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, dennis.joos...@uantwerpen.be 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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