. GaAsFET/pass cavity combo Still not the equivalent box... you're still missing the post active-device stages. I don't need them. But I suppose one could add a 2nd pass cavity AFTER the preamp. I've never had to do that in ~30 years of repeater building. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). Care to publish your results here? Never thought about it much... those who were honestly interested in the subject the last time we rehashed the GLB active device topic here on the RB Group Emailed me direct and we exchanged various notes, diagrams and information. Much of what I have is based on the Pre-selector with the dual-gate mosfet, which I believe is similar to the BF-998 device. Why don't you tell us what you found? I've asked more than once for some reason you're reluctant to publish your results. I can't help but be even more suspicious of the GLB. Lacking the hard data, I'm going to do a little guesswork here: a typical GaAsFET preamp has 17 dB of gain @ 440 MHz. The Simrex preselector has a spec'd overall gain of 8 dB. All other things being equal, the combined loss of the resonators in the preselector would then be 9 dB. Kevin says the distribution is 2 stages before 2 after. If all the stages are equivalent, then the pre-active device loss is 4.5 dB. Assume 0.5 dB NF of the actual GaAsFET device, I come up with 5 dB NF. Am I close? GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... Perfectly fair. Used pass cavities in good condition are plentiful. Can't remember the last time I saw a used GLB or equivalent unit for sale, so I'm simply comparing what's readily available. Yeah, but the numbers are off. To properly compare the two you'd need to use more than one pass-cavity. At least one additional cavity (min) following the active device and to really be honest, more than one trailing BP Cavity. See above, Kevin's post. In many cases, the trailing cavity isn't needed. A single pass cavity usually has enough out-of-band rejection to be totally adequate on its own - no post-preamp filtering needed. The post-preamp filtering can and does contribute in the management (not necessarily the prevention) of high signal levels issues. ..only for RXs that need it. I guess I'm a bit biased because I use real RXs (GEs), so the only protection needed is for the preamp going in front of it. Then again, the fact that post-device filtering is used in the GLB makes me worry about the actual selectivity ahead of that device. If there's only 1 or 2 resonators ahead of it, that's not much protection. A 1/4 wave bottle will provide much more rejection ahead of that first amp, and with less loss hence lower NF. There are 2 resonators in front of the Active Device. The higher Q of a 1/4 wave cavity is obviously better. The honest to thyself person should determine the NF difference, which is probably not a huge amount. Once again, I'm still waiting for the NF numbers. IMO the Simrex amplified preselector is a space-saving compromise, nothing more. Bob NO6B Sure, it's a compromise that works well for what they are. I'd probably (and do) park a Simrex or GLB Pre-selectors in front of less than bullet-proof receivers. Something else to consider: if your less than bullet-proof RX has good sensitivity, a preamp isn't even needed - just throw a pass cavity in front of it. Simple cheap, you'll probably still end up with better sensitivity than if you used the Simrex preselector. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/31/2010 11:28, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier Kevin Custer kug...@... wrote: While the skirts and OBR of the GLB might beat a single cavity, many times it isn't necessary - The better skirts are mucho desired. As you so often like to state, it all depends on the application - in many cases it simply isn't necessary. But there are other solutions: if you want a brute-force window filter they're out there too. I have a few 5 MHz wide 1 dB loss UHF filters sitting on the shelf here that I acquired at the Dayton Ft. Tuthill hamfests. ~$40 each. Again, the loss is low enough that in most cases a leading preamp simply isn't needed. The problem is, like any receiver that has several helicals in cascade before the first active stage, the loss that precedes the active stage has a majority role in the overall NF of the system that follows. It matters little what the quality of the active stage is, because the loss has already determined (for the most part) the system Noise Figure. Kind of makes it look like helicals are a sin doesn't it... Ever wonder why the MVP/MastrII Micors are so deaf compared to more modern RXs? I haven't measured the loss of the UHF helical assembly, but the VHFHB front-end helicals have ~6 dB of loss. In all those radios, their own helicals effectively are all the post-preamp filtering you'll ever need. The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. Depends on the Pre-selector Model and age of the box ... I have GLB units here with two pre-device stages and three post-device stages. And I have versions with a more traditional helical design and others with more of a lumped parts layout. A 2-Meter version I have lots of pictures of has 1 stage of pre-selection and four trailing stages. The active device is an MRF-901. OK. MRF-901 NF @ 2M is ~1 dB, so maybe 2-3 dB NF for the unit. Not bad for VHFHB, but having only 1 little resonator in front of the active device doesn't offer it much OOB protection. Better