[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
shalimr9 at gmail.com
Sun Sep 28 11:24:05 EDT 2014
Late reply... (been busy)
The purpose of the straight drive is to maximize the signal amplitude on
the cable. The test intends to demonstrate that it is perfectly adequate if
only the far end is matched, and yet you get twice the amplitude you would
get with a 50 ohm drive signal.
On the other hand, if the near end is driven like that and the far end is
open, you get very significant ringing even with a modest bandwidth
oscilloscope (100MHz in my case). The longer the cable, the lower the
ringing frequency and potentially the worse the uncertainty on the rising
Tom asked how much cable reflections could bother a 1 PPS signal and the
answer is that it can bother the leading edge a whole lot, no matter what
the frequency is. Of course, if you receive the 1PPS with a serial port
connected to a busy CPU with multiple uS of latency, it won't make much
difference unless the cable is so long as to be impractical.
On Thu, Sep 18, 2014 at 10:00 AM, Said Jackson via time-nuts <
time-nuts at febo.com> wrote:
> Hi Didier,
> You showed the effect of standard end-termination and no end-termination
> both while driving the cable with a mis-matched low signal source
> impedance. These are the same type of plots that Tom posted a link to last
> The interesting plots will be when you set R1 to match the cable impedance
> and then vary R2.
> Sent From iPhone
> > On Sep 18, 2014, at 3:41, Didier <shalimr9 at gmail.com> wrote:
> > Hi Tom,
> > This page will show you the kind of problems to expect:
> > http://ko4bb.com/Test_Equipment/CoaxCableMatching.php
> > Didier KO4BB
> >> On September 15, 2014 8:43:21 AM CDT, Tom Van Baak <tvb at LeapSecond.com>
> >> How important are all these cable / termination / impedance issues for
> >> 1PPS signals? I know ringing and reflections are undesirable in many
> >> applications. But for 1PPS?
> >> I often use pick whatever cable, termination, and trigger level gives
> >> the cleanest edge, the best risetime. What happens to the signal tens
> >> or hundreds of nanoseconds after the edge seems irrelevant to me. Could
> >> one of you RF experts comment?
> >> /tvb
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