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TDS-380 timing compensation failure

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c4757p
Posts: 2
Joined: December 23rd, 2014, 7:12 am

TDS-380 timing compensation failure

Post by c4757p » January 10th, 2015, 12:55 pm

I had to replace the battery-backed RAM module in my TDS-380, and was unsuccessful in preserving the contents, so I had to calibrate the instrument. The voltage reference and external trigger calibrations pass, but when I try to run the timing compensation procedure, it fails the sequence (without explaining the problem). This is, to my knowledge, a perfectly functioning scope otherwise, and the manual doesn't really give any information on what exactly this procedure even does, let alone what might be at fault if it fails.

Error message at boot: https://db.tt/mwkw0OYo

The error log shows this (it wouldn't let me take a screenshot of the log):
101: 4 1 Factory timing cal constants invalid
(Lots of those, it seems to give one every time it boots)

The calibration menu shows the procedure as not being completed: https://db.tt/4FaLzlnm

The only thing I can think of to blame is my test waveform. The technical reference manual specifies that a square wave must be provided, negative-going, 1 kHz, 200mVpp, with a rise time no longer than 10ns.

I didn't have anything that would do that exactly, so I had to cobble together a pulse generator. This is what I fed it.

Note the different scale factors; the rest of the waveform is clean.

Now, is that "bump" about 13ns after the rising edge enough to make the calibration routine decide it can't work with the waveform? (Or is the rise time too fast?) I suspect the bump is due to a slight impedance mismatch, so I could work a bit harder on impedance-matching the pulse generator if I must, but it doesn't really look too bad to me. Is anybody here familiar with what the scope is actually doing during this routine, and why it would fail?

I don't have a screenshot of the actual procedure failure message - forgot to grab one - though if somebody really wants to see it, I can re-run the procedure. It looks like it's the same one it gives whenever the procedure fails, no additional information.

cap
Posts: 397
Joined: May 1st, 2009, 8:16 pm

Re: TDS-380 timing compensation failure

Post by cap » January 15th, 2015, 11:11 am

This may or may not be helpful; I never really worked on TDS300/400’s.

Usually with calibration procedures that are using signal generators, the signal will be terminated with a 50 ohm load. The scope will either provide that 50 ohm termination (if available) or you must use a 50 ohm pass thru termination. If you set up a 1M ohm terminated signal that meets the requirement, then the scope switches to 50 ohms, the amplitude will be well off and quite possible fail that step.

The positive going square wave you show looks appropriate, and I doubt if that would fail, assuming it is a 50 ohm terminated signal; the display is not indicating a 50 ohm internal calibration. I don’t know off hand if this scope has that termination.

Try to get to the error log w/a gpib interface, the how too may be posted here. Probably something like :PASSWORD PITBULL, and then :ERRORLOG:FIRST?, NEXT, CLEAR. Try variations, for example errlog on one scope family may be the same as errorlog on another and so on, wonder why they do that? The error log on your scope may simply say, “failed high frequency cal step”.

Somewhere on your scope there is a true console port that will probably blurt out useful information as the step fails. That too may be described somewhere on this site, maybe you can find it in the schematics. Not certain, it may be similar to what you would find in a TDS500-ish scope, since I think these are both 68000 processor based scopes, probably not the same edge connector though; these interfaces are generally hidden by tek and the other manufacturers. But I thought someone posted on this for these scopes, I don’t recall if it was a complete thought though.

Unless the hosts have made recent changes, the search engine here is barely functional. Spend some hours trying to find what your looking for, or you'll miss it. I would search for all variations of TDS300/400's, errlog, etc.

cap

c4757p
Posts: 2
Joined: December 23rd, 2014, 7:12 am

Re: TDS-380 timing compensation failure

Post by c4757p » January 15th, 2015, 11:39 am

Thank you for the reply!

This instrument, extremely irritatingly for a 400 MHz oscilloscope, has no 50 ohm termination, so that's definitely not it. I'm using an external feedthrough termination (obviously, or else that rise would look much worse :D)

Not sure about the GPIB error log, but I'll keep digging around - with a quick check, I don't see it mentioned anywhere here, nor in any of the manuals. Couldn't hurt to try throwing some commands at it, I guess.

If you're right about the console port, that would be incredibly helpful, thanks! I'll have a look.

Edit: TP604, which is directly connected to the CPU, definitely twiddles at the moment that the calibration fails, and its neighbor TP606 twiddles a bit at boot (they're quiet otherwise) - that look interesting. I'll have to dig out a logic analyzer to capture the twiddles, since this TDS-380 is my only working digital scope at the moment...

konrad
Posts: 1
Joined: November 27th, 2018, 4:48 pm
Country: United States

Re: TDS-380 timing compensation failure

Post by konrad » November 28th, 2018, 5:59 am

c4757p wrote:
January 10th, 2015, 12:55 pm
I had to replace the battery-backed RAM module in my TDS-380, and was unsuccessful in preserving the contents, so I had to calibrate the instrument. The voltage reference and external trigger calibrations pass, but when I try to run the timing compensation procedure, it fails the sequence
Were you eventually able to calibrate this scope? I'm having exactly the same problem. A Wavetek 9100 with option 250 is specified for the calibration. I don't have that instrument, but have downloaded the 3 volumes of its operating instructions. I'm attempting to produce the correct waveform (negative-going, 1 kHz, 200 mV, 50/50 square wave with rise time <10ns) via an H-P 33120A arbitrary waveform generator (which was never meant to be a calibrator).

Specs of the 9100 I'm attempting to meet:

Voltage Accuracy: +/- 0.25%, so +/- 500 uv in 200 mv = 199.5 to 200.5 mv
Frequency accuracy: 25 ppm = 0.0025%, so +/- 0.025 Hz in 1000 Hz = 999.975 to 1000.025 Hz
Aberration: < 1% in the first 30 us

I've trimmed the output (199.8 mv, -0.099 V offset, 1.00005 kHz) of my arb to hit the prescribed values exactly. Unfortunately, the rise-time is too slow at 16ns. The timing cal fails (perhaps due to slow rise-time?). I've also tried jury rigging an H-P 8007B pulse generator where I can hit the rise-time and frequency specs, but probably not the amplitude precisely.

Additionally, I've tried a lot of other things including:

1. Using 1.000025 & 1.000075 kHz.
2. Doing an SPC immediately before the timing cal.
3. Using an 18" coax (terminated, of course) instead of the usual 36".
4. Using an external trigger from generator to scope (although this is not part of the documented procedure).

I need to determine what's important to success, and what's not.

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