[time-nuts] Effect of EFC noise on phase noise
kb8tq at n1k.org
Mon Aug 1 20:08:53 EDT 2016
Ultimately the EFC signal gets to one or more varicap diodes. It likely goes
through a bias or attenuator network to get there. Playing with the resistors
in the network allows the manufacturer to produce parts with consistent EFC
The pinout of your standard OCXO and it’s single ground lies almost entirely
with the OEM customers who buy a lot of OCXO’s. To them a minimum number
of pins means a minimum amount of hassle. Given the choice between a multi
ground part and a single pin, the choice will alway go to the single pin. “We just hook them
all to the same point anyway … why the extra pins?”….. Presented wth an app note
and the explanation of why, the answer is still “we don’t have time to design that in”.
Yes it’s a scary world out there.
> On Aug 1, 2016, at 6:40 PM, David <davidwhess at gmail.com> wrote:
> 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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