KK7UQ PSK IMD Meter
PSK31 is probably my favourite operating mode. It's efficient - PSK31 signals occupy little bandwidth and make long distance contacts possible using low power. It's relaxing - if you want, you can let the computer do most of the work, which is nice when you are tired after a hard day. But it's all too easy to radiate a terrible, distorted signal that wipes out several hundred Hz on either side of you with intermodulation distortion (IMD) products.
Most people try to transmit a clean signal by adjusting the audio drive to the transceiver so that it is just below the point at which ALC starts to register, run the transmitter at half the rated power (or even less) and then hope for the best. Some people don't bother taking care over the audio drive level at all, with the results mentioned above. The trouble is, it's difficult to measure the IMD of a transmission without access to a spectrum analyzer which, not surprisingly, most hams don't have.
Easy to use
But now there is no excuse to radiate a wide PSK signal on the bands. The PSK IMD Meter, originally designed by KK7UQ and sold in the USA by US Interface, is an inexpensive standalone IMD meter that is simplicity itself to use.
The IMD meter is a small plastic box with a plain aluminium front panel having the labels printed in black. It doesn't win any design awards, but it'll do. The meter requires connection to a 12-14VDC supply, though a battery can be fitted internally for use in the field.
To avoid power supply modulation interfering with the results, a separate supply from the main rig supply is recommended. I ordered mine direct from the USA and I received a 115V AC wall wart which is not a lot of use in 230V Europe. But suitable power supplies are readily obtainable in the UK.
The PSK IMD meter requires no connection to the radio. A small telescopic whip picks up the signal off-air. It is very sensitive. With my attic antennas the whip picks up more than enough signal even when fully retracted. As well as making IMD measurements the device can also be used as a sensitive field strength meter.
On the front panel there is a mode switch on the left, which selects the mode of operation, a power switch on the right, which selects the external power supply, battery or Off, and a three-digit red LED display. Above this are two LEDs which give an additional signal quality reading. Green is good, indicating < -23db IMD. Red is > -20db IMD, which is poor. There is also an audible alarm when the signal quality is poor, so you can't fail to notice a problem as soon as it occurs.
When you first switch on the power, if the mode switch is in the FS - field strength - position, the meter self-calibrates. It then displays a relative signal strength reading when a signal is present. It's not an accurate lab quality instrument but the sensitivity is about 1dB per unit - useful for tuning up antennas and similar tasks.
There are two PSK positions, one for PSK31 and the other for PSK63, and they only display a reading when these modes are being transmitted. When they are, you get a direct reading of IMD, in dB. So -34 means the transmitted IMD is -34dB, which is a pretty clean signal. I was not able to check the accuracy of the readings with spectrum analyzer measurements but they correlate quite closely with the readings given by PSK31 computer software receiving the signal off-air.
That's really all there is to it. Just switch the PSK IMD meter on and leave it running whenever you are making PSK31 contacts. It allows you to set up your audio input and transmit power levels to give the cleanest signal your transmitter is capable of, it lets you monitor the signal constantly to ensure that a clean signal is being maintained, and it will alert you instantly if anything is wrong that causes your signal to start to splatter.
My only complaint is that the digital display blinks off and on whenever it shows a new reading. I find that a little annoying, but you get used to it.
Conclusion
The IMD Meter is an excellent, reasonably priced shack accessory that does a very important job helping you to operate within your license conditions by occupying the minimum necessary bandwidth for the mode, and it could not be simpler to use. I just wish it could be made compulsory that everyone who worked PSK31 had one of these PSK IMD meters! And I hope the less than satisfactory purchasing experience reported by some will not deter more people from buying this very useful item of equipment.
Where to get one
This product can be ordered for $139.95 plus shipping from US Interface. As far as I know it cannot be ordered direct from any supplier in the UK or Europe.
