Satgen356 Going Digital Pt4 by GM4IHJ 20 Jan 96 Several readers have asked , " what bandwidth and mode should we use to receive Phase Shift Keying signals at 1200 bps?" Given the sort of filter used in Fo20 to shape its keying pulse, you needed a bandwidth of 2.1 Khz minimum, and you want your receiver switched to accept either CW, or, USB. The other PSK downlink digisats ( Pacsat A016, Webersat Ao18 and Lusat Ao19 ) have provision on one of their downlink frequencies, for a keying mode with a special shape and name. It is called a Raised Cosine pulse, and it consists of the positive going half of a cosine wave raised slightly above zero by the addition of a small DC voltage. The bit shape is almost the same as Fo20s, but it has the advantage that its shape is readily defined using finite hardware. If you have an oscilloscope and can display the keying waveform, you will see that Raised Cosine has the perfect "EYE " shape. Whereas the Fo20 waveform being the product of an RC network , is not quite symetrical. Needless to say, after a period of experiment Ao16, Ao18 and Ao19 have all used the downlink with the Raised Cosine waveform. PSK can if required be used on terrestrial packet communications, provided the other station is also using PSK. Simply operate your TX+PSKmodem in USB mode instead of the standard packet FM mode . If however you try receiving commercial data traffic ( Eg Inmarsat, Marisat etc ), you will find that BPSK bipolar phase shift +90 to -90 , as used by Amateurs, is replaced on most circuits by QPSK which has 4 different phase settings +90 +45 -45 -90. This QPSK cannot be received on amateur radio modems. Not all radio amateur packet satellites use PSK. Some use FSK, Frequency shift keying, because the FM system is simpler to generate , and it has far less stringent tuning requirements. A big plus where doppler shifts at 437 Mhz , reach +8 to -8 Khz on a typical satellite pass. Against these advantages , FM does require a stronger signal . But in going to FM, the builders of Uosat 22 and the Koreasats were able to push the data speed up to the maximum that would be possible with ordinary radio amateur FM equipment. This turned out to be 9600 bps ( 8 times faster than 1200 bps PSK) . 9600 bps just fits inside a bandwidth of +5.6 to -5.6 Khz ie 11.2 Khz. Unfortunately this wide band width is not easily achieved. Radio Amateur FM transceivers used for Voice, feature Pre emphasis and De emphasis circuits in the Tx and Rx respectively to maximise the spread of the modulation across the bandwidth. These circuits are however, no help at all with digital transmissions , so they must be bypassed. In practise this means that the would be FM digital radio amateur has to open up his rigs, and solder in a direct audio signal usually to the transmitter varactor, and, a direct outlet audio straight off the receiver demodulator /discriminator. This sort of action invalidates the equipmant guarentee, so not all radio amateurs have the skill or, the desire to do it. Hence the retention of the slower PSK. Although the next Japanese satellite follow on to Fo20 has provision for both FSK and PSK. These cannot be used simultaneously, so it is to be hoped that they may perhaps be available on different days. Future satgens will discuss more aspects of the FM/FSK option.