[time-nuts] HP 5370B jitter

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jul 14 20:54:10 EDT 2007

From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] HP 5370B jitter
Date: Sun, 15 Jul 2007 12:13:25 +1200
Message-ID: <469966A5.50500 at xtra.co.nz>

> Hej Magnus

Hej Bruce,

> Magnus Danielson wrote:
> > Further investigations have now shown that the 10811 output is clean, just a
> > little 3rd harmonic but nothing to worry about. The INT test point is also
> > clean. I suspect the output drive part, as other outputs to various parts are
> > also free of the 5 MHz. The 10 MHz present detector circuit is currently my
> > main suspect. It consists of a "one-shot multi-vibrator triggered by the 10 MHz
> > signal". Having a RC time-constant of 100 ns makes it a suspect indeed. Probing
> > it (pin 11 on A8U1) clearly shows a waveform wich looks like a 25% 5 MHz with
> > a short spike on it. This little culprit of a detector is infact a wideband
> > comb-generator which contributes its 5 MHz as sidebands to the output 10 MHz.
> > It's only purpose in life is to light a LED only visible to the servicing
> > engineer (me in this case). Thus, making a small modification to disable it
> > during normal operation would improve the quality of the 10 MHz output
> > considerably if I am right. Since I don't do ECL design on a daily basis, I
> > will have to ponder a bit in order to come up with a good method of acheiving
> > this.
> >
> >   
> Try shorting AU8U1 Pins 6 and 7 to ground.
> This turns of the npn output emitter followers driving the longtailed 
> pair which drives the LED.
> This is permitted and nothing will be destroyed or degraded even if the 
> shorts are permanent.

I shorted A8U1 pin 6 by a short jumper from the ground layer to the base of
Q8 since they where nearby each other. This kills the loopback and locks U1A
and thus the whole circuit locks down. I could rewire things a bit since the
OR/NOR gate U1C is not used at all. This would be the neat way of doing it
while the quick and dirty one does the trick.

So, no more 5 MHz with overtones on my HP5370B thanks to this quick fix.

Self-referenced jitter did not change significantly thought. It is actually
slightly higher numbers, but that may be due to heatup or something. I noticed
that during my previous run too.


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