Spectrum Frequency Error with PXIe-5641R and PXI-5600

[Lars.B]

Hello,

following situation: I am using the PXIe-5641R (in FPGA-mode) in combination with the PXI-5600 downconverter to get a spectrum.

After trying out with different span widths I've experienced strange frequency errors in the range span <10M.

To negotiate errors in my program I've evaluated also the NI example of http://zone.ni.com/devzone/cda/epd/p/id/6196 - unfortunately same error in frequency happens there.

What is meant with frequency error, for example:  I have a transceiver which sends FSK with center frequency 443,92MHz. Readings with span-width = 12,5M shows overall good results.

Reading with span width smaller 10M, eg. 3,125MHz, gives unexpected readings. 

With a center frequency of 433MHz and a span of 3,125MHz the transmission is out of range of spectrum. When I add an offset of 2M to the calculated IF-frequency, I get right results.

With a center frequency of 434M I've got readings at 434,92MHz, means at 1MHz too high (after adding an offset of 1M to IF freq readings are correct).

With center frequency 435MHz readings are correct. The limit between correct and wrong is exact at 434,5MHz (resulting IF = 14,5MHz) readings at that center frequency are good, just one step below (434,49MHz) readings are 1MHz too high.

Where is the error hidden? Like I said this behaviour happens also with the NI example.

Thanks for helping!

Message Edited by Lars.B on 05-20-2009 04:26 AM

Message Edited by Lars.B on 05-20-2009 04:28 AM

Message Edited by Lars.B on 05-20-2009 04:29 AM

[Lars.B]

To get closer to the problem: I've managed to get constant right results in editing the VI which calculates the IF-frequency for the IF-RIO after setting center frequency and span of PXI-5600.

But the new values make no sense at all. Documentation says that with spans higher 10MHz step size of center frequency is 1MHz, below it is 5MHz (with lower phase noise). After my modifications it seems that step size is 1MHz at all span widths higher 1MHz and 5MHz at span widths lower 1MHz. 

[Lars.B]

Seems to be a defect in the hardware. Today PXI-5600 isn't programmable and get an error after selftest in MAX.

Another PXI-5600s (got another) behaviour is normal, also with frequency.