put a (gasp) PASS CAVITY in front of it! ;) Every dB of loss ahead of the first active stage ADDS to the system NF - period. This loss can NEVER be recovered no matter how good the preamp is that follows. Kevin Yep. However, In more than a few real world situations you might really need the filter pre-selection a lot more than the most optimum NF. A practical trade of pre-selection for a slightly higher noise figure can and does sometimes make the difference in a usable radio system. OK fine. But again, we DON'T KNOW the noise figure for the device. Furthermore, since the filtering distribution varies with the model, it's very difficult to predict the dynamic range characteristics of the unit. Fun to play with? Yes. Can solve some IMD/overload problems? Certainly. But not a tool for any seriously engineered RF system. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Bob, Thanks for info and links:-) I see where you are coming from.. For me, repeaters are a new deal and I am as dumb as a box of rocks, on the subject..that's why I subscribed to the group. My first repeater project is a portable system, so I have to make it compact for commissioning in hard to access locations, read carry the system on motorcycle. The physical size of high spec filters are too large and prone to transport damage, for my concept. With my Hamtronics low current draw boards, mobile filters with pre-selector, small battery solar panel, 7m squid pole and 4dB antenna with accessories all fit easily on my off road motorcycle, which allow me to reach some good potential sites not accessible by 4x4, if needed. 73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Aug 1, 2010, at 12:00 PM, n...@no6b.com wrote: At 7/31/2010 07:04, you wrote: I don't believe that. But if you want NEW coaxial resonator filters: http://anglelinear.com/filters/coax_filters.html' These are actually a bit smaller than your typical cavity resonator, they're very low loss. Apparently Chip decided to tradeoff a bit of Q to keep the loss low. But with 2 resonators you end up with a very low system NF selectivity only a bit less than the GLB (which is misleading, as I explained in my previous post because some of that selectivity is after the preamp, so it's still partially susceptible to OOB overload). If you factor in the ham discounts, the total for the dual coaxial resonator/preamp combo is a bit more than the Simrex preselector. But we KNOW the NF will be ~1.1 dB, we KNOW that ALL of the preselection will be ahead of the preamp, so with that info we can properly design a RX system. The Simrex units are made in the US, so you still have to pay to have it shipped down unduh. IF the Simrex-GLB product did not perform..the word would spread faster than the Black Plague and no one would buy them Not saying it doesn't work, but I am saying as an RF engineer that it's impossible with the data in hand to properly apply it's use in a repeater system. Bob NO6B
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? Once again, the NF depends on what device arrives in your specific Simrex (aka GLB) Pre-Selector. It wouldn't hurt or be impossible to ask. A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. You're actually comparing two different boxes. The Angle, ARR, Telewave, Hamtronics, yadda-yadda brand Pre-amplifiers are just Pre-amplifiers. The Simrex / GLB box is an RF Amplifier with both Pre and Post filters. The lower realized gain of the Simrex / GLB unit is directly related to the additional filtering included inside the box. The Simrex unit makes a nice solution if you don't have the physical space for a 1/4 wave bottle. But it's not the primary reason why I like them. Then again, I know someone who just ordered an even smaller MtronPTI front-end crystal filter to solve a front-end overload problem from a TX over 1 MHz away from his RX. Crystal filters can be very helpful but people tend to depend on them to cure a Symptom. Not the best solution (higher loss, can't be re-tuned if he ever has to change freq.), but I guess it works for him. Sometimes you need a large brick wall with only a small doorway so they obviously have their usefulness. If you picked the UHF Simrex unit with less gain, more selectivity than the standard model, then the noise factor should be better? The less gain is related to the internal filtering stages, not the device. I would expect the version with the GasFet to have the typical expected NF for that device. Any other available active devices to have Noise Figures commensurate with their typical expected values (for those devices). I doubt it. But without any real NF numbers, it's all guesswork. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). I wouldn't expect a surprise/difference if I ordered the GasFet equipped Simrex Pre-selector. I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... No, your selection should be based on your needs. Do you need a bare RF-Pre-amplifier or do you need an integrated, amplified RF Pre-selector box? GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Reads like you're comparing a new Preamp with a used cavity against the price of a new Simrex (GLB) Pre-selector. That's not really fair... One of the really nice (and mostly overlooked) items about the Simrex (GLB) unit construction (and operation) is the Post (active) Amplifier Filtering (tuned circuits). For more than one real reason they can be one of the most under reported bacon saver in your fry pan. s.
