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.
