[time-nuts] Correcting jitter on the 1 PPS signalfrom a GPS receiver.
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Sep 15 14:14:13 EDT 2014
Hi Dave,
what you are describing is reflected wave switching, which works perfectly
in applications such as the PCI bus. The PCI bus uses it because it lends
itself to extremely low power consumption.
Your scenario does not work with end-termination either if you have
multiple taps, because the taps would have to a) have very high impedance so as
not to disturb the signal traveling down the pipe, and b) not have any cable
length associated with them so as to prevent the edge going down into the
stub and reflecting back from it, and the stub causing a 25 Ohms impedance
drop (due to two 50 Ohms transmission lines in parallel).
With the rise-times of sub 1ns (the CSAC GPSDO has a rise and fall time of
<500ps typically) any cable length becomes an issue, including the couple
of inches of wiring inside the counter and through the T-splitter etc.
Another issue is that the timing of the instruments would be all off because of
the differing propagation delays through the additional cable.
So in the end-terminated world having a stub would only work if the stub
does not have any cable length associated with it and very high impedance.
How many folks distribute their 1PPS signal through a single coax cable to
multiple instruments? This should be done through a 1PPS distribution amp.
I agree though that in the analog video world you don't want reflections
due to even small impedance mismatches because they cause ghosting. In the
digital world where all we care about is that one single rising edge per
second ghosting is not an issue.
My original point was that the video world got it right: use a 75 Ohms
output impedance for a 75 Ohms coax. The 1PPS world did not get it right by
driving a 50 Ohms coax with a 10 Ohms output impedance.
Bye,
Said
In a message dated 9/15/2014 10:43:24 Pacific Daylight Time,
dave.martindale at gmail.com writes:
I will agree that the end termination is optional if you are delivering a
pulse signal to just one input, which is at the far end of the coax.
However, I think there's still a problem with series-only termination when
the pulse signal is daisy-chained through multiple inputs. When you apply
5 volts through a 50 ohm terminator to a 50 ohm cable, the instantaneous
voltage on the coax is only 2.5 V. A pulse of amplitude 2.5 V travels down
the cable, and reflects from the open far end. The reflection travels back
along the cable to the source, raising the voltage from 2.5 to 5 V as it
passes.
A device input located at the far end of the cable sees a single edge of 5
V amplitude, so it's happy. But anything located somewhere along the
cable run sees two edges: one from 0 to 2.5 V, then a constant 2.5 V for a
period equal to twice the delay of the remaining cable, then another edge from
2.5 to 5 V. Depending on the input threshold, this in-between device might
trigger reliably on the first edge, the second edge, or not reliably on
either.
Having proper far-end termination is critical for analog video, where
daisy-chaining is common, and a reflection that's even 1% of the amplitude of
the original signal is likely to be visible as a ghost image. With pulse
signals, maybe it makes more sense to use one cable per device input, input
plus lots of distribution amplifiers and splitters.
- Dave
On Mon, Sep 15, 2014 at 1:13 PM, S. Jackson via time-nuts
<_time-nuts at febo.com_ (mailto:time-nuts at febo.com) > wrote:
Hi Dave,
yes there is a reason.
The "standard" 1PPS signal termination (Thunderbolt etc) used to be 5 Ohms
or less series termination into a 50 Ohms coax (yikes), then end-terminate
to get rid of all the undesired reflections.
Your example below is properly terminating a 75 Ohms coax with a 75 Ohms
series termination. The end-termination then becomes optional and affects
the
signal level at the sink. So if a higher signal level is desired, simply
leave off the 75 Ohms end termination.
But in the case of the Thunderbolt they don't use a 50 Ohms output
impedance, they use something around 5 Ohms. That is the problem here: the
total
impedance mismatch from the very low source impedance into the 50 Ohms
coax.
The reason they do that is so that they can generate a "proper" signal
level that is approaching 5V across the 50 Ohms end termination so that the
signal remains CMOS compatible. Otherwise if they properly terminated the
driver with 50 Ohms they would have a voltage divider and would only
generate
2.5V at the sink.
bye,
Said
In a message dated 9/15/2014 06:04:34 Pacific Daylight Time,
_dave.martindale at gmail.com_ (mailto:dave.martindale at gmail.com) writes:
Is there any reason (other than cost) not to both series-terminate the
source and parallel-terminate the sink?
When I was dealing with analog video, the standard distribution method was
:
1. Buffer amplifier with high input impedance, very low output
impedance, and a gain of 2 (so 1 V P-P input becomes 2 V P-P out)
2. A series 75 ohm resistor from the amp output to each individual video
output. This formed a 2:1 voltage divider with the 75 ohm coax to give
1 V P-P on the cable. It also isolates the loads from each other.
3. A single video signal could be looped through multiple high impedance
loads.
4. 75 ohm parallel termination at the far end of the signal path
(usually on the last device).
This way, every device along the way saw an undistorted copy of the
signal. The buffer amplifier sees a simple resistive load. And any
reflections are absorbed at both ends of the cable.
- Dave
On 15/09/2014 02:04, Fuqua, Bill L wrote:
> A lot of devices have a low output impedance so that the signal can be
split using a TEE adapter with little loss or need for a distribution
amplifier.
> However, the cables must be impedance matched at far end, scope input,
to prevent reflections which are the source of the ringing.
> You can match the impedance at the source and you will get a reflection
which will then be absorbed by the source resistance. One way to do this
> is to get a small 15 turn pot about 100 Ohms put it, in series with the
input source and adjust it until the ringing is gone or you can put it at
the far end
> ,input of the scope, to ground and do the same. But the best solution
is to get a good feed thru 50 Ohm terminator and put it on the input of
the
scope.
> Bill
>
>
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