Re: The GLB Preselector- Preamplifier
> I think we've covered this before, but I'll say it > again: the noise figure spec, the most important spec > on any preamp, is missing! Would you buy a power > amplifier that only spec'd "10 dB gain"? Yes, if it's composite performance includes very good/great 3r Order Intercept Performance. 10dB of known high performance gain can be much better than a higher value of some popular preamplifier that may crap up in high signal environments. > Perhaps these are good preamps nonetheless, but be > advised: you'll never get as good of a noise figure > as using an ordinary GaAsFET preamp with a 0.25 or > even 0.5 dB loss pass cavity in front of it. Not true... the Phempt Device is considered higher performance over a conventional GaAs Fet. Although the low noise figure ranks near number 1 as a desired spec, the application is always a trade-off and a low noise device with poor high level performance is not a good thing. http://en.wikipedia.org/wiki/HEMT http://groups.yahoo.com/group/Repeater-Builder/post?act=reply&messageNum=96145 > Perhaps this isn't so important on antenna noise-limited > VHF bands, but in SoCal on 220 & 440 it is. The only > place I'd use a GLB preselector would be a space-constrained > application where a 1/4 wave resonator simply couldn't fit. A cavity would normally always be considered helpful... but we are/were talking just about the basic composite preamplifier packages. IE no external components... The GLB Pre-selector Preamp also has trailing tuned circuits, and they greatly improve its 3rd order performance. Somehow that fact seems to get lost in these posts. One can simulate the trailing tuned sections with a conventional preamplifier at the cost of the additional cavities or high Q tuned circuits. If you have the luxury of those additional cavities... then we are all playing in the same ball park. I have and use some GLB Pre-Selector Pre-Amplifiers in very high level RF Site locations where conventional GaAs Fet layouts don't fit or perform well... at least not without a lot more cumbersome or costly external cavities. s.
