[time-nuts] noisy varactor diodes

[John Miles]

> The HP sceme in the patent refers to an array of series and parallel
> diodes to reduce noise.
> 
> Just paralleling them does not reduce the noise!

Still, the patent claims seem pretty obvious.  Paralleling n diodes would
reduce their noise contribution by the usual 3 dB * log2(n) factor, but
would also multiply their max/min capacitance ratings by n... which in turn
can be restored to the original value by putting n blocks in series.  Yes, I
can totally see where HP is entitled to a government-enforced monopoly on
this valuable intellectual property.  We wouldn't want to discourage
"innovation," would we? :-P

In practice, low-noise synthesizers made with bulk LC components often use
banks of parallel varactors, but they maintain the necessary tuning range by
switching between multiple inductors.  I'm sure that particular stroke of
genius was patented by someone as well...
 
> The 4 paralleled in the 5065  are to give enough phase deviation for the
> low modulation level applied.  (150mv)
> 
> The symptom was a fluctuating 2nd harmonic meter reading and
> coresponding
> fluctuations at the A7 TP2 test point.

I see.  That symptom isn't one that I've been observing.

> John I could not see the info you refered to?

Here's a direct link to the attachment:
http://www.ke5fx.com/5065a.png

-- john
Miles Design LLC