[time-nuts] Effect of EFC noise on phase noise
brooke at pacific.net
Wed Aug 3 15:11:47 EDT 2016
My first engineering job was working on Tunnel Diode amplifiers at microwave frequencies (when transistors only worked
at audio frequencies). The bias circuit was a 5.1 Volt Zener and a series diode just as your describe for temperature
stability. There was also a BALCO (+ temp coefficient) and VEECO (-temp coefficient) resistor for tweaking the
temperature response. Note that the operating point on a TD is around 1 volt, i.e. between the peak voltage and the
valley voltage. A pot is included to set the stage gain. http://www.prc68.com/I/Aertech.shtml#TDA
When I worked at HP in the Kobe Instrument Division (component test instruments) they had the 4352 VCO tester. It was
used in conjunction with one of the low phase noise frequency synthesizers that typically has a "two man lift" sticker
and the production line had small cranes to lift them. The programmable DC (EFC) supply was specially designed to have
a very low noise level. It's the same programmable supply that is an option on the 4395A. I sure would like to find
one of those to add to my 4395A.
The lesser of evils is still evil.
-------- Original Message --------
> 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,
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