> >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?
Two ways that I know about... the first is to measure it or locate someone else's data as I have also done. About a year or so back when I solicited here on the group for anyone wishing to part with spare GLB units... I found a web page with very well done 3rd order data and his minor circuit (bias) improvements. I then performed and closely verified his data with my own testing. Actually getting into the details of the GLB Pre-selector Preamplifier Operation in high signal levels was quite an eye opener (learning experience). > 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. Maybe your missing two small items... one is the GLB units are offered with GaAs devices and two... I have yet to see other brand amplifiers with the integrated same number and type leading and trailing tuned circuits (inside the preamp box) > >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. Depends if you focus on the preamp/device gain with less of an eye toward the very large signal operation. > > 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. What the signal coming out of the pipe (the amplifiers amplifier) actually is at and above P1dB (The 1 dB compression point) is what I'm more concerned with. An unhappy (or circuit designed to be) GaAs and Phempt device also make a killer mixer (grunge generator). The gain data on paper can be excellent but the buck-shot out the amplifiers tail pipe is what I'm paying more attention toward... There are cases where some bipolar device out perform GaAs Fet in rather harsh 3rd order (overloaded) applications. > >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. Not in every case... what is behind the Amplifier matters. > >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 doesn't... but it can improve it's 3rd order operation. > It may protect the receiver(s) downstream, but they do > nothing to improve the power-handling performance of > the preamp/preselector. If you consider the entire GLB unit as one composite amplifier... yes it can and most often does. If I can easily locate the web page I found the demonstration circuit on... I'll post it here. It is/was quite an interesting read... I found it by looking for the BF981 device information (the active device in Pre-Amplfier circuits like the GLB unit). s.
