AD2 Network analyser

[Malcolm]

Dear experts,

May I know how the AD2 waveform 15 calculates the phase difference in the bode plot of network analyser?

And how does the averaging/average function works? Does it help decrease my signal noise or is there anyway/functions to have a more accurate phase difference reading?

I attached a picture, and I would like to ask if the AD2 is able to capture the phase difference between this 2 signal or is it too noisy?

 

Thanks a lot for reading!

[attila]

Hi @Malcolm

It should be able to measure the phase. The NA can use FFT mag/phase averaging and in the latest beta version added coherent average mode for testing.

See a related post:

 

[Malcolm]

Hi @attila

Thanks for the clarification!

Looking at the related post I do see coherent averaging does improve the accuracy especially when there is huge noise of the same frequency interfering the signal. But this leads me to a question of which case should I use which averaging or is it in all cases the coherent averaging is better?

In your reply to my question, you showed a very small signal with noise may I know how you created that signal in AD2 and what is the SNR of that signal? It seems that with that small signal it is still able to get a decent bode plot which increase my confidence in measuring the phase of my signal example.

Lastly, may I ask what is the maximum current output when using network/wavegen as I see in documents it is 10mA and it says it varies. Does using the 5V auxiliary power supply increases its output current?

Thanks alot for your time!

[attila]

Hi @Malcolm

The Wavegen drive is 10-50mA depending on voltage level, at +/-5V min 10mA and around 0V max 50mA.
The aux/usb supplying affects only the power supply limits.

 

The coherent averaging looks to be slightly better with fixed frequency noise.
In the following pic R1-2 uses coherent average which perfectly overlaps C1-2 using mag/phase.

The 10mV signal is generated by W1 controlled by NA. The C1 SNR is 20...10dB and C2 -40..0dB.

 

[Robin Oswald]

@MalcolmBased on my adventures using the NA, I'm very confident that it should give you reliable results for the scenario shown in your screenshot, especially because I can still rather easily see the sine wave in the response by eye and can pull out a phase difference if really required. I've seen the AD pull out reasonable mag/phase from a lot worse time traces!

The only thing that is a bit borderline (but probably still acceptable) is the amplitude of the blue channel being only a few mV. For your scenario, this should still work, but to improve the measurement it'd be advantageous to either increase the excitation amplitude if you can (i.e. if there isn't any non-linearity limiting you), or using a pre-amplifier before the AD to get the voltage amplitude up into a decent range.

[Malcolm]

Hi @attila

Thanks for the information and detailed analysis! I will try and implement AD2 to my set up and hope it works.

 

Hi @Robin Oswald

Saw your post on coherence averaging and it allowed me to learn a new topic! Thanks to you and @attila , I may try out the beta version for my experiment. 

Thanks for your suggestion, yes I was thinking of using a pre-amplifier, although I am hoping to not add it in first as the accuracy of my experiment relies on an accurate phase information, hence I will try to minimise any components that may add phase noise or too much phase shift. But will add it in when signal is deemed to weak :D.

Once again thank you both for the help and input.

 

[Robin Oswald]

Hi @Malcolm

On 11/27/2021 at 1:22 PM, Malcolm said:

Thanks for your suggestion, yes I was thinking of using a pre-amplifier, although I am hoping to not add it in first as the accuracy of my experiment relies on an accurate phase information, hence I will try to minimise any components that may add phase noise or too much phase shift. But will add it in when signal is deemed to weak :D.

First try without the pre-amplifier, since there's a decent chance that it's already good enough. However, if you do need to add a pre-amplifier you're still going to be fine:

• Easiest: Make the pre-amplifier ~10X faster in bandwidth than the highest signal you care about. Then the phase shifts due to the pre-amp will be a few degrees at most, so probably irrelevant (depending on the required level of precision of course).
• Next easiest: Use the AD2 to characterize the pre-amplifier first, and then account for it's phase shift. This requires a tiny bit more data analysis, but is also fairly straightforward.

Any reasonably pre-amplifier should give you stable phases with little phase noise as long as you don't drive it into saturation, so I think you'll be fine either way.

[Malcolm]

Hi @attila and @Robin Oswald,

Thanks for the help, I am now able to get the phase information that I wanted from the DUT :)

 

@attila May I know what exactly does the average and averaging do for AD2? As I understand the Min periods function has already helped in averaging the results by the amount of cycle. 

 

[attila]

Hi @Malcolm

See the updated Help:

[Malcolm]

Hi @attila

Sorry for my lack of understanding, but I still do not get what is the function for them.

E.g. If I put average as 10, averaging as 50ms and min periods as 32, does it mean that it captures 32 sine periods 10 times for a timing of 50ms and average them out?

Thanks!

[attila]

Hi @Malcolm

The "Min Periods" specifies at least how many periods to be in each capture, at high frequency it could be more than this.
Specifying average of 10 and averaging of 50ms, then in each step it will perform and show the result of at least 10 captures or more if it manages in 50ms.

[Malcolm]

Hi @attila

Thanks for the explanation!

[Malcolm]

Hi @attila sorry for bothering you (or any expert), I posted this question on the labview section of the forum (https://forum.digilentinc.com/topic/22764-labview-for-ad2-spectrum-analyser/)but it seems like that forum is dead already. So I redirect my question to this forum. The question is:

I wanted to control spectrum analyser of AD2 using labview, but it seems like there is no vi for it, hence I used bode analyser vi. I was wondering from the bode analyser labview, how/is there a way for me to control all the controls that is shown in the  image attached?

Also, I encounter a problem in which when I change the amplitude lower than 3V for the bode analyser vi, it will hang. Is there something wrong that I did?

 

The reason why I want to control the spectrum analyser is because I want 'for a step increase in W2, a fast frequency sweep (using W1) is done to get the phase difference between scope 1 and 2, which setting the values for min periods etc will allow me to get a more accurate reading. And for each sweep, I want to export the data to excel. Is there a way to do this via labview/python or any other ways to automate this process?

Thanks!

 

 

Digilent_WaveForms_Bode Analyzer (FGEN and MSO).vi

[attila]

Hi @Malcolm

The Bode VI example probably does not trigger. It is by default set to trigger on C1 0V rising.

The VIs provide raw data and the FFT can be done in LabView.
The WF SDK provides network/impedance analysis functions.

The WF app Script tool can be used to automate things, control the instruments, execute commands, perform data logging and export... like here:

[Malcolm]

Hi @attila,

Sorry, regarding my question I meant to say Network analyser instead of spectrum analyser. So is there a VI for network analyser and if not, the scripting done in Waveform can control things such as min periods, average, averaging in network analyser?

 

For the Bode Vi, the issue was I cannot change the wavegen output amplitude to less than 3V and it works fine for above 3V.

 

Thank you for your time @attila, you have been a great help :)

[attila]

Hi @Malcolm

Dedicated network analyzer VI is not provided but it could be done like the Bode example.
In the example see the "MSO Configure Analog Edge Trigger" for trigger options or remove it run without triggering and also remove the stop condition in the VI for no trigger case.

See the application Help/ Script

 

Edit:

Edited February 21, 2022 by attila