This section was originally titled simply "PSK and Classic Radios " but I have subsequently become interested in digital SSTV transmission and will adapt the content of this page accordingly, in due course. However, in the meantime ...

PSK and Classic Radios

The aim of this page is not to attempt to be a definitive resource for those wishing to know all the various issues around digital communications in general , but instead I intend to describe some of the issues around using this most modern form of communication using equipment designed and built many years prior to these modes being invented.

I assume that the reader has experience of some of the many computer programs available to the PSK operator, these are far too numerous to mention, and as is the way with these things, I strongly suspect that each reader will have his or her own favourites.

My particular favourite (this week anyhow!) is a program called DM780 (short for Digital Master 780) and which is part of the "Ham Radio Deluxe" suite of programs written by Simon Brown HB9DRV which I find powerful and above all intuitive to use, and has facilities which I find very useful.

These facilities are extensive and I will not attempt to describe them in detail here, the best way to find out about the program is to download it and try it, it is absolutely FREE!

I have spent many hours over the last few years monitoring the various PSK frequencies, but it is only relatively recently that I have actually conducted any two-way contacts in this mode, but this has been successfully achieved with the following rigs :- FT-102, FT-107 and FT-897D.  Although at the time of writing I haven't transmitted in PSK mode from any other of my classic rigs, it isn't that unusual to hear stations on the bands using such rigs as FT-101ZD, TS-530, so I will surely try these others in due course.

Although strictly, the FT-897D isn't a "classic" rig as it is still current equipment it is worthy of comment as there are a lot of FT-897 users out there running PSK, but some of whose transmissions leave a lot to be desired in terms of linearity. Poor linearity means excessive spectral occupancy as well as less decodeability - most undesirable!

Interfacing

The first problem to solve is that of connecting together the computer and the radio, usually referred to as "interfacing".

There are many "PSK interfaces" out there in the marketplace, but to date I have managed to get by without using these commercial units, though that is not to say that I disapprove of them, I have just managed without this far (not _quite_ true regarding the FT-897D, but read on ...) Some of these units, and some of the designs for the home constructor which have appeared in many places use "opto-isolated" lines - this is in the main part a bid to provide as much isolation between the RF generated by the transmitter and the sensitive audio input circuitry of the computer. If you experience RF feedback problems, the simple solutions I describe below may not be good enough, and an "isolated" approach may help.

The computer naturally needs a feed of audio from the radio to be able to decode the incoming transmissions.

It is desirable though not essential to be able to provide this independently of the receiver volume control.

For outgoing transmissions the computer soundcard output, usually at what is often called "line level" (up to around a volt of output) sometimes but not always need to be connected to a microphone-level input on the transmitter which is in the millivolt level - this mismatch needs to be resolved before you can dare to transmit that first PSK signal.

The solution to this mismatch in levels between the computer soundcard and the radio input in my shack is an "adaptable box" (Goodness knows how many of these I have made over the years!) containing appropriate audio connectors (normally RCA/phono because they are simple and cheap!) and an audio attenuator made of  three resistors in either the "T" or "pi" configuration to reduce the volt of audio from the computer down to the tens of millivolts needed by the radio (microphone) input circuits. 

There are many esoteric designs for these but my experience is that you really don't need to be all that fussy, and parts from the junk box and a little native cunning will do the job - after all the computer and radio have a pretty wide range of adjustments already available, all that is required is to find a compromise whereby the solution is within the adjustment range of your existing equipment.   Without any kind of attenuator (professionals like to call these "pads") the chances are that the adjustments will be too finicky for practical use.

Soundcards

I should perhaps briefly mention the controversial subject of soundcards in computers.

A while back I built a "SoftRock Lite" Software-Defined-Radio and found that the built-in soundcard in my Dell Dimension 3100 (my "Radio" PC) wasn't really up to scratch for the successful implementation of the SDR software.   Because of this I splashed out and bought a better soundcard in the form of an "Audiophile" unit from M-Audio, who make the Delta 44 soundcard, strongly recommended by the SDR fraternity.  This proved more than adequate for the SDR programs, and, naturally it has become the workhorse for my PSK software.   Had I not got into SDR, I imagine that the on-board Dell hardware would have been perfectly adequate for PSK.  As it turns out, having two soundcards available can prove very useful - you should be able to configure your computer to be able to play your favourite music through your hi-fi whilst carrying out a PSK QSO should you so wish!

