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 <mag...@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 > > > ______________________________**_________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.