Hi
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…. Bob > On Aug 3, 2016, at 10:11 AM, Charles Steinmetz <[email protected]> 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 > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
