Confusing waveform on oscilloscope

[Paul Burridge]

Hi chaps,

I'm getting two different displays for the same waveform. I'm looking
at the output of a slightly dirty 40Mhz oscillator. I've come across
this knob called "holdoff" which when in one position, displays the
wave as a single, sharp, slightly irregular sine wave; but in another
position shows about 4 or 5 distinct, regular sine waves all
superimposed on each other, in phase but with slightly differing
amplitudes. Which is the *real* waveshape that accurately reflects
what the oscillator's actually putting out?
I don't understand what half the knobs and buttons on my scopes are
actually supposed to do. :-/
<sigh....>

 

[Fred Bloggs]

Hi chaps,

I'm getting two different displays for the same waveform. I'm looking
at the output of a slightly dirty 40Mhz oscillator. I've come across
this knob called "holdoff" which when in one position, displays the
wave as a single, sharp, slightly irregular sine wave; but in another
position shows about 4 or 5 distinct, regular sine waves all
superimposed on each other, in phase but with slightly differing
amplitudes. Which is the *real* waveshape that accurately reflects
what the oscillator's actually putting out?
I don't understand what half the knobs and buttons on my scopes are
actually supposed to do. :-/
<sigh....>

The HOLDOFF feature is there to allow you to zoom in on waveform details
with a separate timebase. There are two timebase settings, the
highlighted/intensified portion of the waveform is there to show you the
extent of the HOLDOFF expanded (faster) timebase sweep which should also
be displayed. The "4 or 5" distinct sine waves you observe are due to
the fact that your scope timebase resets and retriggers the sweep on
amplitude level in a shorter time period then the periodic amplitude
modulation of the waveform. You can pick out a better idea of this
amplitude modulation by increasing the timebase to close to an integer
multiple of the suspected period then "uncalibrate" the timebase to
vernier the display into a stable waveform.

 

[Baphomet]

Hi chaps,

I'm getting two different displays for the same waveform. I'm looking
at the output of a slightly dirty 40Mhz oscillator. I've come across
this knob called "holdoff" which when in one position, displays the
wave as a single, sharp, slightly irregular sine wave; but in another
position shows about 4 or 5 distinct, regular sine waves all
superimposed on each other, in phase but with slightly differing
amplitudes. Which is the *real* waveshape that accurately reflects
what the oscillator's actually putting out?
I don't understand what half the knobs and buttons on my scopes are
actually supposed to do. :-/
<sigh....>

Probably, the single sine wave is the proper display. It sounds like the
scope sweep is free running or recurrent (not correctly triggered) when you
are seeing multiple displays of the same waveform.

 

[George R. Gonzalez]

Hi chaps,

I'm getting two different displays for the same waveform. I'm looking
at the output of a slightly dirty 40Mhz oscillator. I've come across
this knob called "holdoff" ....

Holdoff is a very useful but widely un-understood feature.

If the signal of interest is a nice clean repetetive waveform, with each and
every cycle identical, then you don't need Holdoff.

But if you have a waveform that has say, four cycles, all slightly
different, without holdoff your scope will tend to trigger randomly on cycle
1, cycle 2, etc... and all the slightly different size or shape cycles will
be superimposed, leading to a fuzzy looking line.

But if you turn on a bit of Holdoff, at just the right setting the scope
will be coerced into triggering on the SAME cycle of the sequence every
time, so you'l lget a clean and stable display of all X cycles. "Holdoff"
means the sweep is held off from triggering on the next avilable cycle,
instead it waits for the amount of time you've dialed in.

You can do roughly the same thing with the VAR sweep time, to force the
sweep to be within one cycle of the repeat-length. But then you're limited
to seeing exactly one or two or N full cycles. With Holdoff, you can zoom
in on one cycle or less, and the holdoff will add the right amount of delay
so the scope doesnt trigger on any of the cycles you DONT want to see right
now.

If the irregular waveform you're watching has a little extra peak on one
cycle, you can do these tricks too with the trigger level control. By
carefully setting it, you may be able to get it to uniquely trigger on the
one cycle that stands out. But again it's not as stable as using Holdoff to
do the job.


You can send me any unused Tek knobs, my scopes need a few of those
hard-to-find knobs.

[email protected] <[email protected]>
Regards,

George

 

[Ian Stirling]

Hi chaps,

I'm getting two different displays for the same waveform. I'm looking
at the output of a slightly dirty 40Mhz oscillator. I've come across
this knob called "holdoff" which when in one position, displays the
wave as a single, sharp, slightly irregular sine wave; but in another

You should know that the proper etiquette in this group is to therefore
declare that you've discovered FTL, and put up some coax on ebay for
some large amount of money.

 

[Fred Abse]

The HOLDOFF feature is there to allow you to zoom in on waveform details
with a separate timebase. There are two timebase settings, the
highlighted/intensified portion of the waveform is there to show you the
extent of the HOLDOFF expanded (faster) timebase sweep which should also
be displayed. The "4 or 5" distinct sine waves you observe are due to the
fact that your scope timebase resets and retriggers the sweep on amplitude
level in a shorter time period then the periodic amplitude modulation of
the waveform. You can pick out a better idea of this amplitude modulation
by increasing the timebase to close to an integer multiple of the
suspected period then "uncalibrate" the timebase to vernier the display
into a stable waveform.

You're thinking of the delayed timebase function, Fred. The holdoff
control varies the time at the end of one sweep before the timebase will
accept another trigger. If you like, the "dead" time.

It can be useful if there are several events in a cycle that will trigger
a sweep, which can't be selected out by level alone, and you want to trigger
on the same event in each cycle. Using the variable "vernier" time control
can sometimes do much the same thing, but at the expense of uncalibrated
time/division.

 

[Paul Burridge]

[snip]

If the irregular waveform you're watching has a little extra peak on one
cycle, you can do these tricks too with the trigger level control. By
carefully setting it, you may be able to get it to uniquely trigger on the
one cycle that stands out. But again it's not as stable as using Holdoff to
do the job.

Thanks for the comprehensive explanation, George. Most interesting
indeed. I'm sure there's a lot of use I'm not getting out of these
instruments owing to my limited knowledge of how to use them.

You can send me any unused Tek knobs, my scopes need a few of those
hard-to-find knobs.

I'll bear it in mind. I may have an older Tek coming up for spares
shortly. The one I'm currently using is a bit too up-to-date to start
nicking bits off.

 

[Paul Burridge]

You should know that the proper etiquette in this group is to therefore
declare that you've discovered FTL, and put up some coax on ebay for
some large amount of money.

FTL coax, eh? Sounds like a major contribution to speeding up
communications.

 

[Fred Bloggs]

You're thinking of the delayed timebase function, Fred. The holdoff
control varies the time at the end of one sweep before the timebase will
accept another trigger. If you like, the "dead" time.

It can be useful if there are several events in a cycle that will trigger
a sweep, which can't be selected out by level alone, and you want to trigger
on the same event in each cycle. Using the variable "vernier" time control
can sometimes do much the same thing, but at the expense of uncalibrated
time/division.

Ehh- he threw me off with the "highlighted" trace- sounded like dual
timebase sweep B DELAY from A.

 

[Paul Burridge]

Ehh- he threw me off with the "highlighted" trace- sounded like dual
timebase sweep B DELAY from A.

Have we got more than one contributor calling himself "Fred Bloggs"
then? The first reference to a "highlighted trace" (sic) comes from a
Fred Bloggs. Unless there's only one Fred Bloggs that happens to
suffer from multiple personality disorder in referring to himself in
the third-person.