In 2000, I took an interest in the PSK31 digital mode designed by Peter Martinez, G3PLX. This mode is ideally suited to QRP operation, having a bandwidth of only 31Hz. There is plenty of activity on the HF bands. Although I was interested in the technical aspects of the mode, I found operating it to be quite boring, somewhat reminiscent of playing arcade games, so rarely use it nowadays. A collection of useful articles about this mode can be found on the ARRL Website.

	Hardware requirements to implement PSK31 are modest. No expensive TNC or Decoder is required, a PC Soundcard is used instead. I built the simple interface shown on the left. It's primary purpose is to isolate the PC's ground from that of the Transceiver. A pair of 1:1 transformers salvaged from some V32bis modems isolate the RX and TX audio and an optoisolator provides isolation of the PTT signal. I have made up separate interface leads to be able to connect to either the IC735 or IC706mkIIG. Some problems were experienced with the PTT function and IC706. It seems the IC706 needs a very low resistance ground, so the optoisolator was replaced with a reed relay, which provides more reliable operation. I have revisited this issue and found a CNY17-3 will reliably key the PTT. Probably the best component to use is the PS7141-1A solid-state relay.  Suitable Interface schematics can be found on K4ABT's Website, who markets the ISO-Kit range of interface kits. Some of the commercial interfaces are very expensive, considering how few components are contained inside their pretty boxes. I'm currently using a homemade "Prodata" interface from the design by Bob Lewis, AA4PB, in March 2002 QST. This is much better (and cheaper!) than any commercial interface (with the exception of the excellent MixW RigExpert) that I've seen and is designed for use with Icom transceivers. My PC uses a Soundblaster PCI64 card. The PC is a 700Mhz Pentium III with 512Mb running Windows 2000 Professional. The performance is more than adequate for PSK31.
	Software requirements are also modest, as almost all the popular PSK31 programs are freeware. I started with Digipan v1.2, which is quite intuitive to use. The current version of Digipan is 1.8. I have also tried some of the other PSK freeware programs and although they all seem fairly similar, I favour Hamscope, which also runs CW, RTTY and MFSK16 modes and interfaces directly to my logging program, YPLog. Probably the best program for the digital modes is MixW32. This is an incredibly powerful program which supports all digital modes. It will also send CW using paddles via the parallel port This is an example of a PSK31 QSO using Digipan. The bottom window shows the "waterfall" tuning aid. The diamond is centered on the received signal from DL1BBR. Tuning of received PSK31 signals is accomplished by a mouse click on any one of the multiple displayed signals.  Although the PSK31 system has no error checking and correction, the narrow bandwidth imparts a good signal to noise ratio, resulting in a quite robust system.

I've also built the PSK20 kit from Small Wonder Labs. This kit is basically a PSK31 "appliance", being a crystal-controlled 20 metre transceiver optimised for the PSK mode. The PSK20 interfaces direct to a PC and is intended for use with Digipan, although WinPSKse now supports it. The complete transceiver is implemented on a 5.25"x4.5" PCB.   The kit took around 8 hours to build, Dave Benson's documentation was of it's normal high standard, reminding me of the old Heathkit manuals. After a quick visual check, I connected a laptop and antenna to the PCB, then switched it on for a smoke test. Not only did it work immediately, it was happily receiving PSK stations before any alignment. This really impressed me, so much so that it wasn't until the band went dead that I eventually started the alignment procedure. Alignment wasn't too difficult, although I deviated from the procedure set out by NN1G. I had some problems when using Digipan to align the carrier oscillator and found it was much easier to use the spectrum display on WinPSKse to centre the filter passband. After alignment, my PSK20 transmits a steady 2 watts output, as can be seen on the OHR WM2 meter in the photo. Even this fleapower has resulted in many 20 metre QSOs.

PSK31 Frequencies (USB mode normally used)
160m	1838.15
80m	3580.15
40m	7035.15 (Region 1&3, 7080.15 - Region 2)
30m	10142.150
20m	14070.15
17m	18100.15
15m	21080.15  (or 21070.15)
12m	24920.15
10m	28120.15

Links to resources on other PSK31 Webpages

PSK31 Software Links