Digital Modes Meanderings
How Clean is My Signal? – Part 2
Not long after purchasing the KK7UQ IMD Meter, I acquired another piece of equipment that helps to ensure that my digital transmissions are clean. Even though I have occasionally desired one, I have never owned a station monitor. I found a good deal on a clean Heathkit SB-614 Station Monitor and snagged it. Regardless of the brand or model, all station monitors have the ability to monitor the transmitted signal waveform for purity and linearity via a CRT display. The station monitor is nothing more than a dedicated oscilloscope for monitoring transmitter signals being passed through it. Of course any oscilloscope may be connected and used to check your transmit signal. However, few hams will want to leave their test oscilloscope hooked up as a dedicated monitoring scope. That is why a station monitor is usually desirable. It is interesting to view any transmitted digital mode on the station monitor’s scope but the pattern displayed while transmitting PSK signals is the most useful for linearity checks.
A standard test for checking linearity or distortion of a SSB transmitter signal is the “two tone test”. In general, the tones can be any two frequencies within the transmitter audio passband (roughly 300 Hz to 3 kHz). PSK signals generate two audio frequency carriers spaced by the baud rate. Therefore the PSK signal is effectively a two-tone modulation system and requires good linearity throughout the audio path to prevent unwanted side bands from being generated. The SB-614 Station Monitor does a very credible job of helping keep an eye on the condition of the transmitted digital signal. There are other monitor scopes available on the used market but I chose the SB-614 because it had no superfluous features that I did not need or could not use. Being a Heathkit, it should be easier and cheaper to maintain and repair. The SB-614 can generally be found for around $150 and occasionally for much less, making the purchase price an attractive attribute. Prices vary a lot so check around to get the best deal.
A standard test for checking linearity or distortion of a SSB transmitter signal is the “two tone test”. In general, the tones can be any two frequencies within the transmitter audio passband (roughly 300 Hz to 3 kHz). PSK signals generate two audio frequency carriers spaced by the baud rate. Therefore the PSK signal is effectively a two-tone modulation system and requires good linearity throughout the audio path to prevent unwanted side bands from being generated. The SB-614 Station Monitor does a very credible job of helping keep an eye on the condition of the transmitted digital signal. There are other monitor scopes available on the used market but I chose the SB-614 because it had no superfluous features that I did not need or could not use. Being a Heathkit, it should be easier and cheaper to maintain and repair. The SB-614 can generally be found for around $150 and occasionally for much less, making the purchase price an attractive attribute. Prices vary a lot so check around to get the best deal.
Heathkit SB-614 Station Monitor
Below are two photographs of actual CRT displays from the SB-614 Station Monitor. On the left is a properly transmitted PSK63 signal. On the right is an overdriven PSK63 signal. Notice the severe distortion of the signal envelope. The signal is bad enough that it may not even decode properly when received by another digital station. This is an example of what happens if you forget to turn off the speech processor. This only shows the problem with the waveform shape. Don’t forget there are distortion products (side bands) being transmitted up and down the “digital band” at the same time. The IMD measurements were obtained using the KK7UQ IMD Meter.
Good PSK Signal / IMD = - 31 dB Distorted PSK Signal / IMD = -19 dB