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
skipp025 wrote: The less gain is related to the internal filtering stages, not the device. I would expect the version with the GasFet to have the typical expected NF for that device. Any other available active devices to have Noise Figures commensurate with their typical expected values (for those devices). I doubt it. But without any real NF numbers, it's all guesswork. I have measured all the original GLB Pre-selectors and their performance values are very realistic (no surprises). I wouldn't expect a surprise/difference if I ordered the GasFet equipped Simrex Pre-selector. What you are missing is that ANY losses ahead of the first active stage add to the noise figure of the system - directly. So, while filtering exists in the GLB device, so does loss, and this loss is more than what is experienced when using a quality large diameter cavity. Many times Skipp you tell us there is no free lunch, and the same applies to the comparison of selectivity and loss between the GLB and a quality cavity followed by a good active stage. Real world test. Take a Hamtronics receiver (no preamp) and do a basic bench sensitivity test to obtain a baseline. If you find something around -123 dBm your in the right ballpark. Now install a bi-polar GLB preselector/preamp in front and measure the sensitivity again - you'll find you have lost several dB of bench sensitivity - at least 3 or 4 dB. Take the same receiver and add a quality 1/4 bottle with a good preamp (your choice - something with 1.5 dB NF or less) and do the test again. Now, the receiver hears at -123 to -127 dBm (dependent mainly upon the quality of the preamp that follows) because the filter hasn't severely ruined the system NF ahead of the first active stage. Even though the GLB has gain, the noise figure of the design has already determined the sensitivity that will be realized by the receiver that follows. The GLB preselector/preamp should not be considered for adding basic sensitivity, because it's possible (depending on how good the receiver is to begin with) the opposite will happen - however, it will protect a receiver that lacks good front-end filtering, like the Hamtronics. Like everything, the situation helps to dictate what equipment will give the best results. Kevin
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Well, I have decided to buy and try a Simrex Pre-Selector,experiment with it, test it...whatever. At the very least I will learn something from my experiences and it may end up being useful to me, anyway. I certainly appreciate the different points of view here and have learn't of alternative methods and the reasoning behind them. For me, all of it is good information :-) 73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Jul 31, 2010, at 8:53 AM, skipp025 skipp...@yahoo.com wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier Kevin Custer kug...@... wrote: What you are missing is that ANY losses ahead of the first active stage add to the noise figure of the system - directly. What we have here is... failure to communicate... (a line from a famous movie). Actually I was trying to high-lite the active device Noise Figure comparison and the lower spec'd gain value of (at least) the GasFET GLB was mostly from the insertion loss of the Pre and Post Filtering. So, while filtering exists in the GLB device, so does loss, and this loss is more than what is experienced when using a quality large diameter cavity. Sure... but again we are comparing a box to another box and they are not the same device. Please allow me to paste some of the text from a recent post (by me). [pasted text] The Simrex (aka GLB) units are actually amplified pre-selector assemblies, not just plain wide-band Receive Pre-Amplifiers. [end of pasted text] Many times Skipp you tell us there is no free lunch, and the same applies to the comparison of selectivity and loss between the GLB and a quality cavity followed by a good active stage. Correct... and a Simrex GLB Pre-Selector should really not really be directly compared to a cavity followed by a good active stage. The Simrex GLB box is more of a true Pre-Selector layout and contains post amplifier filtering. To better equate a similar layout would have you add at least one or more cavities after the active device. And yes we should clearly acknowledge the hopefully obvious lower loss through a higher Q Quality Cavity. Real world test. Take a Hamtronics receiver (no preamp) and do a basic bench sensitivity test to obtain a baseline. If you find something