How to do IM distortion measurements?

Can anyone give me an overview of making IM distortion measurements to a very low level? I understand the principal, but have no idea what's commercially available, or if there's a good DIY project available. I don't have an SA, other than a sound card, and really prefer old fashioned dedicated hardware based instruments.



2003-02-17 7:38 pm
some soundcards are pretty good, Lynx Studio products are excellent but pricey

being cheap I use the US$130 [email protected] (google [email protected] rmaa for IMD plots)

for higher res with a soundcard there are 2 issues that can be easily improved for simple 2 tone IMD tests

DAC/analog output inherent nonlinearity causing IMD in the test signal

you could use separate channels to create the hi/low sines and separately buffer and add them in a resistor or inverting opamp summer, any nonlinearity should just give harmonics of the sines up to the point the signals combine

ADC/input amp nonlinearity

hi pass filter could remove most of the low frequency sine, passing the hi + IMD products, reducing the low sine amplitude before going into the ADC reduces production of IMD by any nonlinearity inherent in the input

I am currently trying to push measurement resolution with the [email protected] where I'm limited by on board crosstalk - shouldn't be an issue with IMD measurement though - I'm using a US$150 Universal player to serve as source with 192/24 DVD-A test files and just using the [email protected] for ADC