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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Fabio Trevisan
Sent: 05 July 2018 23:35
To: [email protected]
Subject: Re: [TekScopes] 468 DSO Calibration/troubleshooting

OK Collin,
Now it makes some sense that there's no instruction to set the "NON STORE" to Off.
It's set to off at Section #2, step i, when it asks to set storage mode to "NORM".
Fine then... it means that during this step the scope is in full "digital" mode, and the waveform we're looking at is the digitized one.
regarding your triggering problem... well it does seem that it's being caused by something else, because if we think of the entire STORAGE triggering check / calibration instructions, it's rather minimalist.
It basically covers checks of the storage window mode (post trig and pre trig) and the A and B gates, and this one and only adjustment (for Storage NORM DC Bal).
I don't think the jitter is caused anywhere here, and from your description of when you try to align the rising edge of the 50KHz signal to the 2nd graticule line. that it comes out of trigger, that doesn't really make sense.
This fact alone suggests that the triggering point of the digital section is, level wise, completely unmatched to the analog section... Because, if you display this very same 50KHz in analog mode in this time base of 0.1ms, and set the trigger point of the 1st cycle at the 0 crossing... the 6th cycle will be dead on the correct spot (and so every one of the displayed cycles will be crossing the center line exactly at each of the HOR minor divisions).

And talking about your jitter / alias... Well, at 50 points per division (as the storage mode is expected to acquire/display), this 50KHz signal at 0.1ms/div would still have a healthy 10 points / cycle so, a jitter of +- 1 sample would be 1/10th of a minor division and, at this acquisition speed, much lower than the limit of the scope, I wouldn't expect nothing worse than +-1 sample.
Definitely there shouldn't be aliasing at play here either (with this healthy 10 sample / cycle). If much, a jitter of +- 0.05 div.

I will try to dig a little bit on the circuitry to see if I can come with any suggestion... but basically, from this point on, I don't think you're getting the calibration instructions wrong...or that you're missing something. It really seems that something is not working well on the digital triggering section.

KRgrds,

Fabio


On Thu, Jul 5, 2018 at 12:03 pm, Colin Herbert wrote:

Hi Fabio,
The difficulty with looking at a 50-KHZ at 0.1 ms/div is that the triggering
is really not that great and some aliasing is evident. I do see your point,
though and I have given it a go, but the jitter doesn't help and I can't get
any of the rising edges to align with the second vertical graticule line using
the A trigger level. What I have just noticed, though, is that the stable
triggering only takes place between the zero crossing and just after the next
crest and, oddly, it seems to be the wrong slope! That is, with the slope
switch in the + position, triggering is on the falling edge and vice versa.
The non-storage triggering is exactly the way it should be. I think there may
be something wrong, here.

With regard to the storage/non-storage buttons. They are all latched. Pushing
any of the storage buttons (Norm, Envelope, Avg and Save) releases any of the
other three and the Non-stope button. Pushing Non-store releases the storage
buttons. I think that's what you might expect.

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Fabio
Trevisan
Sent: 05 July 2018 19:29
To: [email protected]
Subject: Re: [TekScopes] 468 DSO Calibration/troubleshooting

Hi Collin,
You're welcome... It's more challenging to solve puzzles when you're not
actually on the driver's seat.

Right out of the box I can't see a flaw in your reasoning... I confess I
didn't make the math that 50KHz at 0.1ms/div would give something like 50
cycles...
I agree it seems really silly to try to adjust the crossing of the waveform at
that particular cross-hair, with such dense waveform. 20us/div would really
make more sense in that respect.
Or maybe... they really meant it to be densely packed with cycles, so that we
could "disregard" the rising edge slope (at that density, the rising edge is
almost a vertical line, that we would only need to make sure is aligned to the
vertical graticule).
If there's no mistake in the instructions, it should have - at least -
mentioned that we should be aligning the leading edge of the 6th displayed
cycle, and not just "Align the leading edge"... that's too vague to say the
least.

Now, regarding my doubt whether the procedure is meant to be carried out in
storage mode or not, I have a doubt: How does the "NON STORE" button mode
works (mechanically)?
Is that when we press some of the STORAGE MODE buttons (say, NORM, ENVELOPE,
AVG, or SAVE), does it make the "NON STORE" button to pull out?
That would explain why there's no instruction to explicitly set the "NON
STORE" button to OFF (out).

KRgrds,

Fabio





On Thu, Jul 5, 2018 at 10:07 am, Colin Herbert wrote:

Hi Fabio,

Firstly, thank you for giving this some thought. Some of my observations:

1. Check STORAGE WINDOW Operation. This all works fine, except that when

going

to "d. set STORAGE WINDOW PRE TRIG" the triggering jitter becomes quite
noticeable; it is less so in POST TRIG.
2. Check A and B+ GATES.
Set NON-STORE On; A TIME/DIV 0.1 ms; B TIME/DIV 20 us. All works as

expected,

except for the jitter when in NORM storage mode - Non-storage there is a
rock-solid trace with no jitter at all.
3. Check/Adjust Storage NORM Trigger DC Balance (R126)
Set CH1 VOLTS/DIV 5mV; HORIZ DISPLAY A. Remember that the A sweep is still
at 0.1 ms/DIV and so there are some 50 full cycles of a 50-kHz sine-wave.
Trying to set anything with the A TRIGGER LEVEL is nigh on impossible. I

think

the sweep should be A at 20 us/DIV, showing ten full cycles. The problem now
is that trying to align the leading edge with the second vertical graticule
line at the horizontal centre graticule line results in loss of triggering.
This alignment can be done using the horizontal position control, however.
When aligned in this way, switching between AC and DC Trigger coupling of

the

A sweep shows no movement.

