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

[Charles Steinmetz]

Rick wrote:

> There are a couple of things here that don't add up,
> subject to remembering stuff from a LONG time ago

One source of noise I didn't mention is leakage current in the varactor 
itself.  They are actually pretty noisy devices because of this.  It's 
entirely possible that the varactor noise swamps all the other sources 
of noise.  If that is the case, my conclusion (EFC voltage doesn't need 
to be heroically clean) is unaffected.  Note that it is a known practice 
to bias EFC pins to one extreme, to minimize leakage current and 
hopefully reduce phase noise.

> 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.

Traditionally (1950s through mid-80s), the zener effect predominated in 
"zener diodes" of less than about 5.5v, and the (noisier) avalanch 
effect in "zener diodes" above that voltage.  More recently, there has 
been some slight juggling of the exact number for some newer series of 
diodes.

> 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

It's about 5.5v to 5.6v, so "compensated" zeners (like the 1N82x series) 
were about 6.2v.

> 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.

I don't think the improvement was ever anywhere near two orders of 
magnitude.  I'm not sure it was typically even one OOM.  But in any 
case, the noise figure I quoted is the maximum rated noise for one of 
those golden parts, not just some random ~6v zener diode.

Best regards,

Charles