At 11/27/2009 08:35, you wrote:
>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. ...but the P1dB for the GLB isn't given, so we don't have that either. How do we know (other than anecdotally) what the 3rd order intercept performance of the GLB within its passband is? Yes it has narrow passband characteristics, but my point is that that characteristic can be replicated with superior noise performance using a GaAsFET (or PHEMT) preamp & coaxial cavity filters. > > 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. Perhaps marginally, but insignificant for the purposes of this discussion. I sometimes use the terms interchangeably, since most PHEMTs are types of GaAsFETs. > 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. Agreed. But generally the GaAsFETs/PHEMTS have better P1dBs than bipolars & MOSFETs. >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... If your discussion excludes use of separate filter assemblies ahead of the preamp, then I think we're talking apples & oranges. Again, my point was that the combination of a pass cavity followed by a low-noise preamp will outperform the GLB units. >The GLB Pre-selector Preamp also has trailing tuned circuits, >and they greatly improve its 3rd order performance. How does a filter placed AFTER the active device help improve it's P1dB? It may protect the receiver(s) downstream, but they do nothing to improve the power-handling performance of the preamp/preselector. Bob NO6B