around -123 dBm your in the right ballpark. Now install a bi-polar GLB preselector/preamp in front and measure the sensitivity again - you'll find you have lost several dB of bench sensitivity - at least 3 or 4 dB. Take the same receiver and add a quality 1/4 bottle with a good preamp (your choice - something with 1.5 dB NF or less) and do the test again. Now, the receiver hears at -123 to -127 dBm (dependent mainly upon the quality of the preamp that follows) because the filter hasn't severely ruined the system NF ahead of the first active stage. Even though the GLB has gain, the noise figure of the design has already determined the sensitivity that will be realized by the receiver that follows. The primary land mine in the above comparison is the Now install a bi-polar GLB preselector/preamp. Remember Simrex and GLB offer or did offer a GasFet version of their Pre- selector box. Separate the above in the proper context and the focus should be on the filtering in front of the same type of active device. No one here should discount the higher Q cavity will be the better spec. But again even a high Q cavity with a same or similar active trailing device is still not the same box as the Simrex GLB Pre-selector. You should account for the Simrex GLB integrated post active device filtering. The GLB preselector/preamp should not be considered for adding basic sensitivity, Did anyone make that claim? because it's possible (depending on how good the receiver is to begin with) the opposite will happen - You are correct. however, it will protect a receiver that lacks good front-end filtering, like the Hamtronics. Like everything, the situation helps to dictate what equipment will give the best results. We agree and it's a Friday... Mark today on your calendar. What more could anyone ask for? Kevin cheers, skipp
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Friday, July 30th 2010 - Duly noted on my calendar! LOL - Original Message - From: skipp025 To: Repeater-Builder@yahoogroups.com Sent: Friday, July 30, 2010 6:53 PM Subject: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier Re: Simrex - GLB Pre-Selector Pre-Amplifier Kevin Custer kug...@... wrote: What you are missing is that ANY losses ahead of the first active stage add to the noise figure of the system - directly. What we have here is... failure to communicate... (a line from a famous movie). Actually I was trying to high-lite the active device Noise Figure comparison and the lower spec'd gain value of (at least) the GasFET GLB was mostly from the insertion loss of the Pre and Post Filtering. So, while filtering exists in the GLB device, so does loss, and this loss is more than what is experienced when using a quality large diameter cavity. Sure... but again we are comparing a box to another box and they are not the same device. Please allow me to paste some of the text from a recent post (by me). [pasted text] The Simrex (aka GLB) units are actually amplified pre-selector assemblies, not just plain wide-band Receive Pre-Amplifiers. [end of pasted text] Many times Skipp you tell us there is no free lunch, and the same applies to the comparison of selectivity and loss between the GLB and a quality cavity followed by a good active stage. Correct... and a Simrex GLB Pre-Selector should really not really be directly compared to a cavity followed by a good active stage. The Simrex GLB box is more of a true Pre-Selector layout and contains post amplifier filtering. To better equate a similar layout would have you add at least one or more cavities after the active device. And yes we should clearly acknowledge the hopefully obvious lower loss through a higher Q Quality Cavity. Real world test. Take a Hamtronics receiver (no preamp) and do a basic bench sensitivity test to obtain a baseline. If you find something around -123 dBm your in the right ballpark. Now install a bi-polar GLB preselector/preamp in front and measure the sensitivity again - you'll find you have lost several dB of bench sensitivity - at least 3 or 4 dB. Take the same receiver and add a quality 1/4 bottle with a good preamp (your choice - something with 1.5 dB NF or less) and do the test again. Now, the receiver hears at -123 to -127 dBm (dependent mainly upon the quality of the preamp that follows) because the filter hasn't severely ruined the system NF ahead of the first active stage. Even though the GLB has gain, the noise figure of the design has already determined the sensitivity that will be realized by the receiver that follows. The primary land mine in the above comparison is the