Wes, Welcome to the digital modes. 1967 was a good year - I was also first licensed in '67.
This web site has some interesting information about digital modes. http://f1ult.free.fr/DIGIMODES/MULTIPSK/digimodesF6CTE_en An important parameter is the mean/peak power ratio for each of the modes. This gives an idea of the average output power relative to the maximum rated TX power that can be transmitted without distortion in the TX RF stages. The modes that transmit a single tone at a time with no phase modulation, such as FSK modes, have mean/peak ratios of 1.0. PSK31 has a mean/peak power ratio of 0.79. For PSK31, an average power of 50 to 60 watts should keep the TX out of the nonlinear region of operation for a transmitter rated at 100 watts max CW. In practice most folks use 25 to 50 watts to give a little more margin, to reduce the stress on the PA and because the difference between 40 watts and 60 watts will very rarely make a noticeable difference at the receiving end. In general: Nonlinearity in the TX RF stage will generate multiples of the RF carrier frequency as well as a broadening of the spectrum - particularly for PSK waveforms. Nonlinearity in the TX audio stages will generate multiples of the TX audio frequency. Nonlinearity in the RX RF stages will generate sum and difference products of the RF frequencies of the received signals. Most of these products will be outside the audio bandwidth. This takes an extremely strong signal. A strong signal outside the audio bandwidth can also cause attenuation or blocking of the desired signal. The receiver AGC, turning off the preamp and/or switching in the RX attenuator should eliminate these products. Nonlinearity in the RX audio stages will generate multiples and sum and difference products of the audio frequencies of the received signals. These ghost signals are often seen in my radio that has an early implementation of DSP. ALC activity is an indication that the TX power is approaching the nonlinear region of the TX RF stage. It also modifies the waveform if the ALC response time is not much greater than the time variation of the amplitude of the waveform - the symbol rate or baud rate. Since ALC is designed primarily for voice operation, the response time will not be long enough to avoid distorting PSK waveforms with a symbol rate of 31.25 symbols/second. FSK and AFSK theoretically produce identical waveforms. The difference is that AFSK is susceptible to nonlinearities in the audio and modulation stages as pointed out earlier in this thread. I hope this is helpful. Ed WB6YTE --- In digitalradio@yahoogroups.com, "w1...@..." <w1...@...> wrote: > > Greetings. > > I am Wes W1LIC in Bangor, ME and just joined this group. I run a Kenwood > TS-480SAT to vertical or dipole antennas. I have an old HP desktop running > XP and use either the FLDIGI or PSKExpress software. > > Although I've been a ham since 1967, I am a newbie to the digital modes. CW > has always been my primary mode of operation. I've had several contacts via > PSK31, but now am interested in trying other modes. This week I had my first > contact via OLIVIA, which seems to be a very interesting mode. > > I've always heard and read that on PSK31 we should greatly reduce our power > and not show any ALC indication when transmitting. Does this same advice > hold true for Olivia and other digital modes as well? I'd appreciate input > from some of you more experienced digital ops. > > Wes W1LIC
