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

[Charles Steinmetz]

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