Thanks for the info but I have not forgotten any of what you said. I do not believe I ever suggested using a noiseless oscillator as I also am not aware of such a thing, and if it existed this group would not exist either. I have been making phase noise measurements and trying to achieve very low phase noise at millimeter waves requiring the use of ultra clean fundamental sources for over 20 years. I have had the best minds on this matter working for me or with me on this issue. An audio or a 10 MHz spectrum analyzer today is probably one and the same and the noise floor at DC or at 10 MHz is also the same. Yes, at DC you have to contend with the DC component just like at 10 MHz you have to contend with the 10 MHz carrier component. I keep on saying that the only way to really do it is to multiply your source up to a reasonable frequency like 1 GHz which would raise the phase noise by 40 dB allowing you to make the measurement at DC using the regular phase shift and mixing scheme. - Mike
Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 -----Original Message----- From: Jeffrey Pawlan [mailto:[EMAIL PROTECTED] Sent: Wednesday, April 27, 2005 12:06 PM To: [email protected] Cc: Mike Feher; David Kirkby Subject: Re: [time-nuts] Upper limit on phase noise from two oscillators. On Wed, 27 Apr 2005, David Kirkby wrote: >if one oscillator was perfect (no phase noise at all) also wrote: > If one oscillator was noiseless If you or any one on this list has a lead about where to find a noiseless oscillator, PLEASE let me know immediately. I want one! On Wed, 27 Apr 2005, Mike Feher wrote: > This does not seem to make sense. No matter what you are measuring it is > assumed that the phase noise of the item being measured is greater than the > piece of test equipment doing the measurement, else you would not see it. That is the correct description for using a Spectrum analyser to measure phase noise. I have the HP phase noise measurement utility built into my spectrum analyser and as long as the signal being measured is at least 10dB worse than the noise floor of the spectrum analyser noise floor, the readings are reasonably accurate. But there is no spectrum analyser made that is good enough to measure a precision oscillator like the 10811. Therefore the measurement is done by mixing two of them together and one must phaselock one oscillator to the other and drive the mixer ports at 90 degrees from each other. The resulting output is zero frequency (DC) and an audio spectrum analyzer reads the sidebands which is the summed phase noise of the two. Mike, you forgot that the audio spectrum analyzer does not need to have the dynamic range of the phase noise unless you are measuring quite close to the carrier simultaneously with measuring far from the carrier. If you add high pass filters you can make your measurements in segments so that a modest audio spectrum analyser will work. You may have also forgotten that the carrier becomes a DC voltage since the two oscillators are phase locked to each other. You must use a blocking cap (minimal HPF) to not overload the inout of the soundcard if it is DC coupled. I hope this helps but I need to get back to work now. 73, Jeffrey Pawlan _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
