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From: cyamamot@nunki.usc.edu (Cliff Yamamoto)
Newsgroups: sci.electronics,rec.ham-radio
Subject: RF preamp techniques
Keywords: preamp, splitter, combiner, noise figure, gain
Message-ID: <2038@nunki.usc.edu>
Date: 29 Nov 88 20:22:23 GMT
Reply-To: cyamamot@nunki.usc.edu (Cliff Yamamoto)
Organization: University of Southern California, Los Angeles, CA
Lines: 32

Greetings
	I have an arbitrary question regarding RF preamps, splitters, and
combiners.  Does the following look feasible?
                           ______________
 _____                    | GaAs FET amp |
 \ | /   __________       | 400-450 Mhz  |       __________
   V    |   3 dB   |------|______________|------|   3 dB   |
   |____| splitter |       _______________      | combiner |----> to receiver
        |__________|------| GaAs FET amp |------|__________|
                          | 450-500 Mhz  |
                          |______________|

	Would there be too much noise introduced by the splitter and combiner
to be of any use?  Or is it better to select one amp and operate it at 450Mhz
without the splitter/combiner?  If I use ONE amp, is it possible for an amp to
have a negative dB effect (i.e. attenuation) at the extremes?

Example : if I wished to monitor two signals at 425Mhz and 475Mhz would:
          1) the above diagram using two amps having a noise figure of 0.5dB
             and some gain X dB be better
      OR is
          2) one amp with a 400-500Mhz range, but higher NF and lower gain
             better.  (tuned to center of 450Mhz)

In short my question is : What do you do if you want to pre-amplify a wide
frequency range with high gain and low noise, but DO NOT have an amp with such
a wide BW available.

Cliff Yamamoto
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