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Brand new PXA, LO Unlocked message

Question asked by LLASCA_eesof on Jan 12, 2012
Latest reply on May 17, 2013 by tabbott

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Hi,

We've got a brand new PXA (5 weeks old) reporting error 509 "LO Unlocked". The error only seems to come up (so far) running 89601B/custom OFDM software, it does not seem to persist if I put it at the same center frequency in standard spectrum analyzer mode.

I can't find much help in the documentation, is this a common thing?

I've spoken to another user who said that they had seen this before, returned the instrument and gotten a "no problem found" report.

Lance

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tabbott Jan 12, 2012 1:56 PM
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Hi Lance -

The LO Unlock may only be seen at certain center frequency and Span settings, or may be the result of a missing, or intermittent reference signal (either the internal freq reference or an external reference.

Sometimes the LO Unlock will appear when the instrument runs its power on alignment or when you manually run the alignment by pressing System, Alignments, Align All, Now. Try this since if the LO unlock is detected, we have an easy way to recreate.

We need to be able to duplicate the failure in spectrum analyzer mode. So, if you can look at the 89601B screen and determine the Center frequency and Span.
Once you go to SA mode and enter the exact CF that causes the problem, set the span to 10 MHz or less (try 9 MHz span). Unlock? This sets the phase lock lops a certain way.
Set the span to 11 MHz. Unlock? Again this span uses a different phase lock situation.
Step the center frequency a few MHz or tens of MHz and see if the unlock goes away, or appears.

Also, on your test setup, is the PXA using an external reference? If so, is this a stable reliable reference, and have you “told” PXA the external reference frequency (press, Input/Output, More, Frequency Ref In and view the External Ref Frequency key to assure the frequency shown is the frequency of the external reference signal that is being fed to the instrument . The analyzer cannot auto detect the frequency of an external ref input, so you need to type in the correct frequency. Default is 10 MHz, but can be changed.

The Frequency Reference In can be set to Internal, External and Sense. Where “Sense” will detect if “something” is connected to the EXT Ref In port on the rear panel (but possibly not the correct frequency) and automatically chose the external ref, or if the external reference should disappear, the instrument will no longer sense the external ref and switch to the internal ref.

Enough detail now on references, please tell me if the Frequency Reference In is set to Internal, External or Sense.

Regards -
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- j_sab @ tabbott on May 16, 2013 11:10 AM
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  Hi,
  
  My PXA is a N9030A (50GHz) that just came back from Agilent service for a power source replacement. I use it frequency locked to a E8257D PSG mostly for EMC applications.
  
  I'm having a similar problem with the LO unlocked message, except when my system goes into alignment after the error message it gets stuck in steps 12 - 14 of the alignment cycle. I reboot the PXA and it does not work still...gets stuck again during startup alignment. The only way it will reboot correctly is by disconnecting it from power, the computer and signal generator, and letting it rest for a while before turning back on. I have not been yet been able to determine at what point during our operation the error is happenning. We have owned it for 1.5 years and had never given us this problem before it came back from this service. Any ideas?
  
  Thanks
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  - tabbott @ j_sab on May 17, 2013 3:36 PM
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    Hi J_Sab -
    
    I modified an earlier post to address some things that are critical to solving your issue:
    
    The PXA Frequency Reference can be set to Internal, External and Sense. Where “Sense” will detect if “something” is connected to the EXT Ref In port on the rear panel (but possibly not the correct frequency) and automatically chose the external ref. If the user does not key in the actual external reference frequency via the front panel, a “Reference Unlock, and then an LO Unlock will result.
    If the instrument does have an unlocked LO, the auto alignment will hang up as you mentioned below.
    
    So, possibly when the PXA was serviced, the service tech may have reset reset the External Reference frequency, or may have set the Freq Ref In to External. The default settings are Freq Ref In = Sense and External Ref Freq =10 MHz exactly.
    
    Here is what you can do assuming you are using the instrument with a 10 MHz reference IN signal from the E8257D :
    
    On the PXA press Input/Output, More, Freq Ref In, and chose Sense.
    Then look at the External Ref Freq softkey (4th key down from the top) and note the frequency value. It should be exactly 10 MHz, if not type in exactly 10 MHz. . The analyzer cannot auto detect the frequency of an external ref input, so you need to type in the correct frequency.
    
    What may be happening is the E8257D 10 MHz Ref output may be intermittent, or the interconnect cable between the PXA and PSG is intermittent.
    If the PXA is set to External reference, and the external ref signal goes away, the instrument will show a Ref Unlock / LO Unlock.
    However, if the PXA is set to Sense, when the external reference becomes corrupt, the PXA will automatically switch to internal reference and avoid Prolonged unlocks (the PXA may show Unlock, but then quickly recover after the internal reference is switched in).
    
    Also, it is possible that the PXA has developed a hardware problem causing the LO Unlock.
    The LO Unlock may only be seen at certain center frequency and Span settings.
    Try setting the center frequency to 1 GHz and Span to 10 MHz or less and see if the unlock appears. Then try spans greater than 10 MHz and see if the unlock appears. What you are doing is exercising different phase lock loops by changing span settings. If one of the lock loops is faulty, the unlock will only appear when that loop is used.
    If you find that indeed the problem appears in some spans and not others, this indicates a hardware fault.
    
    Regards -
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LLASCA_eesof Jan 12, 2012 5:54 PM
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Hi,

Thank you for the information.

This problem has occurred a couple of times recently.. I dismissed it the first time, but yesterday it happened about midnight when I was gearing up to make a number of measurements... and after putzing with it for a while of course the problem just went away.

I went through the alignment steps last night and initially it seemed fine after the power on tests, but at least one more time it generated that error.   My first reaction was to put it at the same center frequency and span (last night) which I did, but that yielded no strange behavior; all was well.

I have tried to reproduce the problem, but have not been able to at this time since it started working fine. I have tried the different bandwidths you suggest and not seen any erroneous behavior.

I have no external reference hooked up, but the instrument was setup for SENSE.   I had not changed that setting, but for now I am setting it to INTERNAL since I have no intention of feeding an external reference signal at this time.

What next for diagnosing it?   What can I do when/if it happens again to provide more information?

thanks,
Lance
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tabbott Jan 13, 2012 11:07 AM
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Hi Lance -

We have an EESOF group located here at Agilent in Santa Rosa. Would you happen to be located here? If so, we can see the instrument and press buttons.

Otherwise, since the unit is working OK now, it is difficult to troubleshoot.

Send me the serial number of your instrument and the settings that you think should cause the LO Unlock. We can try them on our analyzer and try a few things here.

Regards -
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LLASCA_eesof Jan 13, 2012 11:14 AM
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Hi,

I've worked with eesof for a while but I'm not an employee (I participate in some of the beta testing and such). At some point during the many forum transitions, somebody added that to my username...

I'll send you a message directly with some of those details.

Lance
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Brand new PXA, LO Unlocked message

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