Hi Jgauthier,
This is a good question. The reason this is called twice has less to do with the purge parameter, and more to do with wanting to discard the first chunk of I/Q samples in the stream. Your assumptions about the purge parameter are correct. There are potentially several FIR filters (depending on settings) that are in the signal processing chain when I/Q streaming. These filters take hundreds/thousands of samples to ‘charge’ and until they do, the I/Q samples will not be amplitude accurate. By calling the function once with the purge parameter, this ensures that the first large segment of I/Q samples subject to this charge time are discarded. This is not necessary, but if you do this, doing this only once after each configure is adequate.
Another interesting component of this, the first call to GetIQ might not purge any samples, as there may not be any samples collected yet on the PC. So the worst case scenario is that the first call is just waiting until some samples arrive on the PC, and the second call is the one that actually purges that first chunk of I/Q to arrive.
Feel free to try just 1 call and see if that affects your application.
Also, waiting some small amount of time (100ms) before calling getIQ after configuring the receiver would ensure that the first call to GetIQ purges the first samples.
We chose to give these samples to the user instead of discard them in the API as they could still contain information just at a reduced amplitude.
If you are using any sort of triggering to acquire your waveform, calling the function twice is probably not necessary.
Regards,
Andrew
