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

Bob Camp kb8tq at n1k.org
Wed Aug 3 18:25:16 EDT 2016


Picking some random 10811-ish numbers:

10 MHz output
5V EFC range
1.6 ppm total EFC range
10 Hz offset from carrier

If you put in 300 nv of noise, in a 1Hz bandwidth, you get around -146 dbc of phase noise. Your
OCXO would be doing very well at 10 MHz to run -135 dbc phase noise at 10 Hz. The same
300 nv RMS gets you -166 at 100 Hz. That’s still pretty well below the expected noise floor. At
1 Hz, you are at -125 and still pretty well below the expected floor. 

I don’t think I’m going to win any “low noise” competitions with a voltage reference that has
a noise density of 0.3 uv / Hz. If your OCXO has less total swing, the noise voltage to generate
the stated phase noise goes up….


> On Aug 3, 2016, at 10:11 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 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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