Urukul frequency instable in combination with external 10MHz clock

simonschey

Hi together,

I am running my Sinara-Box with an external 10MHz GPS-clock.

Clocking is set up as ext0_synth0_10to125.

The Urukul is set up with

    "refclk": 125000000.0,
    "clk_sel": 2,
    "clk_div": 0

Every now and then (I don't exactly know, but it happened at least twice in the last 2 weeks), my Urukul output looks like this (would like to post the video, but don't know how, therefore just a screenshot):

It jumps back and forth by a couple of kHz when viewed on a spectrum analyzer.

Power-cycling the Sinara-box does not change it, but I realized that unplugging and re-plugging the clock solves the issue. Does anyone have experience with this or can help with what might be going on there?

Thanks and BR

Simon

bukehu

Hi!

Is it possible that the cable that connects the external clock to the Urukul is picking up noise/charge? Perhaps the charges disappear when you touch the SMA connector?
And, is the ground properly connected for the chassis?

simonschey

bukehu Thanks for your input. The Kasli-crate is not directly grounded to the Rack it is mounted into. It is only grounded to the experiment via the cable shield of a Fastino. But I would consider that "proper grounding" though.

The clock-signal comes in through a transformer to avoid ground-loops there.

dpn

simonschey I haven't ever seen this, so this is no more than a guess, but: did you every look at the input clock (scope on a splitter/…) when the issue occurred or after unplugging and re-plugging it? Another thing to check might be the signal amplitude, as – in a generic statement about logic circuits – excessive amplitude might in principle lead to latch-up, but this seems unlikely.

Somewhat tangentially, reading out the status register of the PLL chip on Kasli to report clock failures in real-time would be a neat addition.