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

David davidwhess at gmail.com
Mon Aug 1 18:40:44 EDT 2016

Are the EFC inputs all directly DC coupled to the varactor diodes
making them high DC impedance?

I always thought they should bring the varactor or EFC ground out as a
separate pin but I assume that since they do not, ground noise at
least within the oscillator does not limit performance.

In the past when I have had low frequency ground noise, I have either
used an instrumentation amplifier at the load or used a high impedance
current source with a load resistor to ground at the load.  But I have
difficulty imagining either being used to drive an EFC input because
close proximity of the driving circuit allows it to use the oscillator
ground as a single point ground.

I wonder if there is anything to be learned by studying how the old
varactor based (parametric) operational amplifiers were used.

On Mon, 1 Aug 2016 16:32:35 -0400, you wrote:

>If you wire up all the possible circuits and check them all out 
>answer is that big C / small R wins. Big R gets you into resistor noise issues
>and stray pickup. 
>> On Aug 1, 2016, at 4:16 PM, David <davidwhess at gmail.com> wrote:
>> This duplicates the problems encountered when trying to quantify low
>> frequency noise from a voltage reference; it is difficult to make an
>> low frequency high pass filter with lower noise than the lowest noise
>> references and the capacitor is the problem.
>> In Linear Technology Application Note 124, Jim Williams discusses the
>> problems with electrolytic capacitors for this type of application.  I
>> have read that you *can* get away with aluminum electrolytics if you
>> grade them for low leakage and low noise.  The dielectric absorption
>> is also a problem unless you can wait hours for best performance.
>> What about the alternative of buffering the signal with a low noise
>> low input bias current operational amplifier so that a large film
>> capacitor can be used instead?  Is the low frequency noise of a good
>> operational amplifier still too much?  What about a chopper stabilized
>> amplifier without suitable output filter?

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