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