Several operators had observed distortion in my digital transmissions when using the Elecraft KPA-500 amplifier. It was also observed that the adjustment of the TX knob on the SignaLink would not result in any output power variation. Both symptoms are indicative of radio frequency energy in the audio interface between the transmitter and the SignaLink USB radio sound card. This issue has been resolved with the installation of low pass filters on both ends of the cable that interfaces the SignaLink USB 6PM to the Icom IC-F8101-3X.

While a SignaLink USB 6PM is used here, originally intended to have a cable that interfaces to a 6-pin Mini DIN connector, an interface cable was constructed using a discarded Ethernet CAT6 cable, that connects to a 15-PIN male DSUB connector that mates with the IC-F8101 female ACC connector while retaining the original SignaLink USB 6PM JP1 jumper configuration.

On the SignaLink end, the low pass filters are installed onto the jumper connector JP1, with the inductors replacing the jumper wires for the MIC and SPK signals. At the F8101 end of the cable assembly, the filters are installed inside the 15-PIN DSUB connector housing.

The LC low-pass filters use a 100µH chokes and 100nF capacitor, resulting in a filter pole frequency is 50 kHz. This is a sufficiently high pole frequency to avoid impacting the audio frequencies and a sufficiently low frequency to reject radio frequencies. The components are all available from DigiKey.

Testing shows the signal to be clean and adjusting the TX knob on the SignaLink now properly varies the output power level.