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From: dvh@cci632.UUCP (David Hallidy)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: RF preamp techniques
Keywords: preamp, splitter, combiner, noise figure, gain
Message-ID: <23443@cci632.UUCP>
Date: 6 Dec 88 17:55:05 GMT
References: <2038@nunki.usc.edu> <23211@cci632.UUCP> <23212@cci632.UUCP>
Organization: CCI, Communications Systems Division, Rochester, NY
Lines: 54

In article <23212@cci632.UUCP>, dvh@cci632.UUCP (David Hallidy) writes:
> 
> 
> I forgot to add that the Avantek preamp circuit is specified
> from 400-500 MHz. They also, in the same app. note, talk
> about designs for the 900 MHz and 1300 MHz bands.
> 
> Also, the reason you don't want the preamp after the lossy
> splitter (or cable) is because the system noise figure is
> primarily determined by the gain and noise figure of the
> first stage. In this case, the first stage has a 3dB NF, and
> -3dB gain. Running the noise figure cascade on this system,
> with a 0.4dB preamp after the splitter, will yield a noise
> figure cascade somewhere in the 2dB range.
> 
> After the preamp, you can play a lot of games,for the same
> reason- the first stage determines the overall noise figure-
> assuming the preamp has enough stage gain to overcome the
> loss and noise of the following stage(s). Therefore, if you
> have a preamp with a 0.4dB NF and 20dB gain at the antenna,
> you can overcome a lot of loss after the preamp and still
> retain most of that excellent performance.
> 
> I think I got it all out this time.
> 
> 73  Dave     KD5RO
> 


Boy, sometimes I REALLY do it!!!  It was pointed out
to me that the noise figure cascade as described in the second
paragraph of this follow-up is incorrect. IT SURE IS!!!
The correct system noise figure for the scenario described
above is actually (by my now questionable calculations) 3.3dB.
It isn't possible for the system, as described, to have a
noise figure lower than the noise contribution of the first
stage, and since the second stage isn't totally noiseless,
it also has to be factored in (which I did do, just wrong).
In the cascade formula, the gain of the first stage is required
in the denominator of the part of the equation relating to the
second stage contribution. I did include it, but as a decibel
expression, not as a RATIO- since the equation works in raios,
this really changes the result!!!  As I said, sometimes I
really do it.

Bottom line is, put the preamp at the antenna and you won't
have any problems. 20% bandwidth is no problem to achieve
and you should keep an excellent system noise figure across
the 400-500 MHz range.

I hope this is finally right.  (THANX GLEN FOR THE EMAIL
POINTING THIS OUT)

73   Dave    KD5RO