PSK31 Data suppressed SSB carrier tests with AF 1500Hz higher

PSK31 Data suppressed SSB carrier tests with AF 1500Hz higher. Carried out as a result of an observation from another station using an SDR who had observed the unwanted carrier near the dial frequency.

First image shows 5W tune signal, with Kenwood TS-2000 transmitter USB dial frequency of 5.3623 MHz and AF 1500Hz higher (5.3638 MHz). Screenshot of received signal using Flex-3000 SDR with attenuator in place due to close proximity of Tx and RX antennas.
Note both suppressed carrier level and AF signal levels within light grey coloured SSB bandwidth. That suppressed carrier level should be 50dB or better and not be visible.

Carrier Test 5W

Second image with PSK31 5W signal. Other parameters remain the same.

PSK31 Carrier test 5W

Third image with PSK31 25W signal. Other parameters remain the same, but severely overloading the SDR receiver.

PSK31 Carrier test 25W

Fourth image with RX antenna disconnected and SDR receiver not now overloaded. The constant carrier in the display at below 5.362 MHz is a sproggy, there all the time as an artifact possibly of a computer power supply. The transmitted AF carrier is shown at 5.3638 MHz with a break of transmission to show not  related to other artifact. No sign of my USB dial frequency suppressed carrier at 5.3623 MHz.

PSK31 test no antenna

Update 31st January 2013.

Extensive further tests have proven very interesting, the unwanted carrier artefact is around 31.25 Hz above the actual suppressed carrier frequency. It is not apparent on TX carrier only, but appears when the signal is modulated by the PSK31 (31.25Hz) audio. My friendly radio guru believes this problem is caused by either RF feedback from the TX antenna getting back into the wiring between computer and transceiver OR a PSU earth loop. Some further testing required to see which of the two, but easily cureable.

Update 12th February 2013

Well after extensive further testing the problem was neither ground loops or RF feedback, but linked to the PC soundcard not being driven sufficiently high. The soundcard level had been set too low at 10/100 whereas when set to 50/100 or higher the unwanted artifact disappears.

73 de Philip G0ISW

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