Now install a bi-polar GLB preselector/preamp. Remember Simrex and GLB offer or did offer a GasFet version of their Pre- selector box. Separate the above in the proper context and the focus should be on the filtering in front of the same type of active device. No one here should discount the higher Q cavity will be the better spec. But again even a high Q cavity with a same or similar active trailing device is still not the same box as the Simrex GLB Pre-selector. You should account for the Simrex GLB integrated post active device filtering. The GLB preselector/preamp should not be considered for adding basic sensitivity, Did anyone make that claim? because it's possible (depending on how good the receiver is to begin with) the opposite will happen - You are correct. however, it will protect a receiver that lacks good front-end filtering, like the Hamtronics. Like everything, the situation helps to dictate what equipment will give the best results. We agree and it's a Friday... Mark today on your calendar. What more could anyone ask for? Kevin cheers, skipp
[Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier The Simrex (aka GLB) units are actually amplified pre-selector assemblies, not just plain wide-band Receive Pre-Amplifiers. There's a reason why these units offer/spec 8dB Gain versus the others un-protected preamps. The less obvious gain when compared to traditional GasFet and BiPolar RF Preamplifiers is due to the internal integrated pre and post filtering. What does it all mean... If you have some types of IMD grunge problems when trying a regular GasFet/BiPolar Preamplifier, chances are your results using the Simrex/GLB pre-selector might be better. The pre and post filtering inside the Pre-Selector is a big deal. s. Jack Chomley ra...@... wrote: This unit looks like the answer for me..I think! I am in the process of building 2 repeaters on 70cm for possible emergency use by our Amateur Radio Club. Both will be very portable, I have Vertex VX-2200 radios for one, Hamtronics T304/R306 boards for the other. In keeping the units very portable, I am using these filters http://www.polarelectronicindustries.com/model.php/model_id/983/ These filters are being used due to portability, but I have reservations about performance IF my systems end up sited close to other equipment. Would the Simrex unit offer advantages when used with my filters? I would look to getting the higher selectivity spec, in preference to gain. 73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Jul 30, 2010, at 6:20 AM, Steven M Hodell st...@... wrote: More great info from Frank @ SIMREX. Steve ~ KA1RCI - Original Message - From: Frank Neuperger To: fr...@... Cc: Steven M Hodell ; maqui Mike Aquilino ; sa...@... Sent: Thursday, July 29, 2010 4:15 PM Subject: Re: [Repeater-Builder] Fw: TKR750 -Preamp Steve, That did not post to the list because I don't have membership. I would appreciate if you would forward to the list on my behalf. Regards Frank On 7/29/2010 3:58 PM, Frank Neuperger wrote: Steven, Thanks for the vote of confidence. We sell them to Hams (Ham frequencies) for $275($25 less than 2-way dealers) as a gesture to Gil Boelke (silenet key) who developed the preselector and founded GLB (now owned by SIMREX). Note that the standalone preselector is still made in house by staff that were trained by Gil and on tooling built by Gil. It is also the front end of every SNRDS-II radio. Same radios that are part of many FAA and other Federal systems. Standard gain is 8 db For +$25, you can order it with ~4 dB gain and steeper skirts on the filter or ~11 dB gain and gentler slope on the filter skirts. For extreeme interference, one usage of the preselector has been to place a coax stub notch filter between the antenna and the preselector. I have done ~3MHz spacing of the notches of the comb for a marine radio BASE application using ~ 35?? feet of LMR 400 for my stub. One of the notches was placed directly onto the frequency of collocated marine base station giving us an extra 22dB of suppression at that frequency. Using excessive length of coax resulting in a comb allows the overallbandwidth to repeak steeply and with minimal loss between the notches of the comb. A single notch approach (not a comb) when placed close to your desired carrier will often have unacceptable insertional loss at your desired frequency.The comb notch filter is just extra coax and minimizes the insertion loss close to the notch. . The calcs for length are not too difficult. Warmest Regards Frank Neuperger Simrex Corporation VE3FNZ - Original Message - From: Steven M Hodell To: Repeater-Builder@yahoogroups.com Cc: sa...