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
