The Octoclock outputs are TTL gates that deliver 5 volts into a high
impedance. Such gates aren't designed to drive a 50 ohm load, and they
will show a significant voltage drop in that case. Just how much drop
depends on how much current the gate can source (ie, what its source
impedance is).
The Octoclock spec sheet doesn't specify a load impedance for the PPS
outputs; it just says "5 volts". (The 10 MHZ output is spec'd at 50
ohms, but not the PPS outputs.)
For historical reasons, there isn't a defined standard for PPS signal
levels or termination, and this makes things "interesting" to say the
least.
In most cases, a 5V pulse through 50 ohm coax terminated into 50 ohms
will cleanly trigger a 5V logic gate at the far end, but the margin can
be small. If the far end has a 3V3 input, it will usually work very well.
Using a higher load resistance will increase the voltage, but may cause
ringing in the coax and that could result in triggering issues. Often,
something like a 100 or 200 ohm termination is a good compromise that
delivers sufficient voltage without causing too much ringing.
Hope this helps.
John
----
On 8/18/25 09:44, [email protected] wrote:
Hi,
Is it possible there is an error in the datasheet of the OctoClock-G
regarding the 1 PPS output?
It states that it has an output of 5 V, however, when measuring with an
impedance-matched scope, I only get about 2.5 V out. When switching to
1M input on the scope, it is around 5 V.
Datasheet: https://www.ettus.com/wp-content/uploads/2019/01/
Octoclock_Spec_Sheet.pdf
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