@... Sent: Thursday, July 29, 2010 2:34 PM Subject: Re: [Repeater-Builder] Fw: TKR750 -Preamp Another very good choice for this application is the GLB Preselector / Preamp from Simrex Corp. I have several of these deployed on 144 / 220 / 440 Amateur repeaters and they all perform very well. They can be tuned to maximize gain or selectivity per your requirements and their support is outstanding. http://www.simrex.com/site/products/special.htm PRESELECTOR PREAMPLIFER Low cost solution to interference, intermodulation and desensitization problems. Helical Resonator Design. Preselector Specifications (pdf 63k) Preselector Example Diagram (pdf 10k)
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/29/2010 14:59, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier The Simrex (aka GLB) units are actually amplified pre-selector assemblies, not just plain wide-band Receive Pre-Amplifiers. There's a reason why these units offer/spec 8dB Gain versus the others un-protected preamps. The less obvious gain when compared to traditional GasFet and BiPolar RF Preamplifiers is due to the internal integrated pre and post filtering. What does it all mean... If you have some types of IMD grunge problems when trying a regular GasFet/BiPolar Preamplifier, chances are your results using the Simrex/GLB pre-selector might be better. The pre and post filtering inside the Pre-Selector is a big deal. Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. The Simrex unit makes a nice solution if you don't have the physical space for a 1/4 wave bottle. Then again, I know someone who just ordered an even smaller MtronPTI front-end crystal filter to solve a front-end overload problem from a TX over 1 MHz away from his RX. Not the best solution (higher loss, can't be retuned if he ever has to change freq.), but I guess it works for him. Bob NO6B
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
On Jul 30, 2010, at 9:31 AM, n...@no6b.com wrote: At 7/29/2010 14:59, you wrote: Re: Simrex - GLB Pre-Selector Pre-Amplifier The Simrex (aka GLB) units are actually amplified pre-selector assemblies, not just plain wide-band Receive Pre-Amplifiers. There's a reason why these units offer/spec 8dB Gain versus the others un-protected preamps. The less obvious gain when compared to traditional GasFet and BiPolar RF Preamplifiers is due to the internal integrated pre and post filtering. What does it all mean... If you have some types of IMD grunge problems when trying a regular GasFet/BiPolar Preamplifier, chances are your results using the Simrex/GLB pre-selector might be better. The pre and post filtering inside the Pre-Selector is a big deal. Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. The Simrex unit makes a nice solution if you don't have the physical space for a 1/4 wave bottle. Then again, I know someone who just ordered an even smaller MtronPTI front-end crystal filter to solve a front-end overload problem from a TX over 1 MHz away from his RX. Not the best solution (higher loss, can't be retuned if he ever has to change freq.), but I guess it works for him. Bob NO6B If you picked the UHF Simrex unit with less gain, more selectivity than the standard model, then the noise factor should be better? I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... No such thing as a free lunch :-) 73, Jack VK4JRC
Re: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
At 7/29/2010 16:52, you wrote: Yes, but the NF is unpublished unknown. I therefore assume it's poor, maybe 3 dB? A Chip Angle GaAsFET @ UHF has 0.5 dB NF. Put a 1/4 wave pass cavity with 0.5 dB loops in front of it you have a narrow 1 dB NF front-end. I'll take 1 dB NF over 3 dB NF any day. The Simrex unit makes a nice solution if you don't have the physical space for a 1/4 wave bottle. Then again, I know someone who just ordered an even smaller MtronPTI front-end crystal filter to solve a front-end overload problem from a TX over 1 MHz away from his RX. Not the best solution (higher loss, can't be retuned if he ever has to change freq.), but I guess it works for him. Bob NO6B If you picked the UHF Simrex unit with less gain, more selectivity than the standard model, then the noise factor should be better? I doubt it. But without any real NF numbers, it's all guesswork. I guess it comes back to price too, a 1/4 wave can plus pre-amp will cost more money... GaAsFET preamp is ~$130. I don't think I've ever paid more than $50 for a pass cavity, so the total is ~$100 less than the GLB unit. Bob NO6B
