[time-nuts] Question about an HP5370B TIC

[John Miles]

> As long at there is always a positive or always a negative time
> difference between the two 1 pps signals the result makes perfect sense.
>   There appears to be about a 2 to 3 usec jitter in the 1pps from the
> GPS (this is expected given the design of the receiver).
> 
> If I put the 5370B in the +/-TI mode I expected to see both + and -
> small time differences when the two signals are closely aligned - and
> mainly I do.  Typically the difference is + or - a usec or less.
> Occasionally however the 5370B displays very near to a +1 or -1 second
> (999.999 xxx ms or -1.000 000 xxx s for example) reading.  Needless to
> say this plays havoc with trying to measure a mean value of the
> alignment.  The 5370B is clearly measuring the "outside" between the
> pulse pair when I want it measure the "inside" (or short time interval)
> between the pulse pair.
> 
> Is this normal behavior?  Is there a setup that I am missing to make it
> do what I want?

>From what I've seen it's best to stick with +TI mode for measurements like
this.  You don't want the counter making decisions about which half of the
interval is "short" and which half is "long" -- it just doesn't add any
value to the measurement.  

With 1-pps on both input jacks, your oscillator should remain within +/- 500
msec of GPS for a very long time, so you should be able to set up a
combination of delays and/or edge polarities that will result in an
arbitrarily-long set of phase measurements that never come anywhere near the
0 or 1000-msec points where the phase will wrap.  (Even if they do wrap,
it's not a big problem as long as you aren't using averaging on the
counter.)

-- john, KE5FX