Cheap and nasty USB Soundcards for SDR work

This post is a work in progress.

This pic is the spectrum of the Griffin iMic with no input source connected.

This pic has a stereo pair from the  KX3's I/Q output connected to the Griffin iMic, but with no antenna connected to the radio. The iMic is in line input mode.

In this pic the iMic is in Mic input mode.

Note that in the latter two scenarios with the receiver connected to the soundcard, I'm using a cheap stereo line isolator to ensure there are no ground loops, as I also have a serial connection between the laptop and the radio which gave major problems without the isolation in the I/Q path.

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Update.

So, I've received the Behringer UCA202 I ordered the other day.  This gave very poor image rejection when immediately hooked up to my KX3's I/Q output.  After some dabbling in gnuradio-cmpanion (grc), I used the UCA202 line output to feed a complex signal into its line input with a 5kHz tone.  I'm only getting about 35dB image rejection when asking the UCA202 to eat its own dog food.

The image rejection gets worse, as expected, when the I and Q signals are independently disconnected or swapped.  This confirms that the signals are wired correctly. In the screen shot above, the peak hold value shows the image peak with only one of the I/Q pair connected.

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Some comparitive tests between the Griffin iMic (white model) and the Behringer UCA202, whereby both are forced to consume their own dog food.

The  first test uses a loopback cable to feed the line in from the line out.  No signal is sent to the line out of the soundcard.

Here is the Griffin iMic

and here is the UCA202

Now for test 2. In this test, a 5kHz sinewave at (-20dB full scale) is fed as a complex I/Q signal into the soundcard's line output, via the loopback cable and into its line input.

Here's the Griffin iMic (white)

 and here is the UCA202.