Role of IMD
The role of IMD is still useful. When viewing the trace of very strong signals, well-supressed additional sidebands may be seen above the noise floor. In this case, an IMD reading may assist in determining their status.
If the IMD reading is about –25db or better, then any additional sidebands can be assumed as below the interference threshold, disregarded, and a Q9 report given.
If the IMD reading is about –20db or worse, then the additional sideband modulation is undesirable. This may cause interference to other stations, and an appropriate value from the quality table can be applied to the report.
It is suggested that IMD readings only be given to the other operator when requested, and then only as a supplementary report. This is because IMD is only valid for strong signals at idle, and after careful adjustment of receiver settings..
Further IMD information…
Inter-Modulation Distortion (IMD) is a report often exchanged during a PSK QSO as a figure of merit for the received signal. It is widely assumed that a very good IMD report for an idling signal is around -30db, a poor report around -20db, with the worst possible at -10db. Howard (Skip) Teller KH6TY who developed the first panoramic PSK31 transceiver and Digipan gives the following advice:
“…IMD is the measurement of the first pair of unwanted sidebands to the desired ones. If the signal to noise ratio is not around 26 db, you will start to measure the noise instead of the unwanted sidebands, which will be under the noise threshold. Also, if there is any distortion in the receiver, you create a false IMD reading. Whenever I measure a station’s IMD, I have to be sure the S/N is good, and then I reduce the RF gain of my transceiver until the IMD stops falling and starts rising. At this point, I believe I have almost eliminated IMD caused by the receiver distorting the signal.
Many, many times, I have observed stations giving an IMD reading that is obviously way off, for reasons of background noise or receiver distortion. In some cases, I measure IMD under 26 db, whereas the station is reporting numbers in the -22 range, leading the other operator to believe his transmitter is distorting, when it is actually very clean.
On my bench, feeding a local strong signal, I routinely measure under -30 db on my own designs, and often under -36 db. Before the output stage is fitted, I will typically measure IMD under -40 to -43 db. The highest quality transceivers will usually show IMD readings of better than -30 or -32 db when signals are strong, but you seldom see such reports offered…”
KH6TY added that operators can reliably measure their own transmitted IMD with Digipan operating in full duplex mode, in conjunction with a separate monitor receiver. A cheap or second hand receiver is quite adequate in this case.