Coerced sampling rate using PXI-5142 in OSP mode

[David R]

When using the PXI-5142 (as a part of the PXI-5661 vector signal analyser) in OSP mode I am unable to get a sampling rate of 20MS/s.
As I understand it, in OSP mode, the IF signal is downconverted to baseband at the full sampling rate of the digitiser (100MS/s), and then low-pass filtered and decimated.  When I request a sampling rate of 20MS/s (divide by 5) it is coerced to 25MS/s (divide by 4). The next closest sampling rate is 16.667MS/s (divide by 6).
Is it possible to get a sampling rate of 20MS/s?

Thanks,
David

Using Labview 8.0 / NI-RFSA driver / Windows XP

[JoeFriedchicken]

Hi David,

The restriction on decimation is divide by 2n, not divide by n. The reason for this is the digitizer is sampling a single input waveform and extracting an I waveform and a Q waveform, effectively halving the sampling rate.

In other words, an input waveform sampled at 10 MHz will yield an I waveform sampled at 5 MHz and a Q waveform sampled at 5 MHz. When you specify the I/Q rate, you're giving the final sample rate of just the I and Q waveforms (5 MHz), so the driver then tells the hardware to sample at twice that to give you the rate you want.

Asking for a 20 MHz I/Q rate asks the driver to sample at 40 MHz, which isn't an integer divisor of 100 MHz, so the rate will be coerced up to 50 MHz, giving you a 25 MHz I/Q rate.

[David R]

Cheers, thanks for that explanation

David