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Agilent 54835A scope (4 channel 1GHz / 4Gs/s) repair & uphack

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Jwalling:
And then I find it.
https://community.keysight.com/message/69764

I'll put it here in case it moves again. Hope I don't mess this up...

Keywords: 54835A 54845A 54846A Understanding trigger specifications.

QUESTION:

--- Quote ---Understanding trigger specifications
Question asked by deninnh on Jan 22, 2015
Latest reply on Jan 23, 2015 by deninnh

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I've been using scopes for over 30 years up to the latest and thought I knew them fairly well.  I've purchased several high end scopes in the past and have to admit that trigger specifications were not high on my list of concerns.  They just worked !

After acquiring a personal 54846A which will not trigger above 1GHz on a clean sine wave (Agilent source), I decided I should read the spec.

It was surprising to find that trigger sensitivity for this 2.25GHz scope is only spec'd up to 1GHz.

So then what happens ?

Funny thing is that it *will* trigger on very fast events like an avalanche pulser or airborne ESD event (from a distance of course) with risetimes below 200ps.  But it just can't handle a sine wave.  I've looked at the source output and it is very clean spectrally.  On another 3GHz scope, the sine wave is perfect.

Is the scope broken?  All channels are identical in behavior.  Deleted cal files and ran full cal with no change.  Unit is very clean and has the last software version offered and also the last, fastest motherboard.

I realize that current  models have much improved triggering.  On another unmentioned brand of scopes I've always been able to trigger right up to the specified scope bandwidth before aliasing and other things kick in.   
--- End quote ---

REPLIES:


--- Quote ---Generally, the HW Trigger BW of scopes, in the past, has been approximately half the scope BW. There are also sensitivity specifications, so depending on the signal magnitude, you can trigger on higher BW signals. Newer Agilent/Keysight scopes have a "Trigger Sensitivity" button, that can give higher BW triggering.

That said, there are also digital scopes that 'fake' a higher BW. They use a hidden 'Auto' trigger mode (which forces a trigger if there's no trigger event within a short period of time), then look for an edge near the center of the trace in SW, and move that to the middle of the display. If it happens fast enough, it is indistinguishable from a 'real' trigger. It's very hard to determine if this is being done.

You don't indicate the brand on the 3 GHz scope, or the frequency of the sinusoidal signal you used to test it, so I can't comment on that part .

Al

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    deninnh
    deninnh @ algoss on Jan 23, 2015 12:38 PM
    Hi Al,

    Thanks for the quick reply.

    I was applying a clean sine wave between 1GHz and 2.5GHz, but suspended the BW test of my "new" scope when it wouldn't get past 1GHz.  (it will actually trigger at all up to around 1.2GHz but with much jitter on the trace).

    Last night I ran the trigger sensitivity test in the service manual and it passes, but tests are only at 100MHz,  500MHz and 1GHz.

    The "other" was a TDS694C and it delivers a cleanly triggered waveform almost up to 3GHz (its BW).

    SInce there is no spec beyond 1GHz for this model, I may not have a leg to stand on for returning this scope to the dealer.

    We have a 3000X (1GHz) and 4000X (2.5GHz) at work and I noticed that the trigger spec no longer mentions at what frequency (as far as I recall).  Likewise for the other brand.  It is Y range sensitive but not frequency dependent.  These models are converting me to Agilent but my home lab budget says "old Agilent".

    So, if this scope triggers "ok" at 1GHz, would you say that was normal for the model?

    Thanks for your help.

    Den
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        algoss
        algoss Employee @ deninnh on Jan 23, 2015 12:56 PM
        Yes, I would say that since it triggers at 1GHz, it is working as specified. The fact that it's "jittery" at 1.2 GHz is proof, at least to me, that it's a real trigger, not one of the "fake" triggers I mentioned.

        The 3000X and 4000X only talk about trigger BW in terms of trigger sensitivity at different bandwidths.

        For many measurements that people are making with higher speed scopes, Edge trigger BW becomes less important. When looking at jitter, for instance, the trigger location is not part of the equation at all. The only thing that matters is the timebase, and the location of the edges.

        Al
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            deninnh
            deninnh @ algoss on Jan 23, 2015 3:27 PM
            Thanks again Al. 

            I decided to be practical about this and try it in a real world application, USB HS verification.  The trigger was good enough but of less importance since the test is done on data from a single sweep, triggered by a pulse width condition.

            It worked flawlessly and blows away the old TDS in terms of ease of use and fast storage (good old 1GB flash drive under Win98).

            Den
--- End quote ---

nctnico:
I'm aware of that limitation. It still is a good buy IMHO. I bought the 54835A specifically for looking at digital signals and it does that OK. It already has done some work to pay for itself.

eurofox:

--- Quote from: Jwalling on November 02, 2017, 12:46:34 pm ---Here's the 54846A. Just two resistors need to be de-populated, so my drawing is at least correct for the 54846A.

--- End quote ---

I have a fully working 54845A that pass all test and calibrate without problem and I did the change to make it a 54846A.
It detect that it is a 54846A but testing give a lot of error and finally crash!

This mean that software is different and maybe attenuators and other parts …

It could be as well that during installation of the software that is encapsulated in the installation software different versions are installed depending of the resistor selecting the model.

Jwalling:

--- Quote from: eurofox on November 03, 2017, 11:12:19 pm ---
--- Quote from: Jwalling on November 02, 2017, 12:46:34 pm ---Here's the 54846A. Just two resistors need to be de-populated, so my drawing is at least correct for the 54846A.

--- End quote ---

I have a fully working 54845A that pass all test and calibrate without problem and I did the change to make it a 54846A.
It detect that it is a 54846A but testing give a lot of error and finally crash!

This mean that software is different and maybe attenuators and other parts …

It could be as well that during installation of the software that is encapsulated in the installation software different versions are installed depending of the resistor selecting the model.

--- End quote ---

It's not the software. I have one Symantec Ghost image that works in all the scopes in this series with the FIC motherboard.
The scopes are 54810A, 54815A, 54820A 54825A, 54835A, 54845A, and 54846A.

eurofox:

--- Quote from: Jwalling on November 04, 2017, 09:43:20 am ---
--- Quote from: eurofox on November 03, 2017, 11:12:19 pm ---
--- Quote from: Jwalling on November 02, 2017, 12:46:34 pm ---Here's the 54846A. Just two resistors need to be de-populated, so my drawing is at least correct for the 54846A.

--- End quote ---

I have a fully working 54845A that pass all test and calibrate without problem and I did the change to make it a 54846A.
It detect that it is a 54846A but testing give a lot of error and finally crash!

This mean that software is different and maybe attenuators and other parts …

It could be as well that during installation of the software that is encapsulated in the installation software different versions are installed depending of the resistor selecting the model.

--- End quote ---

It's not the software. I have one Symantec Ghost image that works in all the scopes in this series with the FIC motherboard.
The scopes are 54810A, 54815A, 54820A 54825A, 54835A, 54845A, and 54846A.

--- End quote ---

It is working in the 548446A mode not calibrated, I did not check the performance to check if the modification work, the software ask to do a calibration what I try and fail on all channels, I start again with a test and got a lot of errors and finally the software crash.
It cross my mind to replace the 0 ohm resistor with a switch wired to the back of the scope because it is not a good idea to "play" with the board due to EMC and connector issues but just put the resistor back, 1.5Ghz is not that bad  :popcorn:

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