NI 5142 phase response

[RChoudhary]

We are trying to measure the IQ phase response of NI 5142 ,we are sweeping input signal using signal generator and then giving IQ input to digitzer using splitter .According to digitizer spec IQ phase error is 0.018 deg but it is increasing by 0.1 deg with every offset in freq and for low power levels it is even worse.Is there any specific setting we need to use for getting constant pahse error across frequencies?

and while using get frequency response vi given given under ni scope palette it is showing error attribute or proeprty not supported but for NI 5142 since it supports on board signal processing so i am not sure why this error is coming. Attached is the vi in which we are trying to use the get frequency response.

[RChoudhary]

Hi everyone,

We conducted some experiments and found  for a specifc power level and frequency we are seeing variation in phase error if we vary the vertical offset of digitzer, is there any setting we need to consider while measuring the IQ phase error of digitizer.There is too much variation in data for phase response at low pwer levels e.g. -55dBm. Are there any input settings thatwe need to consider while measuring the phase response of NI 5142 digitizer?

[Nathan-P]

I'm not sure where you are pulling the IQ phase error specification from, but it is not documented in the NI 5142 Specifications.  We actualy do not test IQ phase response, or provide IQ Phase response data.  While it is possible to use both channels for I and Q data, the NI 5142 was actually designed to handle complex waveforms, or single channel acquisitions, with the IQ data combined.  The reason for this is because the CH0 and CH1 inputs are not phase matched, neither in hardware, nor corrected for in calibration.  This means that there will not be a constant phase across frequencies, unfortunately. 

The phase response will vary based on power level, frequency, coupling, channel gain settings, noise/antialias filter settings, etc.  Any setting that affects the analog path will cause variations in phase response between the two channels.

This being said, it is reccomended to use a single channel of complex data instead of IQ on separate channels.  This is the configuration used when paired with our NI Downconverters.  

[RChoudhary]

Hi Nathan-P

yes we got to know from ni support that there is no phase error spec for NI 5142. 

Thanks for reply.

We have one more doubt, there is one VI Get Frequency Response.vi to get phase and amplitude but whenever we are trying to use it is showing attribute or property not supported. i have attached the vi in previous thread. Is there any special setting required to use this VI.

Thankyou

[Nathan-P]

That function is only supported on the NI-5622.  I will file a bug report to have our documentation updated.  Sorry for any inconvenience. 

Regards,

Nathan

[@Ld3N]

Hi Nathan-P,

Are there any available digitizers other than 5142 that can be used for this IQ measurements?

Looking at PXIe-5645R which has an IQ ports, the phase mismatch between channels is around 0.1 to 0.5 degrees. http://www.ni.com/pdf/manuals/373914b.pdf

Would there be an alternative digitizer having this feature?

thanks.

[Nathan-P]

The PXie-5645R is probably your best option for digitizing separate IQ signals.  If they were combined in the same signal, that would open the possibilities for several of our other devices like the NI 5622, or NI 5624R, etc.