IMD Meter is working. Or is it?

I do a fair amount of PSK-31 operation. Good transmitted signal quality is important to keep bandwidth to a minimum, which, after all, is one of the impressive features of this mode. Quality is measured in terms of IMD (intermodulation distortion) and there are basically two ways, I think, to check it. One is to transmit an idle signal and get the person you are in QSO with to read off the IMD from their PSK-31 software. The other is to use a device at the transmitting end to measure the IMD.

One such device is the IMD Meter from KK7UQ. It samples your transmitted signal through a telescopic aerial and has a microprocessor that measures and displays the IMD. The fact that it is available as a kit also appealed to me. I ordered one from the USA and, within a week, it had arrived.

Construction was pretty easy (good documentation), even though there are a lot of components on small PCB and my eyesight is not as good as it once was. And I was delighted when, switching on the meter for the first time, it ran through its computerised self check without a hitch: everything flashed and bleeped like the manual said it should.

Then I tried to measure my PSK-31 transmitted signal. There are two steps in this: set the meter to measure field strength (to make sure you are close enough to the transmitter) before switching to IMD mode for the actual measurement. The meter gave some weird results. The field strength flipped between zero and full scale without any clear correlation with output power from my transceiver, position of the IMD Meter and so on. It just felt a bit flaky.

Back to the manual, which has an excellent step-by-step troubleshooting guide. This pointed me to checking various voltage points on the PCB and making sure that there were no dry solder joints in the RF area of the board. I couldn't see anything glaringly wrong, but I resoldered the joints just to be sure.

When I retested the meter, things had settled down. The field strength reading was a lot more stable, and I got a valid IMD reading when I switched to that mode. Only problem is, I get the same very good IMD measurement whichever band I test on and no matter how badly I try to generate a poor signal by increasing audio drive from my PC to the transceiver. Call me pessimistic, but I can't believe that my setup is that perfect :-)

Anyway, next stage is to try the IMD Meter further away from the transceiver (so far, I have been testing on my shack desk) and see if I get some variation in results.

Two things I have learned so far. Picofuses are expensive. These are little in-line fuses, about the size of an old-style 0.5 watt resistor. The PCB includes one to protect the power supply from a short on the board; for example, a short caused by measuring spot voltages when troubleshooting and shorting two IC pins together :-) Obviously, there wasn't a spare in the kit, so I checked out various suppliers to see where I could get a replacement. I saw one site offering them for close to £7, so I decided to take my chances and replace the blown fuse with a wire link. Fingers crossed.

The other thing I have learned is that QuickMix is a really handy piece of software. I installed this a while ago when setting up my soundcards to work PSK-31, RTTY and so on via the Tigertroincs interfaces. It allows you to save multiple different profiles of sound control settings and reload them at will. Ideal for returning to optimal settings after deliberately trying to overdrive the transceiver.

I'll post the results of the further tests when I have them. Meanwhile, here are some photos of the IMD Meter (internal and external) and the mod I made to attach a hinged, detachable aerial in place of the fixed one supplied with the kit.