One further point regarding noise from the EFC voltage: The varactor in
the oscillator will necessarily have a rather high resistance in series
with it, which adds a certain amount of unavoidable Johnson noise.
Also, the "other end" of the varactor is not generally grounded --
rather, it is connected (through a high-ish resistance) to an internal
reference voltage, which has its own noise.
Taking the HP 10811 as an example, one end of the varactor is connected
to an internal +6.4v reference through 100k ohms, and the other is
connected to the external EFC voltage, also through 100k ohms. The
resistors alone set a noise density floor of about 90nV/sqrtHz at the
oven temperature, while even a "low noise" 6.4V zener diode operated at
1mA has a noise density in the low thousands of nV/sqrtHz (in this case,
filtered by 11k ohms and 6.8uF). The filter has a LP characteristic
with a corner frequency of ~2Hz, but (1) the noise below that frequency
is still a very real concern for phase noise, and (2) it's only a
one-pole filter, so the 10Hz noise is still in the 1000nV/sqrtHz range.
The point of all this? The external EFC voltage doesn't have to be
heroically quiet to remove it as a practical source of phase noise.
That said, be careful about radiated fields and poor PSRR inducing or
coupling voltages onto the EFC line -- such interference can be
thousands of times larger than the Johnson noise, and can cause spurs on
the oscillator output.
Best regards,
Charles
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