> Great! Too bad the manufacturer didn't do this;
> would've saved you & others the trouble of having
> to characterize a brand new component.
I actually ended up verifying what another person on
a similar path did before me.
> 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.
We were talking about two different directions. Yes to
all the above if the topic is pre-amplifiers and external
cavities.
A GLB Pre-Selector Pre-Amplifier at a high RF Site
parked bare naked between a duplexer and a receiver can
out perform some bare preamplifiers. If you have the
luxury of the extra typical High Q band-pass cavities
then the NF will be better... but again the 3rd order
performance might not be and what happens after the
active device plays a much more important part of the
realized high signal level performance.
> Only if the GaAsFET preamp is maldesigned. Some
> will break into oscillation at different source/load
> complex impedances.
The key is the manufactures data sheet for the device
and the completed pre-amplifiers real world measured
3rd order performance.
> 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.
The web page I mentioned before goes through examples
of said... it was a very decent text about Fet Bias schemes
for realized improved 3rd order performance. I'm getting
motivated to go search through my notes, which are of
course buried under other "urgent" paperwork.
> So yes if you look far & wide enough you'll find
> exceptions to just about everything.
Yep, and when the supporting information is included it's
sometimes hard to quickly change conventional thinking.
> >Not in every case... what is behind the Amplifier matters.
>
> Behind? I assume you mean the RX (after).
Yes and no... I mean any selective networks between the
pre-amplifier output and the receivers first active device.
In specific the band-width and Q of those selective
circuits. For reasons other than the first line of
conventional thinking... the realized performance of
a receiver front in harsh signal conditions is directly
related to the receiver front end Q, the Band-width and
preamplifier operation/event in front of it through
the rx front end filtering and any preamplifier trailing
filter networks.
> 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.
Receiver/antenna system design and construction can be
just this side of voodoo magic.
> 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.
Closely related but not necessarily the same. Where
they are different in high level operation can be
and sometimes is a big factor in the system performance.
> 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.
Nope, the trailing stages do contribute to the
GLB IMD Performance. The out of band issue is an
additional side benefit.
> 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.
The key issue is the band-width of the above radio
front end circuits, which are fairly wide in the
real world.
> FWIW the BF981 is a dual-gate MOSFET, not a GaAsFET.
Yep, but the GLB trailing tuned circuits would also
improve the realized GaAs Fet device equipped 3rd
order performance... as would properly set cavities
between a pre-amplifiers output and the receivers
input. The external high Q band-pass cavities in
front of a lower Q wider band-width receiver front
end are always better.
I suspect the point being missed here... is when a
preamplifier is generating spectral buckshot in a
very toxic condition... the filtering after
the active device improves/reduces what the receiver
sees/has to deal with. Mucho better to have high
Q cavities if you can but the GLB trailing tuned
circuits are better than the nadda of a bare preamp
dumping a wider mix of poop direct into a receiver
front end.
No one wants an overloaded preamplifier condition
but there are interesting ways to manage and improve
the situation. The GLB preamplifier design is one
such layout that receives little credit for doing
the mentioned. I speculate the reason for the original
GLB trailing tuned network part of the design was at
least min two-fold. To deal with improving the high
signal level and the out of band performance.
> I assume Aria switched to a GaAsFET for the
> current-production unit?
When I last looked in... it was an option/choice.
OK, back out the door...
chow for now
s.
