[time-nuts] 5>10 doubler

Richard (Rick) Karlquist richard at karlquist.com
Fri Jan 30 11:57:53 EST 2015

On 1/29/2015 5:41 PM, Alexander Pummer wrote:
> And the narrow notch for the harmonic is not required anyway, since the
> fundamental is fare enough, therefore a high Q  LC trap will work
> better, also with the setting of the biasing af the active devices the
> Alex KJ6UHN

When I designed the 5071A RF chain, I used five cascaded frequency
doublers to go from 10 MHz to 320 MHz.  I definitely used traps
to reduce the 10, 30, and 40 MHz spurs (using 10->20 MHz as an
example).  It was no easy thing because I could only use coils of 
moderate Q (less than 50) and I needed at least 80 dB suppression.  You 
might wonder why I needed to reduce 40 MHz spurs in the 20 MHz
output.  It turns out (little known fact) that the if I drove
the 20->40 MHz doubler with 20 MHz contaminated with 40 MHz
harmonics, it would degrade the spectral purity of the 40 MHz
output.  Strange but true.

The 5071 filters are basically cascaded notch filters, as
opposed to band pass filters.  Doing this allowed me to
have zero adjustments.  Previous atomic clocks used narrow
high-Q filters that had to be tuned up, and were temperature
sensitive.  The production engineers had to constantly stay
on top of these filters because they were so temperamental.
OTOH, the 5071 filters just work.  There was never even a
production change to them AFAIK.  The key to getting the notch
filters to work was to use 2% components, and use two coils
and or two capacitors together to get around the fact that
the standard values are quantized to 10%.  Additionally, I
measured each tank circuit in situ on the PC board and tweaked
it to take into account the de facto board parasitics.

Rick Karlquist N6RK

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