ALC and related issues

Most radios use some kind of ALC to ensure that transmissions are not over-modulated.  A side-effect of this feature is the compression of the signal, which in some cases (ie increase in "talk power") is desirable for some modes of communication, notably speech.

However, as with many things in life, what you gain on the swings, you lose on the roundabouts.   Compression brings with it distortion products, and in the case of PSK, many of the distortion products are "in-band", and can seriously mar your signal quality, both in terms of decodability, and of spectral occupancy (ie your transmission can get W - I - D - E).   Where you are able to monitor the ALC circuitry in your rig, please do so and ensure that you are not causing a lot of ALC action with your audio drive from the computer.    Ideally you should have enough signal from the computer just NOT to move the ALC metering - it practice I have found that just producing the smallest of deflections of the ALC meter is fine, but if the ALC is kicking over significantly, you are probably overdriving your transmitter.

It may be worth mentioning at this point what my "rule of thumb" is for the maximum amount of power you should transmit in digital (I am really thinking of PSK) modes.    In my view  you should allow a minimum of 6dB "headroom" to ensure acceptable linearity of your signal.   In other words, a100W SSB transceiver should not transmit more than 25W of RF, as measured on a mean power reading power meter.   Scale this figure up or down as appropriate - ie for an FT-817 running at 5W SSB, the power meter should be reading no more than 1.25W on PSK (mean power, not peak power).

PTT issues (keying of the transmitter)

The proper way to key the transmitter from the computer is directly to command the PTT line via some suitable interface which my be via the serial or parallel port of the computer (many modern computers don't have these at all), or by using VOX control.  My preference is to use the latter, though there can be a problem with the padding down of the audio signal between computer and transmitter dropping below the range over which the VOX circuitry can operate.   Where this is a problem, my solution is to connect a manually operated switch to the appropriate PTT port on the transmitter though I have to admit this is a tad crude, and I may well implement this in a different way in the future.

After all this talk, let's look at some real radios and how they might be connected up for PSK (interfaced!)

FT-102 and FT-107M

Discussed together because their connections to the outside world are similar.

Connection FROM radio TO computer :-  [TX] AF OUT RCA/phono jack (independent of AF Gain control) - connects directly to AF IN port on soundcard

Connection FROM computer TO radio :- [TX] PATCH (in) RCA/phono jack via Adaptable Box with 20dB (approx) T-pad then connected to AF OUT port on soundcard.

PTT :- [TX] PTT RCA/phono jack via screened flying lead to manual SPST switch to key transmitter when required.

FT-897D and FT-817ND

Discussed together because their connections to the outside world are similar.

The DATA port on which can be found the audio in and audio out signals is a standard 6-pin mini DIN socket, CAT control (amongst other things) is available on the 8-pin mini-DIN socket marked ACC.

I make use of an LDG Electronics RCA-14 Breakout Box - with a lead terminated at each end with at 6-way mini DIN connector connected between the breakout box and radio.

This useful little box is designed to break out both the 6-way and the 8-way connectors on the radios to the outside world by converting them into simple RCA/phono sockets. In this implementation only the 6-way (DATA) DIN connection from the radio is needed, and as the radio when selected to "DIG" mode via the DIN connections on the rear is expecting basically "line-level" signals, there is no need for any attenuation in line with the AF drive to the radio.

Connection from radio to computer :- RCA/phono lead from port 13 on the RCA-14 to the AF input port on the soundcard. (This picks up pin 5 on the radio's DATA port).

Connection from computer to radio :- RCA/phono lead from AF output port on the soundcard to port 9 on the RCA-14. This injects the line-level signal into pin 1 on the radio's DATA port.

RCA-14 connected via the 6-way mini DIN lead to the DATA port on the radio.

For both these radios the "DIG MODE" is set to "PSK31-U" on the appropriate menu (#26 on the FT-817ND and #38 on the FT-897D).

PTT on the FT-897 via VOX, though I find the sensitivity has to be turned up quite high. (Menu #40 set to 95)

Note that the VOX functionality with the FT-817ND is NOT available in DIG mode. At the moment I have to use my "manual PTT" box connected to port 11 on the LDG breakout box (connects to pin 3 on the radio's DATA port). A more elegant solution would seem to be called for. Another item added to my "to-do" list!


Page last updated :- Nov 11 2013