pxie 5673E modulation

[Andy_Hinde]

This sounds like an S21 phase / insertion phase measurement you would use a VNA to measure. To measure the phase shift of the DUT you need a phase -coherent Tx and Rx system, and would need to know the phase of the stimulus signal at the input to the DUT and at the output of the DUT. You will have trouble making an accurate measurement with a VSG and a VSA without vector calibration capabilities that a VNA would provide. 

[kylelove2902]

Yes, it is an S21 phase measurement. Is it a random phase relationship between Tx and Rx? I'm just thinking that because I need to do the averages about 50000 times, the phase relationship might be canceled out if it is random, and then I can get the phase I want.

Thanks!

[kylelove2902]

I also find the discussion here https://forums.ni.com/t5/RF-Measurement-Devices/Synchronise-NI-RFSG-NI-RFSA-with-NI-TCLK/td-p/767803. Is it possible to use TCLK to fix the phase?

[kylelove2902]

Hi,

I have a problem with pulse generation. Actually, I want to generate a Gaussian pulse, so  'RFSG Pulsed Data.vi' is not the case I want. I think 'RFSG Arbitrary Waveform Generation.vi' can help me generate the Gaussian pulse, but I have no idea how to change it to the Gaussian pulse. 

Thanks a lot!

[Andy_Hinde]

If you need to measure insertion phase/S21 phase, you need to use a vector network analyzer (VNA). National Instruments offers the NI 5632 VNA along with calibration kits that are used to perform S parameter measurements.

VNAs are designed specifically to have phase coherent transmit and receive subsystems (i.e. shared Tx and Rx LOs) which are needed for S parameter measurements, particularly phase. A random Tx/Rx phase relationship will just cause random measurement outputs. This is not a situation where multiple measurements of a random process will help average out to a statistical mean.

[Andy_Hinde]

The example shows you how to write an arbitrary waveform to the NI 5673 in order to generate an arbitrary RF output. The pulse output shows how combining some signal (in this case sine tones, which are DC values at baseband IQ) with off time (all 0s) can create a pulsed waveform. If you want a Gaussian pulse instead of a regular CW pulsed output, you need to replace the IQ data representing the sine tones (I=1s, Q=0s) with the IQ data representing your Gaussian pulse, so your task is really figuring out how to create a complex Gaussian waveform at baseband/IQ.

I found a function in LabVIEW for creating a Guassian Monopulse, I don't know if this will help you or not. I attached a screenshot of the help.

[kylelove2902]

Hi,

For the Gaussian pulse, I find that I can use the for loop to generate a Gaussian array and then put them into the I components, and set all Q=0s. The demodulation results give me the Gaussian pulse, so I think it is correct.

For the part of VNA, because I don't have that device in the lab now, I need to find out other solutions. My professor told me that I need to let pxie 5673E and pxie 5663E have the same trigger, so I'm just thinking to connect pxie 5450 PFI 0 with pxie 5622 PFI 1. Also, I find out there is an example "RFSA List Mode Loop Synchronization with RFSG (Multiple Waveforms) ". In this example, there is a step they call synchronize NI-RFSA and NI-RFSG by using trigger vi. Do you have any suggestion for this example?

Thanks!!

[Andy_Hinde]

The best you can do is to perform a crude transmission response calibration and measurement with the 5673 and 5663 using multirecord acquisition mode of NI-RFSA. When you perform a multirecord acquisition using RFSA, all records acquired will retain a common phase response. Multirecord acquisition is a mode of RFSA that allows you to collect N records of data that are considered part of a single acquisition. You would need at least two records, one for calibration and at least one for measurement. You can use a Software Trigger to manually fire a trigger via software, since you will need to change connections manually in between calibration and measurement. Make sure you set the acquisition timeout long enough so that it doesn't timeout while you are manually changing connections.

You will need a cable coming from the 5663 and 5673 and an adapter to connect the cables together.

You will need to:

• Connect the Tx and Rx cables together
• Begin generation of the RF CW stimulus
• Configure a multirecord acquisition and a software trigger in RFSA
• Initiate the multirecord acquisition and generate a SW trigger for RFSA with RFSA Send Software Edge Trigger.
  • This captures your reference or calibration measurement. This IQ data should be converted to polar for the initial phase value.
• Remove the adapter and insert your device under test (DUT).
• Generate a SW trigger for RFSA with RFSA Send Software Edge Trigger.
  • This captures your device measurement. This IQ data should be converted to polar and compared to your initial measurement.

This approach has several flaws:

• The adapter is present during calibration and removed during measurement. Ideally the calibration adapter would be removed and replaced with another adapter of the same exact electrical length that allows mechanical mating with the DUT. Otherwise you are inserting error into the measurement equivalent to the length of the adapter.
• This simplistic approach won't be able to discern between a phase shift of x deg, 360+x deg, etc... You could do this at several frequencies to get a better idea of this, because at the end of the day it sounds like you want group delay, which is the derivative of the phase as a function of frequency. One frequency point won't get you this.
• You are not using a VNA with vector calibration, which corrects for several types of systematic errors, one of which is impedance mismatch at the DUT interfaces. You will have reflections at the device connectors that will create standing waves and change the phase of the waveform you measure. 

Use this approach and leave the adapter in place between the > 2 measurements at first to make sure you are getting the same phase measurement each time. Then you can insert the DUT and see how much the phase shifts. Your measurement uncertainty will depend on how you account for the adapter and the match of your DUT connectors, among other things.

[kylelove2902]

Hi,

Thanks for your advice. In my program, I can generate a CW wave by Pxie 5673E and demodulate it by Pxie 5663E, so I can get I and Q are two DC lines. However, every time I restart the program, the I and Q  DC lines will shift because they have some phase relationship, or in other words, they don't have the same trigger.

My first step is using the program to let them have the same trigger, so every time I can get the same IQ values. I'm trying to use the software trigger, but I have no idea about this. Is there any example vi can help me?  I also see the example "RFSA Synchronization With TClk and RFSG VI"  which is in the RFSA file. In the example, it uses the tclk vi, but they set it at the reference trigger, so they cannot have the same trigger. It also says if one needs to use the tclk, then it should change to start trigger. Do you have any suggestion about this?

Thanks!

[kylelove2902]

Hi, sorry I need some help to figure out what I'v done is correct or not.

Because I want 5673E and 5663E to start together,  I will use the niRFSG export signal VI to send a digital trigger to the RFSA. However, I don't have a smb cable now, so my FIRST QUESTION is if it is correct to send a digital trigger from  5673E PFI0 to 5663E PFI1.

I also want that the trigger is repeatable for the repeatable measurement ( I need to wait  40 microseconds to do the measurement again), and my Second Question is how to do this thing. I thought that I can let the RFSG to export a trigger to RFSA and then create a software reference trigger in RFSA to let it know to send trigger by itself every 40 microseconds, but I'm not sure if it is correct to put two triggers together. The idea is in the file.

Any suggestion is appreciated!