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
