Satgen 416 Signals in Noise by GM4IHJ (BID SGEN416) 15 Mar 97 A great deal of exciting amateur satellite operating involves attempting to read very weak signal in noise or, signals which regularly fade into noise. Lots of "AIDS" have appeared over the years, and it may be useful to review history ,past and present . The problem has been with us since the first days of radio, and though shunned by modern operators, the old fashion 600 ohm telegraphers headphones were in fact so narrow in their frequency response that they acted as a very useful audio filter. Noting that theory says - reduce the bandwidth to the minimum to exclude as much unwanted noise as possible. Even today , if you have a pair of these old phones you can attach them to your modern Rx low impedance audio jack via a reversed loud speaker transformer from an ancient thermionic tube Rx , and get excellent results. But as far back as the 1930s . Led by American communications receiver manufacturers, expensive crystal filters were begining to appear. By 1945 military versions of former amateur equipment all had xtal filters and some special military receivers were also using relatively low frequency IF amplifiers. Although much better than nothing however , the crystal filters had a habit of ear splitting "ringing" if excited by too much gain or an adjacent strong signal, and the low IF was prone to nasty image interference. Some relief from ringing , albeit expensive, was available in the mechanical filters of the top class Collins receivers, but no real break through occured until the advent of the transistor and its offspring the microchip, permitted complex circuitry without large size, to appear in the form of analog electronic filters. A typical example which has stood the test of time being the one manufactured by Datong. Whilst similar designs for the Home Brew brigade have featured regularly in the pages of QST, and several commercial receiver manufacturers have introduced continuously variable bandwidth filters into their receiver designs ( Eg the JRC NRD 535 series ). Now in the 1990s a further dimension has been added with the introduction of digital filtering techniques. These appear in two basic forms - the Fast Fourier Transform narrow band (2Hz) audio spectrum analysers of the AF9Y type , which tell you a signal is there visually but may not give you an audible output which you can read. Or ,in the form typified by W9GR's DSP II, which being an audio listening aid has much wider bandwidth than the inaudible FFTDSP . So what does the modern operator settle for. Naturally opinions vary , but the digital filter alone has a very square sided response and it can like the old crystal filter suffer from aliasing when strong adjacent frequency signals or QRM are present. This can be reduced by using IF bandwidth control in the receiver prior to the digital filter, and still further relief may be obtained by having an analog filter of the Datong type or similar in circuit after the digital filter. Admittedly a catch all solution , but it does seem to work for both satellite and EME weak signal work. While at IHJ the visual display of spectrum analysis from the FFTDSP4 software is also kept up and running , to give early warning . Noting that a complex 3 filter system takes a lot of adjustment before you can read the signal, and having the FFTDSP4 display is the best way to alert you to the fact that something is there before you go listening for it in the audio.