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SA7—2
SA7 INTRODUCTION:
A "preamplifier" gives an initial boost, measured in decibels, to the level of a signal before it is processed by
tuned circuits, filters, amplifiers, mixers, detectors or other circuit stages. Preamps may be designed for
either RF or audio use and may be either tuned or broadbanded. The SA7 is indeed a "general purpose"
RF pre-amplifier. While we realize that most users understand the term "broadband", we understand that
some of you would appreciate some clarification. In short, "broadband" means you can use the SA7 to
boost the RF level of any signal from the AM broadcast band through VHF and UHF FM and TV bands all
the way into the 1300 MHz microwave spectrum. This is possible because the SA7 does not limit or restrict
the RF input by means of tuned circuits or filters. Therefore, ALL signals reaching the input are amplified
equally. This amplifier differs from the Ramsey PR-10, 20 and 40 models for 144 MHz, 220 and 440 MHz
respectively, in that those models include hi-Q tuned circuits to "pre-select" the desired frequency range,
restricting the strength of other signals, out of band. A "pre-selector" is a pre-amplifier with a tuned input.
We'll show you how to use the SA7 as the heart of such a pre-selector configuration, either fixed or tunable,
if that is the practical application which is of most interest to you. So,"broadband" is a mixture of good news
and bad news, though it's mostly good news if you understand what such a circuit can and cannot do.
Because it boosts all signals equally, its successful operation relies on the input tuned-circuit selectivity of
the receiver whose performance you wish to boost. If the receiver does not have much ability to reject
unwanted signals, such signals will be boosted just as much as the desired signal.