Digital Modes Meanderings
How Clean is My Signal? – Part 1
After all the station equipment upgrades I was left wondering if anything else in my station needed my attention. I decided that there was indeed something that I could do to further improve my digital mode station. Up to this point, I had never had a poor signal quality report. I had always strived to ensure that my signal was clean. But how clean was my signal? The Alinco did not have a way to monitor the ALC so there was some uncertainty at times. With the Kenwood TS-590S I can now verify that my transmitted audio signal was not being driven into the ALC zone. But I still wondered if maybe my signal had possibly been border line at times for some other unknown reason and no one had reported anything. I have witnessed that exact situation on the waterfall. Sometimes there has been a station obviously having a signal quality issue and not a single digital station came forward to alert the station having the problem. I think some hams don’t want to offend another ham (especially if it is a DX station) by telling them that they have a bad signal quality. I understand that no one wants to be the “bad guy” and inform someone they have a problem. But they are not doing the offending station (nor the amateur community at large) any favors by not reporting the problem. The dirty station could get a citation from the FCC or at the very least a bad reputation with his fellow amateur radio enthusiasts. The station with the bad signal quality needs to know so the problem can be corrected. If it only prevented the station with the poor signal quality from getting contacts then that would be its own deterrent. However, usually the poor signal is causing grief to a large section of the “digital band” and at times the entire (3 kHz) “digital band”. You don’t have to be abusive with your reporting of a poor signal. Just the facts and a friendly word about hoping the guy locates the problem quickly (while off the airwaves preferably). Perhaps even a friendly suggestion as to what the problem may be, if you think you know. We are radio amateurs, but no one wants to broadcast an amateur-ish quality signal.
I remember back in 2005 that a neat piece of equipment was being talked about that could check your PSK31 (or PSK63) signal quality right off the airwaves! It was called the KK7UQ IMD Meter. I had wanted one back then but I did not have the extra funds to allocate to what was at the time a luxurious expenditure. The KK7UQ IMD Meter is a very simple to use piece of equipment. Essentially you turn it on, transmit a PSK31 or PSK63 signal and it analyses the signal for IMD (intermodulation distortion) quality. It displays the IMD reading with a LED readout in dB units. The more negative the reading the better the signal quality. The example reading in the photo above is an excellent IMD at -31 dB. No physical connections are needed to either the computer or the radio. Even better is that it can check the signal quality with either an unmodulated or modulated PSK carrier. So you can constantly monitor your signal quality. No more relying on other hams for signal quality reports. You can check for yourself at any time.
The KK7UQ IMD Meter only works with PSK31 and PSK63 digital modes. However, if you set up your station for clean signal transmissions while using PSK modes you can be nearly certain other digital modes will be clean as well. The KK7UQ IMD Meter is a very useful tool to monitor and check your digital signal quality and is a recommended addition for any digital mode station. I finally acquired one of these excellent pieces of equipment for my digital station in 2014. Incidentally, it was with this meter that I discovered just how clean the TS-590S is while transmitting digital modes. On 20 meters, while transmitting a PSK31 signal with the ALC meter pegged full scale, the Kenwood TS-590S runs at an IMD of -34 dB! That is a phenomenally clean signal under those conditions. All the other bands run at a similar performance level as well. So I don’t worry too much if the transmit audio drive level peaks over a couple of bars on the 590’s ALC meter. I have plenty of headroom for running a clean digital signal. But I would not have known this without the KK7UQ IMD Meter.
For the record, the test I just mentioned was conducted on a clear 3 kHz range of frequencies away from the traditional PSK frequencies just in case the test results were not positive (i.e. not a clean signal). The tests were kept to a brief couple of on-the-air checks. The same test procedure was done on all the other bands too with similar IMD readings. You can run initial tests into a dummy load as there can be enough RF leakage to still drive the KK7UQ IMD Meter and get valid IMD measurements. However, at some point you will still need to do actual transmissions on the air with a real antenna to make sure there is not RF feedback getting back into your station setup which can also cause your digital signal to be unclean or non-linear. I DO NOT recommend running at a maximum ALC reading regardless of the IMD measurement. You should at all times strive to keep the drive signal out of the ALC range. It’s all about that LID thing again. Remember, don’t be a LID.
The KK7UQ IMD Meter only works with PSK31 and PSK63 digital modes. However, if you set up your station for clean signal transmissions while using PSK modes you can be nearly certain other digital modes will be clean as well. The KK7UQ IMD Meter is a very useful tool to monitor and check your digital signal quality and is a recommended addition for any digital mode station. I finally acquired one of these excellent pieces of equipment for my digital station in 2014. Incidentally, it was with this meter that I discovered just how clean the TS-590S is while transmitting digital modes. On 20 meters, while transmitting a PSK31 signal with the ALC meter pegged full scale, the Kenwood TS-590S runs at an IMD of -34 dB! That is a phenomenally clean signal under those conditions. All the other bands run at a similar performance level as well. So I don’t worry too much if the transmit audio drive level peaks over a couple of bars on the 590’s ALC meter. I have plenty of headroom for running a clean digital signal. But I would not have known this without the KK7UQ IMD Meter.
For the record, the test I just mentioned was conducted on a clear 3 kHz range of frequencies away from the traditional PSK frequencies just in case the test results were not positive (i.e. not a clean signal). The tests were kept to a brief couple of on-the-air checks. The same test procedure was done on all the other bands too with similar IMD readings. You can run initial tests into a dummy load as there can be enough RF leakage to still drive the KK7UQ IMD Meter and get valid IMD measurements. However, at some point you will still need to do actual transmissions on the air with a real antenna to make sure there is not RF feedback getting back into your station setup which can also cause your digital signal to be unclean or non-linear. I DO NOT recommend running at a maximum ALC reading regardless of the IMD measurement. You should at all times strive to keep the drive signal out of the ALC range. It’s all about that LID thing again. Remember, don’t be a LID.