> >The key is the manufactures data sheet for the device
> >and the completed pre-amplifiers real world measured
> >3rd order performance.
>
> This reply appears to be in the context of one designing
> their own preamp. I was thinking in terms of commercially
> available preamps.
So was I..
> >Receiver/antenna system design and construction can be
> >just this side of voodoo magic.
> I think this is where/why we seem to be missing each
> other's points. To me it's not magic at all: it's simple,
> straightforward engineering. If you have a ballpark antenna
> noise temperature, RX noise temperature (deduced from 12
> dB SINAD), knowledge of nearby (in freq. & location) potential
> "problem" radiators, you can calculate the necessary hardware
> (gain, required noise figure & filtering) to get the most
> noise-free signal detected.
Sure... but not all the variables are known in stone so
you described a conventional conservative side. I described
a dual series Rx Preamplifier layout with gain well in excess of
your 14dB value and how both commercial and amateur radio
installations all about Northern California have been using
it for quite some time (decades) with excellent results.
> I suppose the more unknowns you have in the above
> equation, the more it becomes "voodoo magic".
Actually, I would say the more variables because a lot
of the unknowns change quite a bit.
> >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.
>
> Irrelevant; we're really getting off track on this one.
> Neither P1dB nor 3rd order intercept of an amplifier is
> affected by filtering placed after it.
Actually it is... but it's beyond my desire to get into
that detail here. However as stated before... tuned circuits
after the preamplifier can and do help manage the IMD
reaching the receiver front end.
> >Nope, the trailing stages do contribute to the
> >GLB IMD Performance. The out of band issue is an
> >additional side benefit.
> Incorrect. If a large signal is presented to the
> active device, how do tuned stages on the OUTPUT of
> the active device protect it from overload? They can't.
They do to a degree... but I'm not going to bother to
explain it in detail here. The issue with the trailing
tuned circuit (regardless of type) is the filtering
of non F-center products when they occur. Management
of unwanted signals regardless of type. If you want to
go into the details of how the trailing cavities effect
the preamplifier operation, then we might eventually
consider moving this portion of the topic over to the
Yahoo rfamplifiers Group.
> >The key issue is the band-width of the above radio
> >front end circuits, which are fairly wide in the
> >real world.
>
> The measured 3 dB bandwidth of a G.E. MVP front-end
> helical resonator assembly is 1.8 MHz. I don't
> consider that "fairly wide".
Your reported MVP front end value is not bad relative to
some of the other receivers... but some of the more popular
repeater receivers don't have even the 1.8 narrow front-
end and the gross IMD buckshot of some overloaded
preamp conditions with little or no trailing internal
or external filters would more directly affect the
receivers performance.
I seem to remember the Micor front end being at least
a few MHz wide. And I'd be willing to bet not all of your
repeaters are MVPs.
> >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.
>
> No, they don't. They only improve the 3rd order
> performance of whatever is downstream.
Bingo... but also in reality the trailing filters
would also contribute toward the preamplifiers 3rd
order performance.
> If the preamp is generating "spectral buckshot",
> you're already screwed. The typical symptom here
> is on-channel crap being generated by the preamp,
> which of course cannot be filtered out.
Not in cases were the buckshot is managed. You
assume the products are primarily on-channel. The
data I saw demonstrated indicated the IMD was
not primarily on channel and managed with trailing
filters. I didn't think this up but I was impressed
with the demonstration and the applied information
in the real world.
> > 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, the correct solution is to not allow the nonlinearity
> in the first place.
Sometimes you don't have a choice and/or the benefits of
managing the preamplifier system can far outweigh the
detractors/negatives.
> Letting the preamp generate "poop" & then filtering the
> off-channel garbage you've already generated in that
> preamp is a poor solution;
Ah, now were getting close... now assume the preamplifier
generates really bad unwanted products in extremely overloaded
conditions that don't occur most of the time. When the
buckshot flies for relatively modest times... the trailing
internal/external filters (regardless of location) would help
a good receiver better deal with the event.
> the in-band garbage generated in the preamp goes right on
> through.
And now we ask how much F-center and close adjacent in band
garbage actually gets to the receiver front-end and how well
do the receiver(s) handle this event? With the right hardware
layout a lot better than you might assume at first glance.
> >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.
> Well, IMO it gets all the credit it's due.
> No published NF + no published P1dB or 3rd order intercept
> = suspect product.
For the more serious person these values can be fairly
easily measured...
s.
