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.
