[time-nuts] Why using HP5370 ext-ref is (maybe) a bad idea

Bob Camp lists at rtty.us
Sun Mar 2 18:24:13 EST 2014


Do you have any idea how “clean” your 200 MHz signal is? The manual suggests getting it to -65 dbc for subs using a spectrum analyzer. That’s pretty far down. I seem to recall the adjustment process being a bit tedious (lots of back and forth). 


On Mar 2, 2014, at 5:29 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> I have spent another evening playing around with the 5370 and the
> conclusion is pretty ironclad now:
> Running a 5370 with ext-ref locked to input frequencies is simply
> a bad idea and should not be done.
> Running it on the internal OCXO works fine.
> Running it on another frequency *not* locked to the input frequenc
> also works fine.
> In both cases the errors are statistically well-behaved, and can
> be treated with normal statistical methods, including the built-in
> STD-DEV function.
> But feeding ext-ref a frequency which is locked to the input frequencies
> causes the errors to become systematic, and they can no longer be
> treated as statistically well-behaved.
> For instance:  The length of the coax to ext-ref suddenly affect
> your TI measurements, because it shifts the phase between the 200MHz
> and the input signal.
> I tried tuning up the A21 200MHz synthesizer to the best of my
> ability, and it clearly made a difference, the phase pattern
> of errors shifted around, but the errors did not get any smaller,
> they just moved.
> I also tried disconnecting the "10 MHz present" circuit, that
> didn't change the magnitude of the errors either, but did shift
> the phase of the peak noise a couple of degrees.
> Looking at some old notes from years past which just didn't make
> sense, does now.
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
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