[time-nuts] PN/AM and 1.5Hz spur from frequency doubling?

John Miles john at miles.io
Thu Jan 19 06:11:46 EST 2017

You may find that the behavior varies quite a bit depending on what you put between the maser output and the doubler.  I've seen one case in particular that generates a ton of PM spurs, specifically an HP 11721A doubler driven by the 5 MHz output of a 5061A Cs standard.  The 5 MHz output uses a narrowband transformer with a parallel-tuned primary, so it looks highly reactive at frequencies other than 5 MHz.  I'm not sure what's really going on, but the 11721A (which is being run below its frequency spec in this case) doesn't seem happy at all ( http://www.ke5fx.com/11721a.png ).  Adding an SLP-5 lowpass filter in front of the 11721A makes the spurs go away, as does an isolation amp.

The 1.5 Hz spur in your plot looks like leakage from a nearby 10 MHz source, presumably a free-running OCXO that's a bit off frequency.  The rest of it looks like an open coax shield or something.  (If you're using a coaxial balun, try removing it.)  It's possible to get 2-3 dB of excess noise from a multiplier beyond the expected 20*log(N), depending on source and load characteristics, but I haven't seen anything that bad in the absence of other problems.

-- john, KE5FX
Miles Design LLC

> >> I'm seeing +20-30 dBc/Hz of excess AM/PN, as well as a strong 1.5 Hz spur
> >> created by frequency doubling from 5 MHz to 10 MHz.
> >> https://goo.gl/photos/GFx9tQoxrSmyzUQo8
> >> The input amplitude to the doubler should be just above the
> recommended 11
> >> dBm.
> >> What's going on??
> >>
> >> thanks!
> >> Anders

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