[time-nuts] Effect of EFC noise on phase noise

Richard (Rick) Karlquist richard at karlquist.com
Wed Aug 3 12:30:48 EDT 2016

Around 35 years ago, I worked with the guys
who designed and manufactured the 10811.  There
are a couple of things here that don't add up,
subject to remembering stuff from a LONG time

1.  Back in those days at least, there were
vendors who supposedly specialized in providing
low noise zener diodes.  The particular breakdown
voltage of zener diodes was important.  IIRC,
at low voltages, it is a true "zener" diode and
at higher voltage it is merely an avalanche diode.
The physics are somehow different.  There is also
a "magic" voltage where the tempco happens to be
+2mV/degree C, in which case you can cancel it
out with a series junction diode.  I believe they
even sold combination diodes with both the zener
and the temperature compensation diode in one
package.  6.4V is not far from the magic voltage,
FWIW.  Anyway, what I was led to believe is that
certain JEDEC 1N___ part numbers, with suffixes
indicating noise properties, from particular
vendors had much lower than average noise.  Thus
if a run of the mill zener diode has 1,000's of
nV/sqrtHz of noise, these "golden" diode might
have only 100's, or even dozens.  At one time I
had some copies of some fairly detailed lab notebook
pages detailed research by engineers that I
have a lot of confidence in.

2.  In all the work with 10811's, E1938A's, and
the 5071A, there was never any indication that
the zener diode and associated resistors, etc
made any contribution whatsoever to phase noise or Allan
deviation.  If there was any such effect, I can
guarantee that we would have heard about "hero
experiments" proving it, and we would see
10811's with "EFC-ectomies" used as references
in phase noise and AD test systems.

An indication of the level of scrutiny during
the 10811 design, I submit 2 examples:  there
is a 10 Meg resistor across the crystal that
supposedly prevents DC charge from building
up in the crystal due to cosmic ray hits
There was an extensive witch hunt that found
ultimately that lubricating oil in the piston
trimmer migrated around and caused "aging".
I'm not sure what the fix was other than not
lubricating the cap.  Maybe a different type
of oil.


On 8/3/2016 7:11 AM, Charles Steinmetz wrote:
> One further point regarding noise from the EFC voltage: The varactor in
> the oscillator will necessarily have a rather high resistance in series
> with it, which adds a certain amount of unavoidable Johnson noise. Also,
> the "other end" of the varactor is not generally grounded -- rather, it
> is connected (through a high-ish resistance) to an internal reference
> voltage, which has its own noise.
> Taking the HP 10811 as an example, one end of the varactor is connected
> to an internal +6.4v reference through 100k ohms, and the other is
> connected to the external EFC voltage, also through 100k ohms.  The
> resistors alone set a noise density floor of about 90nV/sqrtHz at the
> oven temperature, while even a "low noise" 6.4V zener diode operated at
> 1mA has a noise density in the low thousands of nV/sqrtHz (in this case,
> filtered by 11k ohms and 6.8uF).  The filter has a LP characteristic
> with a corner frequency of ~2Hz, but (1) the noise below that frequency
> is still a very real concern for phase noise, and (2) it's only a
> one-pole filter, so the 10Hz noise is still in the 1000nV/sqrtHz range.
> The point of all this?  The external EFC voltage doesn't have to be
> heroically quiet to remove it as a practical source of phase noise.
> That said, be careful about radiated fields and poor PSRR inducing or
> coupling voltages onto the EFC line -- such interference can be
> thousands of times larger than the Johnson noise, and can cause spurs on
> the oscillator output.
> Best regards,
> Charles
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