Let me give a voice from the off:
- slew rate isn't as relevant for the Octoclock as clock/time source for USRPs – these
latch the PPS state on the rising edge of the internal clock, which might or might not be
derived from the 10 MHz from the Octoclock and cleaned up after. That's why you'll find
the schematics [1] feature simple cascaded 7400 series inverters
- you're right, stating explicitly that this is the unloaded output might have been nice,
but then again, since characteristic impedance doesn't matter for a signal that's DC for
>= 100 ms, and since it's "explicitly" omitted, I think that's a fair assumption. You'll
even find in the USRP X300 schematics (sheet 12) that the port is about 50 Ω-loaded for
sufficiently high frequencies, but at DC is effectively 3 kΩ-terminated. Flat frequency
response was seemingly not a design goal, but rapid dampening of overshoot at low DC power
was.
Cheers,
Marcus
[1] https://files.ettus.com/schematics/octoclock/
On 8/26/25 7:01 PM, John Ackermann N8UR via USRP-users wrote:
Agreed that it would be good for the datasheet to have mentioned this, as it's a
frequent cause of confusion.
While SMA connectors are designed for 50 ohms, it's really not safe to rely on that to
identify signal characteristics. After all, most oscilloscopes use 50 ohm BNC
connectors while their input impedance is 1 Mohm or more.
As I mentioned, there is no standardization for PPS signals characteristics. As an
example, older HP atomic clocks have PPS output on a BNC connector that is well over 10
volts into 50 ohms (unloaded it's way higher). They did that to increase the slew rate
to get more precise triggering in an era before modern logic gates were widely used.
But the width is very narrow (~20us) so the total power in the pulse isn't that high.
Most modern systems use logic gate outputs don't always consider what the load will be
and that can cause issues. For example, running a u-blox GNSS timepulse output into
coax cable does not work well at all; the source impedance is quite high and you don't
get reliable triggering at the far end of even a fairly short cable regardless of
termination. You need to buffer the signal before feeding it into coax.
I've designed several products with PPS outputs and use three 74AC04 gates in parallel
to provide 5 volt no-load outputs with enough drive capability to deliver at least 3
volts into 50 ohms. That has worked very well driving many types of counters and other
devices.
John
----
On 8/26/25 11:49, [email protected] wrote:
Hi John,
Thanks for this interesting insight!
However, I would think it is measured at 50 ohms as the outputs of the PPS on the
OctoClock-G are SMA connectors. As far as I am aware, SMA connectors do not have a 1
Mohm impedance.
I found that inverters are placed before the PPS outputs, which have a minimum output
of 2.4 V and a typical output of 3.4 V. The input of the USRP B210 (which takes in the
signal from the OctoClock-G) has an input range of 1.8 V to 5 V. So, I am not ruling
out that mismatching or matching at 1 Mohm can work too.
So it is not really an error in the datasheet. But, it would have been nice if they had
added an extra row in the table, pointing out at which impedance this was measured,
similar to the 10 MHz signal.
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