Have I achieved what is required, or am I again missing something?

It is a pity that Reed Dickinson hasn't seen this thread, as I think he is a
bit of an expert on the 468 and he certainly holds them in some respect.
However, I repeat my thanks to you, Fabio.

Regards, Colin.

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Fabio
Trevisan
Sent: 05 July 2018 17:09
To: [email protected]
Subject: Re: [TekScopes] 468 DSO Calibration/troubleshooting

Hello Collin,

Regarding my last statement (the P.S.), that this check should be made in
"non-store" mode, I'm giving second thoughts at it.
I`m not sure yet, but I just found out that there are two "NORM triggering

DC

balance" adjustments... one on the analog side and another one on the

storage

side...
So, it seems weird that we could even be able to adjust the STORAGE NORM
triggering DC balance in any mode that isn't a storage mode.

On the other hand, I may still be correct on my initial assumption, because
the flow of the instructions and the explicit instruction to set "NON STORE"
to ON, at Section #2, step g.
I`m only not sure if that adjustment (R126) would be even doable while in

NON

STORE mode...

I`ll get back to it as soon as I can dig a little bit more on the manual.

Rgrds,

Fabio



On Thu, Jul 5, 2018 at 08:23 am, Fabio Trevisan wrote:

Hello Colin,

I don't own a 468 and have no particular experience with it, but I have

the

manuals (in PDF form) and since your doubt is related to understaing of

the

instructions, I decided to give it a try and see if I could make sense out

of

this part of the instruction that you're doubtful at.

At this step, the only thing that doesn't seem really clear to me is why

it

asks specifically to align the leading edge of the sine-wave to the 2nd
graticule line (why the 2nd.???)..
But, regardless of not understanding that, I think the purpose of this
calibration step is clear... Is to make a "differential" assessment of the
triggering level shift, between using AC triggering coupling and DC

triggering

coupling...
So, basically what it is asking for, is for you to make sure that the
triggering point doesn't move between AC and DC coupling, when you're

actually

feeding a signal that already swings about 0V...
I mean, if the input signal is swinging about 0V (i.e. if it doesn't

contain

a

DC level), changing the triggering coupling from AC to DC should not make

any

difference, and therefore, the triggering point, whatever it was set to

before

switching to DC, the wave form must remain in the same position.
If the waveform shifts horizontally, than it means that the triggering

point

"seen" by the triggering circuit have changed when switching over from AC

to

DC, which it shouldn't.

Giving second thoughts to the instruction, it's not much different than
adjusting the Norm DC triggering balance of a 464 (which I own), and

probably

the same as the 465 (more common).
the only difference is that, for the 464, we're asked for looking at the

very

beginning of the sweep, and not at the 2nd vertical graticule line (i.e. 1
division after the sweep actually started).
This difference in procedure (from the non store scope), doesn't actually
matter to what the DC balance adjustment is doing (which is nothing more

than

shifting the DC level of the actual signal being fed to the triggering

input

buffer, with the intent to making it match with the level of the same

signal

when AC coupled).
The only reason I think they're doing that (adjusting at the 2nd vertical
graticule line iso at the beginning of the sweep), is because this point

is

about the actual point on the display where the triggering point of the
digital stored signal is supposed to be displayed.

P.S. Also note that, although this adjustment step is called "Check /

Adjust

STORAGE NORM Trigger DC balance", by following the instructions coming

from

the previous steps, I understand (it's my conclusion though) that this
adjustment is not meant to be performed in storage mode.
The previous section, "#2 Check A and B+ gates", at step g. it asks you

to

set the "NON STORE" button to ON position... and then it doesn't ever ask

you

to change it to OFF, so, by the flow of the procedure, the #3 check is to

be

performed in NON STORE mode.
Therefore, this jitter that you mentioned you're getting on storage mode
should not interfere with you performing this adjustment (because it's

done

in

NON STORE mode).

Now, talking about this jitter... I don't think it has any relation to

this

adjustment, and it's probably not going to be fixed by setting this

adjustment

right.

I hope I was able to help you somehow.

KRgrds,

Fabio




On Tue, Jul 3, 2018 at 10:08 am, Colin Herbert wrote:

I have got through the calibration/adjustment procedure up to "Storage
Triggering". My problems now are twofold. I can't understand Section #3

-

"Check/Adjust Storage NORM Trigger DC Balance" (P4-89 in the manual).

Apart

from the fact that the storage triggering has a deal of jitter, I can't

get

step (e) to make sense. I am supposed to "use the A TRIGGER LEVEL to

align

the

point where the sine-wave's leading edge intersects the center

horizontal

graticule line with the 2nd vertical graticule line". Because of the the
jitter and the fact that the triggering fails when I adjust the A

TRIGGER

LEVEL, this is impossible. Am I doing something wrong (quite possible),

or

is

the manual giving me the wrong information? Help!
If anyone has an idea of how I can eliminate the jitter, that would be

nice,

too. There is no jitter in "NON STORE" operation. I was hoping it would

just

be something simple like a switch needing cleaning, or an IC needing
reseating, but nothing has given me any clues so far...
Colin.