IQ demodulation board suggestion

[hykwei]

Hi guys, I am looking for a high speed NI board that can perform the quadrature demodulation (for a 20 MHz signal), I have looked through the web but so far I can only find 5620, 5660 and 5142.

cos(wt) ----> Device under Test (DUT) -----> Acos(wt + theta)

cos(wt) is the original tranmitting signal (20 MHz). After it is passed through the DUT, amplitude change (A) and phase shift (theta) is then introduced into the output signal (the frequency still remains the same.) The purpose of this setup is to measure the amplitude change and phase shift, relative to the original signal.

Thus I need a simple I/Q demodulation board that can perform this job. Is there any NI board that one can recommend?

Or is there a board that can generate the driving signal and do the IQ domodulation at the same time? I think 5620, 5660 and 5142 can only perform I/Q demodulation process...

Each board costs quite a lot! Therefore I want to make sure that I am buying the right thing...

Thanks for the help!

[Chris_W.]

Hello,

It sounds to me that this would be a perfect application to use the PXIe-5641R or PCI-5640R. This is an IF Transceiver that has 2 IF inputs and 2 IF outputs. This can generate the signal you would like to pass through your DUT and measure the signal back directly and the signal through your DUT to compare the phase, if that is what you're looking for. The 2 IF inputs can be used used simultaneously to compare the phase of those signals. I hope this helps.

Chris W

[Andy_Hinde]

Hello,

There are three digitizer modules that contain Onboard Signal Processing in the form of a Digital Downconverter (DDC): the PXIe-5622, PXI-5142, and PXI-5620. Each of these will perform quadrature demodulation in firmware. 

The PXI-5660 is composed of a PXI-5600 RF downconverter and a PXI-5620 digitizer, so the PXI-5660 is not a good option for you if you only need a baseband digitizer.

 All three are capable of acquiring a 20 MHz wide signal. Where that 20 MHz signal is centered in frequency, and whether the digitizer must be DC coupled will impact your options, so if you post more info about that I can help narrow down your choices.

As for options that include both transmit and receive, the above comments about the 5640R and 5641R are correct. 

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Message Edited by Andy Hinde on 01-28-2010 11:00 AM

[hykwei]

Hi Chris and Andy.

Thanks so much for your speedy response, I am very appreciated!

I just had a look of those devices...

Chris, do you maybe know the difference between 5640 and 5641? They seem to be the same to me, since they all have the same spec for their input/output. It is also stated in the datasheet that 5640's driver can be used for 5641 as well?

Andy, I dont think the system will require AC couple function. Actually, the main information that I would like to extract from the output signal is the phase difference (compare to the driving signal). This phase shift is very tiny (0.1 degree~0.5 degree). I have built myself an analog IQ demodulation circuit, but its output noise is too large to detect this tiny phase shift accuratly! Thats why I am now thinking of the digital I/Q technique and maybe NI (which is a well known instrumentation specialist) can provide me the best solution! Although they are all very expensive...

Thanks alot for your help,

Kent

[Chris_W.]

Hi Kent,

the 5641R and 5640R are basically the same device. The 5641R is the same except ported over to the PXIe bus platform. The 5641R also uses a larger FPGA, which allows a little more powerful processing flexibility.

As a note on your comment to Andy's suggestions. The 5641R and 5640R are also AC coupled inputs and outputs, so you actually have a band limited generation/acquisition. For example, the lower cutoff frequency of the generation band is around 250kHz, which you can see in the 5641R specifications document.

I would peak through the specs to see if these sound good for you. Let us know if you have any questions.

Chris W