rfmx chp behavior clarification (19.0)

[wei-smart]

Background:

• customer complain use ni vst get different power in different iq rate, problem is confirmed by me in Hisi lab.

Problem is reproduced by rfmx chp example with rfmx 19.0 in 5840

After escalation ni community, problem is not reproduced outside.

https://forums.ni.com/t5/RF-Measurement-Devices/rfsa-captured-power-is-different-in-different-iq-rat...

Debug:

• Problem is NOT reproduced in 5646 then check different between 5840 and 5646. 

Root cause:

• default setting of LO offset in 5656 is manual and value is 0 Hz. BUT default LO offset for 5840 is automatic;  in different span,  this value will be changed automatically.

Current situation:

• problem is clarified, current workaround is fix LO offset.

Question：

• For various VST， is it possible to align default setting in rfmx?
• is it possible to improve power stability for different LO offset?
• RFmx SpecAn instr recommended acq type property read out value is always IQ. But RFSA is spectrum.
• For 5840, Rfmx SpecAn CHP span higher than 300MHz, instrument will be switch to spectrum which is not necessary and this property can't be change to IQ from either rfmx or rfsa property. 

[YouZi]

Hi smart,

• For various VST， is it possible to align default setting in rfmx? is it possible to improve power stability for different LO offset?

        The Freq offset of both 5646 and 5840 will be tuned automatically at different SPAN setting. But the 5840 will have several jump value as attachment document. The difference is owe to instantaneous Bandwidth difference between 5646 and 5840.

• RFmx SpecAn instr recommended acq type property read out value is always IQ. But RFSA is spectrum.

       What's the stuation when RFSA is spectrum type, but RFmx SpecAn is IQ type?And there is no acq type property at RFmx SpecAn property class. Can you offer your example VI? 

• For 5840, Rfmx SpecAn CHP span higher than 300MHz, instrument will be switch to spectrum which is not necessary and this property can't be change to IQ from either rfmx or rfsa property.

        Can you offer your example VI? How can you monitor the acquisition changing？

[YouZi]

Hi smart，

RFSA spectrum mode, delivers to the API a spectrum assembled of many acquired smaller spectrum, that are not centered at the LO frequency of the instrument. This is done automatically and without your control, but what that means is that your tone will lie at different frequencies in the instantaneous BW of the HW under different conditions. 

This is why SA Peak variation across Span. And It is common phenomenon for VST.

[YouZi]

Hi smart,

• For various VST， is it possible to align default setting in rfmx? is it possible to improve power stability for different LO offset?

 5840：      

5646：

As before compareing picture, the Downconverter Freq offset of both 5646 and 5840 will be tuned automatically at different SPAN setting. This is done automatically and without your control, both 5840 and 5646, but what that means is that your tone will lie at different frequencies in the instantaneous BW of the HW under different conditions. 

NI recommends using the LO Leakage Avoidance Enabled property instead of the Downconverter Frequency Offset property. The LO Leakage Avoidance Enabled property automatically configures the Downconverter Frequency Offset property to an appropriate offset based on the bandwidth or span of the measurement.

Example is as attachment (SA Test.vi).

• RFmx SpecAn instr recommended acq type property read out value is always IQ. But RFSA is spectrum.

         RFmx SpecAn instr recommended acq type property returns the recommended acquisition type for the last committed measurement configuration. And RFSA acquisition type configures the session to either acquire I/Q data or to compute a power spectrum over the specified frequency range. They are different concept.

• For 5840, Rfmx SpecAn CHP span higher than 300MHz, instrument will be switch to spectrum which is not necessary and this property can't be change to IQ from either rfmx or rfsa property.

        RFmx SpecAn instr recommended acq typei is not acquisition type. Can you offer your example or core API to show how you monitor the acquisition changing on Rfmx SpecAn CHP Test?

[wei-smart]

Hey Youzi, Thanks, Nornally, we can get rfsa acq type from property below, thanks.

[YouZi]

Hi Smart,

• RFmx SpecAn instr recommended acq type property read out value is always IQ. But RFSA is spectrum.

         RFmx SpecAn instr recommended acq type property is only used when the T=RFmxInstr Initialize NIRFSA VI or the RFmxIntr Initialize VI is called with option strings "AnalysisOnly =1", as below. Or else, the property is meaningless.

• For 5840, Rfmx SpecAn CHP span higher than 300MHz, instrument will be switch to spectrum which is not necessary and this property can't be change to IQ from either rfmx or rfsa property.

        As the example on attachment, it can't reappear your phenomenon. 

     

[YouZi]

 Hi Smart，

• For various VST， is it possible to align default setting in rfmx?
• is it possible to improve power stability for different LO offset?

        The spectrum mode spectral stitching frequency plan of RFSA is not something that we can control or query.         

        If you desire to have less variation across BW, then you need to perform an action which will force the acquisition to not use spectral mode of RFSA, and then you can better control and offset for specific LO frequencies.

[YouZi]

Hi Smart，

1. At Spectrum mode, the LO frequency of the instrument is done automatically and without your control, but what that means is that your tone will lie at different frequencies in the instantaneous BW of the HW under different conditions.   

2. At IQ mode, you can better control the offset for specific LO frequencies.

3. At RFmxApecAn CHP Test, the aciquisition type is related with instantaneous BW. When Span is lower than instantaneous BW, the aciquisition type is IQ mode; Or else higher than instantaneous BW, it is spectrum mode. You can refer to the attachment example VI.

I hope it is helpful to you~