[time-nuts] Correcting jitter on the 1 PPS signalfrom a GPS receiver.
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Sep 15 13:13:54 EDT 2014
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.
In a message dated 9/15/2014 06:04:34 Pacific Daylight Time,
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
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.
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
> 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
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