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
