At 11/27/2009 13:08, you wrote: > > >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).
Great! Too bad the manufacturer didn't do this; would've saved you & others the trouble of having to characterize a brand new component. > > 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) I didn't miss that at all. You seem to be missing my point that the leading, small resonators in front of the GLB will either degrade the NF far more than a 1/4 wave coaxial resonator, or offer far less out-of-band rejection. > > > 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. Only if the GaAsFET preamp is maldesigned. Some will break into oscillation at different source/load complex impedances. I've also seen some bipolar designs (the Kendecom RX front ends come to mind) where the 1st RF amp is biased at a rather high DC current value. Apparently the goal was to trade off NF for more dynamic range. So yes if you look far & wide enough you'll find exceptions to just about everything. > > >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. Behind? I assume you mean the RX (after). I don't worry about my RX's dynamic range - I use GEs :) But if you're not, more pass cavities, or even your GLB preselector, after the preamp are an easy fix, since loss is less of an issue there. > > >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. Well, again we're talking (OK, typing) but simply not communicating. P1dB & 3rd order intercept are closely related, as they are both measures of a widget's dynamic range/linearity, & I use the terms somewhat interchangeably for the purposes of this discussion. > > 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. Again, the ONLY filtering that will improve an amplifier's resistance to IMD is filtering on its input, not its output. The tuned stages that are after the GaAsFET in the GLB serve only to protect the following device (RX or another preamp) from overload by out-of-band signals. In the case of a good commercial RX like our GE Mastr IIs & Motorola Micors, this is almost always unnecessary, since they already have integral high Q (& lossy!) helical resonators. > 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). FWIW the BF981 is a dual-gate MOSFET, not a GaAsFET. I assume Aria switched to a GaAsFET for the current-production unit? Bob NO6B
