Xref: utzoo sci.electronics:4352 rec.ham-radio:7074
Path: utzoo!utgpu!watmath!clyde!att!rutgers!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: RF preamp techniques
Keywords: preamp, splitter, combiner, noise figure, gain
Message-ID: <1252@unccvax.UUCP>
Date: 30 Nov 88 17:20:50 GMT
References: <2038@nunki.usc.edu>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 47

In article <2038@nunki.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes:
> 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  |
>                           |______________|

	Feasable and doable, but probably not as good as a well
designed 400-500 mHz amplifier.

	You should be able to build a reasonable GaAs preamp
with an 0.5 - 0.7 dB noise figure with conventional microwave
transistors. The first splitter will have an insertion loss
associated with it (-3 dB); add it to your noise figure cal-
culation. That signal is gone **forever**.  As long as your
 amp is quieter than 3.5 dB, the single amp is better.

	Also, without good design, there will be differing
propagation delays through the two amplifiers. When the 
two signals finally do sum in the other combiner, you
may find marked gain reduction for some frequencies and
excessive gain for others.

	If you **must** listen to 425 and 475 mHz, you 
could do separate amplifiers for each frequency, and switch
between them. This will give some improvement, due to lower
noise bandwidth and the ability to obtain a conjugate impedance
match for the antenna system at the amplifier input.

	The broadband amp stinks in one respect, though;
if you have a strong RF source nearby (say, a TV station
on channel 18) it will be overloaded. Conventional slotted
line tecniques can null out that junk, though.

	There are numerous other considerations, but the
fact that there is attenuation in the input of the first
amplifier simply adds that attenuation to the first stage
noise figure.

York David Anthony
WKTD Wilmington, NC