[time-nuts] Fury Interface Board: How about TI OPA277?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Nov 2 20:25:18 EDT 2007

John Miles wrote:
>>> The Achilles' heel of this technique is the statistical temperature
>>> fluctuations experienced by individual resistors.
>>> ...
> Seems debatable whether it's worth worrying about details like this when the
> EFC signal is just going to go straight into a 20K-100K resistor in the
> OCXO, isn't it?
> I just use a simple resistor network to drop the bipolar DC EFC control
> voltage from my 11729C to the level needed by the 10811A in my 8662A, and I
> have no problem making measurements at the 10811A's specified noise floor,
> at least from 100 Hz and beyond.  I suppose 1/f and popcorn noise is still a
> concern, but surely that's dominated by the relatively-large resistance
> between the EFC control loop and the varactor.  No?
> -- john, KE5FX
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In the 10811A one of the 100K resistors and the varicap capacitance
(100pF) act as a low pass filter with a 16kHz cutoff (the 0.1uF cap and
the other 100K resistor form a low pass filter with a 16Hz cutoff
frequency) as far as the noise voltage across the varicap is concerned
so there is no need to go overboard in filtering the EFC voltage as the
(SQRT(kT/C)) voltage noise due to the internal RC filter is around 6.5uV
rms across the varicap. This noise is independent of the actual resistor
value although the noise spectrum depends on it.

If one is only using a 10811A or a 10544A then an SSM2143 (or equivalent
configured as a 2X differential amplifier) plus another opamp an a 5V
reference will suffice to translate the Fury's 0-5V EFC signal to a -5V
to +5V signal with adequate stability and sufficiently low noise whilst
allowing remote sensing of the Fury EFC ground and the 10811A / 10544A
internal EFC ground nodes.


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