measuring uBITX IP3 with FLDIGI

Rod Davis <km6sn@...>

Hi All,

Some time ago I was curious about the quality of embedded sound cards in my old
Dell laptops, so I connected two of them together with audio cables, and ran
FLDIGI on both of them,  in PSK31 mode.That mode shows IP3 information.

I simply used audio-to-audio connections, with no intervening TX and RX. 

I was stunned to see that IP3 was in the -80 dB region! I was impressed
with the quality of the built-in soundcards.

That means it is possible to get a coarse IP3 measurement on a uBITX
using a laptop and a reasonable quality auxiliary receiver, listening to PSK31 from the uBITX.

The measurement would certainly be good enough to give IP3 indications
when adjusting TX RF drive levels, and mic gain, and soundcard output levels.

Be careful not to overload the aux receiver, and turn off the AGC.

Another possibility is to use an audio spectrum analyzer program on the receiving
PC to see the IP3 and IP5 peaks.

Free test equipment!


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