[time-nuts] Current state of optical clocks and the definition of the second

Bob Camp kb8tq at n1k.org
Thu Jan 15 07:47:42 EST 2015


I guess the question becomes how low is low. 

If it’s a 50 ohm system 

If the power level is rational

If you are at room temperature 

There are some limits on how low low can be. 

You have a -174 dbm  / Hz thermal floor. AM or PM noise can only be 3db better than the thermal floor. At a power level of 1 watt, that’s a -204 dbc / Hz limit. You will spend some time correlating to that level. You also may need to play a bit with the input circuits to handle the 1W without damage. At a somewhat more common 100 mw, the limit is -194. People have been measuring phase noise in the > -190 dbc / Hz range for at least 20 years now. Correlation may take a week at some offsets. Time will be longer or shorter at other offsets. As with anything else, the more money (correlation channels)  you throw at the problem, the quicker it will go.  Numbers in the -180 vicinity with normal gear, offsets, and FFT windows are an overnight run sort of thing. 

> On Jan 14, 2015, at 9:47 PM, Mike Feher <mfeher at eozinc.com> wrote:
> Hi -
> I agree with what you stated, however, I am not sure that at real low levels they are actually discernible. Regards - Mike 
> Mike B. Feher, EOZ Inc.
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960 office
> 908-902-3831 cell
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob Camp
> Sent: Wednesday, January 14, 2015 6:14 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Current state of optical clocks and the definition of the second
> Hi
> More or less by definition:
> AM noise has the sidebands in phase, PM noise has the sidebands out of phase. PM adds to no envelope power, AM adds to the envelope power. If you have purely random noise, half of the power is AM, half is PM by this approach. If you have what is effectively a SDR (high speed ADC(s), decimators, cross correlation …) doing your phase noise measurement, figuring out sidebands and phase is part of the process. With an old style single mixer approach, you switch your operating point on the mixer.
> Bob
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