Published on CCW reflector 31 October 2013
The design for the PA board is now finished. The Sentry SDR transceiver will use four Mitsubishi RD16HHF1 mosfets in push pull parallel.
I’ve just spent an afternoon measuring the transmit IMD performance of the prototype transceiver both as a transverter driver and transceiver.
The transmit third order IMD of the 1 mW transverter output at full power is -35 dB. At half power it is -48 dB. This checks that the USB sound card, transmit I/Q filter, upconverter and driver amplifier are working correctly and the output is very clean.
Testing the PA at 12V rather than 13.8V the transmit third order IMD at 25W peak output is -30 dB. Running at 13.8V will improve the IMD by a few dB but I wanted to test the worst case scenario.
What do these figures compare to? In recent QST reviews of amateur transceivers an Elecraft K3 had a third order IMD of -27 dB and the Flex Radio Flex-1500 came in at -22 dB.
The tests I’ve run used HDSDR as the SDR software with Spectrum Lab generating the two tone signal of 700 and 1900 Hz using Virtual Audio Cable. Measurements were taken both with a HP spectrum analyzer and a SDR-4+ receiver.
I also tried some digital mode programs generating PSK signals with similar results.
I’ve also run the transceiver and prototype PA on permanent transmit at full power for three hours so it should stand a few RTTY contests!
I should get the PA and low pass filter PCB board designed over the weekend and then it’s just some firmware testing before I can finish the main board PCB design.