Why PSK31 over FM is nonsense

The current issue of ARRL’s magazine QST features an article about PSK31 on 2m in FM. This article is going to show why this is an unnecessary waste of bandwidth and therefore nonsense.

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.

Envelope of a PSK31 signal, carrier: 1 kHz

Envelope of a PSK31 signal, carrier: 1 kHz

A BPSK modulated signal with a data rate of 31.25 baud will result in a bandwidth of 31.25 Hz [1]. 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.

180 degree phase shift at amplitude minimum of a PSK31 signal

180-degree phase shift at amplitude minimum of a PSK31 signal

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 [2]. The 17 dB bandwidth, so to speak.

The formula reads as follows:

B = 2(\Delta f + f _m)

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:

[1] M-ary PSK calculator, KF5OBS: http://jaunty-electronics.com/

[2] Carson bandwidth rule, Wikipedia: http://en.wikipedia.org/

10 thoughts on “Why PSK31 over FM is nonsense

  1. Doing PSK on FM is a waste. How many FM transmitters hold that tight a frequency tolerance?
    With a 1PPM tolerance @ 30MHz. you are +/- 30Hz.
    With a 1PPM tolerance @ 146MHz. you are +/- 146Hz.
    With a 1PPM tolerance @ 446MHz. you are +/- 446Hz.
    I would be more prone to try MT-63 or Olivia on SSB. I don’t know if these are approved for Amateur Radio but we use them in MARS on HF and they will work weak signals when you can just barely hear them with the human ear! I agree with Mark/N5RFX that digital operation should be regulated by bandwidth and not mode. The ARRL needs to mount a charge and make this happen! For EMCOMM work 2 meter SSB/Digital in large Metro areas like the DFW area where I live could be a real shot in the arm passing Emergency traffic. Any mode that can connect directly to the internet like Winmor/Paclink or text that can quickly be converted to a mode that goes to the internet. The EMCOMM movement of both the ARRL and CQ magazine is going to save our hobby!

    Jim/WB4GWX TX

  2. For the most part I agree that psk31 over FM is a waste of bandwidth. Your analysis clearly shows that from a purely technical perspective.

    The one use of this mode that I do believe has merit is as a quick ‘field expedient’ method of moving text files between locations during emergencies. All that is required is a computer with sound capability, a thumb drive with Digipan, and a handheld radio.

    This is of course not as elegant as the Winmor/Paclinc option, but for locations where the resources do not exist for other text transfer modes, this quick solution has been proven to work well.

    Other than a little short practice to become familiar with the setup mentioned above for potential emergency use, using PSK software over FM is a bad idea due to the wasted bandwidth. We should however keep the ‘tool’ in the toolbox for those times when it fits the situational need.

    J. Nordlund – AD5FU
    Tactical Communication Specialist
    Preparedness and Response Branch,
    Arkansas Department of Health

    • John,

      packet radio is perfect for that. Why limit yourself to 31 Baud if you can have 1200 Baud just as easy? The system requirements are the same, computer with sound-card and one of the many free AFSK programs. Particular for the transfer of files there are software packages out there supporting TCP/IP over packet radio. As a matter of fact, almost every Linux / Unix system supports TCP/IP over AX.25 by default. There are also packet radio based chat clients, etc. Very easy to use stuff which serves you so much more efficient than PSK31 over FM. Packet also has handshake and error correction means that PSK31 simply doesn’t have.

  3. AMSAT has toyed with PSK31 on a couple of satellite missions.

    The premise is that the uplink is PSK31 SSB mode and the downlink is FM mode. Several different carriers on the uplink are demodulated as different carrier tones to present to the downlink. Thus four or more simultaneous channels are available.

    I think there was limited success but the concept was valid.

  4. Jim; in FM mode, the receiver is also FM. The PSK31 tones are simply a subcarrier. Frequency tolerance of the radios is a non issue.

  5. > That means our resulting FF signal is 19,999,968.75 Hz wider than necessary.

    Do you mean 19,968.75 Hz? That said, the qualitative analysis is right on.

    • Just because there is a software for it, doesn’t make it less nonsense. Also, I didn’t criticize PSK over 2m, I criticized PSK over FM.

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