PSK31 is a digital transmission mode designed for minimum bandwidth and great performance even in weak signal conditions. Sending a PSK31 signal over the air using Frequency Modulation (FM) completely eliminates both the narrow bandwidth and the robustness in weak signal conditions.
PSK31 is a general term for all kinds of M-ary Phase Shift Keying (PSK) modulation schemes with a baud rate of 31.25 baud. This mode is commonly used to transmit short text messages such as a quick description of one’s station. The most common modes are BPSK31 and QPSK31. As the names suggest, BPSK31 uses Binary Phase Shift Keying (BPSK) and QPSK31 uses Quadrature Phase Shift Keying (QPSK). For this article, I am going to focus on BPSK31.
A BPSK modulated signal with a data rate of 31.25 baud will result in a bandwidth of 31.25 Hz . Generally, most articles speak of 31 Hz because it’s easier to read, write and remember. But this bandwidth is of course only true if you indeed do transmit a BPSK signal, not FM. Now I know what you are thinking, “Why BPSK? I use SSB for PSK31!”
When a transmitter in Single-Sideband (SSB) mode is used and a M-ary PSK (BPSK, QPSK, etc.) signal is applied to the audio input of such said transmitter, the SSB transmitter will merely act as upconverter and emit a M-ary HF signal. If one applies a BPSK modulated 1 KHz tone to the audio input of a SSB transmitter set to 14.070 MHz upper sideband (USB), this will result in a BPSK modulated 14.071 MHz signal. Therefore, the overall bandwidth of the HF signal is equal to the bandwidth of the original BPSK IF (audio signal).
One thing that many amateur radio operators seem to have a problem with is understanding that the audio fed into the transmitter is already a modulated signal. It is not the modulation information, but the modulation information modulated on a carrier already.
What happens if a BPSK modulated audio signal is connected to an FM transmitter? Carson’s bandwidth rule should be able to shed some light on this question. Carson’s bandwidth rule is used to determine the bandwidth of a FM signal in which 98% of the energy is contained . The 17 dB bandwidth, so to speak.
The formula reads as follows:
B = 17 dB bandwidth (98 %)
Δf = FM deviation
fm = highest frequency in the modulation signal
A normal audio voice signal in most commercial and amateur radio sets is limited to only transmit the range between 400-3000 Hz. Therefore, the maximum frequency we need to consider for bandwidth calculations is 3 kHz.
The FM deviation is around 5 kHz for signals with 25 kHz channel spacing and around 2.5 kHz for 12.5 kHz wide channel spacing (narrowband FM).
Using a normal audio signal with a maximum frequency of 3 kHz, the bandwidth according to Carson’s formula would result in 16 kHz for the 5 kHz deviation and 11 kHz for 2.5 kHz deviation. Please note that the actual bandwidth is wider than this and additional guard band between channels is needed. In a practical set-up an entire 20 kHz channel is occupied.
If a PSK31 modulated audio signal is now used to modulate the FM transmitter, the bandwidth will behave accordingly. Assuming a 1 kHz audio carrier, the resulting FM bandwidth will be 12 kHz for 5 kHz deviation and 7 kHz for 2.5 kHz deviation. Since most amateur radio sets have a deviation of 5 kHz, let’s stick with the 16 kHz bandwidth.
That means our resulting FF signal is 19,968.75 Hz wider than necessary. Or in other words: the resulting signal is occupying 64000 % of the necessary bandwidth. But that’s not all, the weak signal advantages disappear now, as well. This is because they will stay the same as the weak signal capabilities of a “regular” FM signal. Therefore, conclusively PSK31 over FM is nothing but a reductio ad absurdum. Or, in plain English, nonsense.
I think the reason why PSK31 over FM is becoming so popular is because few understand what they are actually doing and what is going on inside their radio. For most, it’s just “you connect your sound card to your radio and you have this super cool digimode, it’s that simple.” Well, quite obviously, it’s not that simple.
Now I am sure a few people will complain about this article and say PSK31 over FM is still useful to get a taste of PSK31 and to get your feet wet. No, you don’t test drive a bicycle to see how flying an air craft feels like either. In the same fashion, I doubt anyone would seriously pursue doing CW over FM by keying an audio tone off and on (other than for CW training purposes).
And last but not least, if you decide to actively participate in this nonsense, at least get your facts straight. You did not do “PSK31,” you had an FM QSO, nothing else. Your QSL cards, QRZ.com log book entries, etc. need to read FM, not PSK31. However, your emission designator changes from F3E (FM voice) to F3D (FM data).
Links and Sources:
 M-ary PSK calculator, BalticLab: http://baltic-lab.com/
 Carson bandwidth rule, Wikipedia: http://en.wikipedia.org/