7B71 time base jittering at 50ns/div, but not other speeds.


John Griessen
 

On 4/16/19 4:36 PM, Raymond Domp Frank wrote:
Back to your issue: The horizontal starting position on the screen is never an indication of time with regard to the trigger moment, especially at different higher-speed sec/div. settings.
OK, thanks for explaining that the different starting positions are as designed. Please look at this trace though:

https://www.ecosensory.com/tek/tek-7B71_trouble-7.jpg

Did you see the 1 div wide blur of starting positions that is a random process happening? This can happen in several
sweep speed knob positions and is drastically different than the others, and not very useful for measuring pulse widths against.

That is not normal, is it?


 

On Wed, Apr 17, 2019 at 01:05 AM, John Griessen wrote:


That is not normal, is it?
Oh yes, it is absolutely normal. The slope should be much steeper for measuring pulse widths or slopes: Consider the fact that with your setup a minimal difference in trigger voltage means a relatively large difference in time. As a result, you're mostly "measuring" trigger voltage. With a steep slope, the trigger voltage is not as critical.
The "jitter" that you're seeing in the slope is a result of the fact that even an extremely small variation in trigger voltage (nothing is perfect) causes a horizontal shift of the image. Adjust the trigger voltage very carefully and you'll see the trace (slope) shift horizontally. The jitter will be less with some settings.

Raymond


John Griessen
 

On 4/16/19 8:00 PM, Raymond Domp Frank wrote:
Adjust the trigger voltage very carefully and you'll see the trace (slope) shift horizontally. The jitter will be less with some settings.
No, this is a symptom I am asking for help with. The jitter is constant at all trigger levels on 50ns/div, but not other speeds.


 

On Wed, Apr 17, 2019 at 05:59 AM, John Griessen wrote:


The jitter is constant at all trigger levels on 50ns/div, but not other
speeds.
With "not at other speeds" you mean not at faster speeds either? Use enough brightness to judge that, since the jitter may not be evenly spread.
Related to what I said before about trigger level: Do not expect a jitterless display with an on-screen relatively high-speed slope of significantly less than 45 degrees at the trigger point. The 'scope and time base aren't made for that and the image contains little information. You can't decide if you're seeing timebase jitter, trigger jitter or jitter in your signal. Read up on triggering in general and with regard to your setup, especially minimum size of the on-screen signal for reliable triggering at a certain horizontal speed.
I really have not seen or read any indication that something's wrong with your 'scope or time base.

Raymond


John Griessen
 

On 4/17/19 4:53 AM, Raymond Domp Frank wrote:
The jitter is constant at all trigger levels on 50ns/div, but not other
speeds.
With "not at other speeds" you mean not at faster speeds either?
Yes, of the four fastest speeds, 50ns/div is always jittery +/- 1/2 div.
20 ns is usually solid, with maybe 1/2 trace width jitter.
100ns/div and 200ns/div randomly can have excess jitter.

A 7B70 does not do this, has maybe 1/2 trace width jitter up to fastest, (for it), 20 ns/div
7B10, 7B15, 7B92A do not do this either.


John Griessen
 

On 4/17/19 4:53 AM, Raymond Domp Frank wrote:
The jitter is constant at all trigger levels on 50ns/div, but not other
speeds.
With "not at other speeds" you mean not at faster speeds either?
Yes, of the four fastest speeds, 50ns/div is always jittery +/- 1/2 div.
20 ns is usually solid, with maybe 1/2 trace width jitter.
100ns/div and 200ns/div randomly can have excess jitter.

These show this symptom:

https://www.ecosensory.com/tek/tek-7B71_trouble-6.jpg OK (100ns/div)
https://www.ecosensory.com/tek/tek-7B71_trouble-7.jpg random (50ns/div)
https://www.ecosensory.com/tek/tek-7B71_trouble-8.jpg good (20ns/div)

A 7B70 does not do this, has maybe 1/2 trace width jitter up to fastest, (for it), 20 ns/div
7B10, 7B15, 7B92A do not do this either.


 

On Wed, Apr 17, 2019 at 03:46 PM, John Griessen wrote:


20 ns is usually solid
You seem to say that it too sometimes displays the same behaviour, which is only natural, as may be deduced from what I wrote earlier.
You are looking at a very slow ramp/edge, triggering a time base at about its top speed at almost top amplitude level with a vertical sensitivity of only 2 mV / div.
Such a slow ramp provides very little information in your measurement setup. It may be an interesting experiment.
It is possible that some imperfect/varying contact causes the effective trigger voltage to vary by a few 100 microvolts, which would cause the behaviour that you see. If you'd see the same effect with a steeper slope with a larger amplitude at the same time base speeds, *that* would mean something and would point specifically to a problem in the time base.

Raymond


Jim Ford
 

I've been following this thread with some interest because I have a 7B71 myself.  Planning to try it in 7904 when I get a chance.  With 7A26 and new-to-me 7A11's.  Maybe this weekend.  I will post my results. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Raymond Domp Frank <hewpatek@gmail.com> Date: 4/17/19 7:43 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 7B71 time base jittering at 50ns/div, but not other speeds. On Wed, Apr 17, 2019 at 03:46 PM, John Griessen wrote:>> 20 ns is usually solid>You seem to say that it too sometimes displays the same behaviour, which is only natural, as may be deduced from what I wrote earlier.You are looking at a very slow ramp/edge, triggering a time base at about its top speed at almost top amplitude level with a vertical sensitivity of only 2 mV / div.Such a slow ramp provides very little information in your measurement setup. It may be an interesting experiment.It is possible that some imperfect/varying contact causes the effective trigger voltage to vary by a few 100 microvolts, which would cause the behaviour that you see. If you'd see the same effect with a steeper slope with a larger amplitude at the same time base speeds, *that* would mean something and would point specifically to a problem in the time base.Raymond


John Griessen
 

On 4/17/19 9:43 AM, Raymond Domp Frank wrote:
If you'd see the same effect with a steeper slope with a larger amplitude at the same time base speeds,*that* would mean something and would point specifically to a problem in the time base.
Ok, I can try that. I'll have to get another pulse generator than the calibrator to be steeper slope. Later today.


 

On Wed, Apr 17, 2019 at 04:56 PM, Jim Ford wrote:


I've been following this thread with some interest because I have a 7B71
myself.  Planning to try it in 7904 when I get a chance.
To Jim & John:

I just tried a 7B71/7B70 set in a 7904 (they were really meant for the 7704(A) but work fine in most any 7xxx mainframe).
I've always found the trigger setting knob in these time bases (7B5x and 7B7x) a bit finicky in more critical situations. Triggering on a < 1 ns edge at 20, 50 and 100 ns / div. with about 4 div. signal amplitude was no problem but it wasn't difficult to have some jitter by playing with the trigger level adjustment knob. It was slightly different for different speed settings. All to be expected. Difficulty in removing the jitter with a reasonable amplitude (> 1.5 div on screen for a reasonably fast signal) and reasonable steepness of the slope would indicate a time base problem, *unless* trying to trigger too close to the amplitude extremes.

Raymond