[time-nuts] Pulse width for start signal for HP5370
jim77742 at gmail.com
Sun Sep 4 13:23:12 UTC 2011
Is your trigger set at x10 or x1? If x10 that would narrow your range quite
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.
On 4 September 2011 18:19, Magnus Danielson <magnus at rubidium.dyndns.org>wrote:
> On 04/09/11 09:07, Tom Van Baak wrote:
>> 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
> 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
> 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.
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