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How to wait for spectrum analyzer averaging to complete before reading

Question asked by rftester on Nov 9, 2015
Latest reply on Dec 2, 2015 by WDrago

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Hi,

I am trying to read the channel power from a PXA using the commands shown in the attached file. There is quite a bit of averaging applied and the last transaction tries to read the power before the averaging is complete, resulting in the error also shown in the attached. If I insert a wait transaction before the last line that is longer than the averaging time it works ok.

I thought the *WAI command in line 11 was going to prevent the analyzer from processing more commands until the averaging sweeps were done but I must be missing something.

Any advice on how to wait until averaging is done before reading, without inserting a wait that will add unnecessary measurement time when not needed?

Thanks,
Don K.

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Mike Nov 10, 2015 4:01 AM
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Interestingly I have recently been struggling with a PSA and the same issues - surely it should know when it has finished a measurement / alignment or the like? It seems not. Even the examples in the manual, apart from missing a step or two, don't give enough time. That's the process of clearing status registers, writing in various masks to enable bits, calling *OPC to request when things are complete and then polling for the result... a complete waste of time in my experience.

In the end, and to my abhorrence, the only way we could make the semi-automated measurements reliably was with some ugly waiting times. We contacted Keysight and they suggested 'searching the web' and looking at the examples. Surely they should know how to do these things reliably and without unnecessary wait times?

If you pursue this further and find a solid way to wait for the analyser, I'd be interested to know how you did it. Otherwise a computed wait time based on sweep time, number of sweeps to average 'plus a bit' would be a way forward.

HTH, Mike Watts
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- rftester @ Mike on Nov 10, 2015 9:20 AM
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  Hi Mike,
  
  Thanks for your reply.
  
  I was able to tweak one of the examples of polling and masking the status byte for the alignment completion until it worked for me. It's been quite a while since I did this, I don't remember where I got the example or why it didn't work without modification. We also have PSAs, I'm pretty sure it works fine for them too. I am attaching the code.
  
  I thought the alignment was a special case, and that this kind of thing was not necessary to just wait for a measurement result. I guess a similar thing can be done for finding the end of the averaging sweeps but it just seems like that shouldn't be necessary.
  
  I'm going to use the wait time for now so I can move forward, but am keeping this in mind for future work. Likewise, I would appreciate it if you shared any advances you make on this.
  
  Don K.
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  - lib_PXA_Cal.vlb.vee22.9 KB
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  - Mike @ rftester on Nov 12, 2015 2:12 AM
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    Hi Don,
    
    Thanks for the example. Interestingly I didn't use the Spoll method. I thought it was really a bit of a legacy as there is no option to do any 'proper' interrupt handling, not that that is a problem with today's fast computers... maybe there is a new opportunity to set that off in a separate thread if things do need to be processed in the meantime.
    
    Instead
    (a) I found there needed to be a command "*SRE 32 " inserted before ":CAL:ALL" and
    (b) the check was a query of *STB? looking for the bit weight 32 as yours does
    
    Next time I have a spectrum analyser to play with it I will certainly see if there is a difference in the methods.
    
    Best Regards,
    
    Mike
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    - WDrago @ Mike on Nov 23, 2015 7:44 AM
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      Here's how I do it using '*OPC?' and it works just fine. See attached screenshot.
      
      'chanNum' and 'averages' are variables in my code.
      
      BTW, if you have a large number of averages you will have to increase the timeout for the instrument or you will get a timeout error while waiting for the READ TEXT to receive a response from the OPC command.
      
      -Bill
      
      Edited by: WDrago on Nov 23, 2015 8:42 AM
      
      Edited by: WDrago on Nov 23, 2015 8:44 AM
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      PNA-wait-for-averages.jpg28.6 KB
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      - rftester @ WDrago on Dec 1, 2015 2:24 PM
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        Thanks for posting your example Bill.
        
        I was not able to recreate it exactly with my instrument, but similar attempts to put the *OPC? command into my test were not successful. In my application I found that having the *OPC? command included never hurt anything, but sometimes the measurement would work, or not, depending on the parameters. In other words, I thought the *OPC? was working until the parameters changed such that the averaging time became quite long.
        
        I have implemented the serial polling of the status byte, the same as used to determine the completion of an alignment, and it works well. If using the *OPC? or *WAI commands will somehow do it I have been unable to make it work. There is an example (not Vee) of "serial poll for sweep complete" in the X-series signal analyzer programming examples, the existence of which suggests that this, or something like this, is needed.
        
        Thanks for the heads up about the timeout, I had also run across that.
        
        Don K.
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        rftester @ rftester on Dec 2, 2015 8:06 AM
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        Well, I posted too soon about the serial polling working well. After power cycling the test setup the register did not behave the same way the next day.
        
        I'm a little red-faced to admit it, but my problem with the *OPC? was the instrument timeout Bill mentioned. I knew about issue, and thought I had it set, but importing a library file changed it. After getting that sorted out the *OPC? and *WAI commands are working as expected.
        
        Thanks for the help, programming is a secondary job for me so I was happy to find these user forums to help me out.
        
        Don K.
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        WDrago @ rftester on Dec 2, 2015 10:08 AM
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        Glad it all worked out for you, Don. We all make mistakes...
        
        -Bill
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How to wait for spectrum analyzer averaging to complete before reading

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