[time-nuts] Pulse width for start signal for HP5370

Jim Palfreyman jim77742 at gmail.com
Sun Sep 4 13:23:12 UTC 2011


Paul,

Is your trigger set at x10 or x1? If x10 that would narrow your range quite
a bit.

Apart from the oscilloscope suggestions above, make sure your 1PPS is
terminated appropriately. If you don't have 50R termination and it is
required you will get a lot of ringing on the upstroke of the 1PPS and that
can cause problems too.

Jim Palfreyman


On 4 September 2011 18:19, Magnus Danielson <magnus at rubidium.dyndns.org>wrote:

> On 04/09/11 09:07, Tom Van Baak wrote:
>
>> Paul,
>>
>> Do you have an oscilloscope handy? When doing precision
>> time interval measurements it's useful to take a look at what
>> the counter inputs actually look like. Use a BNC tee and a 1M
>> scope probe (so as not to artificially load the counter input).
>>
>
> I tend to do the same. Also, some errors is not due to the wrong frequency
> but due to interference, and you can't see that very good with a counter,
> but on a scope.
>
> Here a short-hand of observations and probable causes:
>
> 1. too high frequency (unstable)
>
> May be trigger error, usually bad slew-rate on the slope or slope of other
> direction. In really bad cases double frequency can be seen.
> Injected noise has also been seen.
>
> 2. too low frequency (unstable)
>
> May be trigger error, usually a trigger on the bad slew-rate peak, so
> moving tricker back to safe ground high slew-rate is needed.
>
> 3. unstable
>
> Beyond the above cases, you can also have ringings or other similar
> features of the signal which can cause bad timing triggers etc. High
> slew-rate is where you want to trigg, if you just can avoid other trigger
> problems.
>
> Low-pass filters can help to clean up trigging.
>
> It is a good exercise to train the trigging skills on oscilloscope, since
> you will get fuzzy and/or double/tripple/whatever images.
>
> So, in all these a good scope gives you clarity and a good feel for the
> signal.
>
>
>  For pulse inputs it should help you set the DC trigger level
>> appropriately. I usually use 0.5 or 1 or 2.5 VDC. But seeing
>> the actual waveform helps me decide.
>>
>> For CW inputs you will probably want to set the counter to
>> AC coupling and zero volt trigger.
>>
>> Some counters give you a choice of 50R or 1M termination.
>> Be careful with that. In some cases the waveform is much
>> cleaner with 50R. In other cases 50R puts way too much
>> load on the source and you trigger level or risetime suffers.
>>
>
> All of the above matches my experience.
>
> Doing measurements properly is an art. While I am sloppy, it is mostly from
> lazyness. I can even get called in to the lab just to "find" the signal with
> the scope they already hooked up (usually they made the error of going for
> higher frequency rather than lower frequency offending signals... so that I
> find within minutes with just turning time-base and tuning trigger).
>
> Also, if you have it, at some times a spectrum analyzer can help you to
> understand what happens... such as unexpected harmonics. The spectrum
> analyzer has better dynamics than the scope.
>
> Cheers,
> Magnus
>
>
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