Stephan Sandenbergh wrote: > Thanks, > > I'm familiar with the designs you posted to measure voltage noise ect. on > you home page. These, with some modification, mainly removing the blocking > caps, seems that it would do the trick. > > Cheers, > > Stephan. > > On 10 March 2011 10:50, Bruce Griffiths<bruce.griffiths@xtra.co.nz> wrote: > > >> For conventional phase noise measurements at offsets in the (10Hz, 20kHz) >> range one can use a sound card with a low noise preaamp. >> Suitable sound card preamps with lower noise floors than Enrico's or >> Wenzel's designs can be built using readily available components. >> Wider bandwidths ( up to 1MHz or so) are not difficult to achieve. >> >> Bruce >> >> >> Stephan Sandenbergh wrote: >> >> >>> Hi, >>> >>> Cross-correlation a very clever idea! Thanks for the reference - Rubiola >>> got >>> some good sources of reference on his home page. >>> >>> One thing though - for a phase-noise kit one will probably need to replace >>> the ZCD with a low-noise amplification stage of around 80dB to be to allow >>> sampling at ADC voltage levels? >>> >>> Cheers, >>> >>> Stephan. >>> >>> On 8 March 2011 22:28, Magnus Danielson<magnus@rubidium.dyndns.org> >>> wrote: >>> >>> >>> >>> >>>> On 03/08/2011 07:46 PM, Stephan Sandenbergh wrote: >>>> >>>> >>>> >>>> >>>>> Hi, >>>>> >>>>> I recently noticed something interesting: The DMTD measurement gives a >>>>> set >>>>> of phase values x(t). From which fractional frequency y(t) is >>>>> calculable. >>>>> So >>>>> now it seems viable to plot the spectrum, Sy(f) and if you scale it >>>>> properly >>>>> you arrive at Sphi(f). If I'm not making a gross error somewhere the >>>>> math >>>>> seems to check out. But, I'm wondering is there a physical reason why >>>>> this >>>>> isn't valid? >>>>> >>>>> I have not seen this being done anywhere - so I assume there is. >>>>> However, >>>>> it >>>>> seems possible to plot Sphi(f) for 1Hz< f<100kHz when having a vbeat = >>>>> 100kHz sampled for 1 second. >>>>> >>>>> I'm familiar with the loose and tight phase-locked methods of measuring >>>>> phase noise, but am quite curious to know if phase noise from a DMTD >>>>> measurement is a valid assumption. >>>>> >>>>> I would guess that if the frequency domain phase noise measurement >>>>> requires >>>>> phase-lock then the time-domain measurement requires as well. However, >>>>> here >>>>> in lies my real interest - two GPSDOs are phase-locked (not to 1Hz, >>>>> something far less I know) so can it be possible to measure GPSDO Adev >>>>> and >>>>> phase-noise using a single DMTD run? Am I making a wrong assumption >>>>> somewhere? >>>>> >>>>> >>>>> >>>>> >>>> An architecture not completely different to the DMTD architecture is used >>>> in phase-noise kits. Instead of having two sources and one intermediary >>>> oscillator is instead there one source and two intermediary oscillators. >>>> The >>>> oscillators is locked to the carrier frequency rather than an offset. The >>>> mixed down signal is then cross-correlated to get the spectrum. >>>> Increasing >>>> the averaging factor and the spectrum can be suppressed below that of the >>>> intermediary oscillators. Since the two intermediary oscillators have >>>> uncorrelated noise, the external noise is what correlates over time. This >>>> technique is simply called cross-correlation. Such a cross-correlation >>>> setup >>>> can run very close to the carrier in terms of offsets. >>>> >>>> In contrast will a DMTD with it's offset frequency be problematic at low >>>> offsets since the positive and negative offsets noise will not occur at >>>> the >>>> same frequency in a DMTD setup. Consider a a DMTD with a 10 Hz offset, >>>> pointing a spectrum analyzer on 100 Hz will measure the down-converted >>>> average of carrier+(100-10) Hz and carrier-(100+10) Hz, thus carrier+90 >>>> Hz >>>> and carrier-110 Hz. >>>> >>>> Creating a mixed-mode setup for phase-noise/DMTD will however be >>>> possible. >>>> >>>> So, DMTD as such is relatively limited, but add an RF switch and another >>>> oscillator and you get a cross-correlation phase-noise kit. >>>> >>>> To turbo-charge the phase-noise kit use a quadrature combiner and >>>> amplitude >>>> adjustment to create a interferometric mixdown, working around part of >>>> the >>>> mixer limitations. Enrico Rubiola has writen about this approach. >>>> >>>> Cheers, >>>> Magnus >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>>> >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >>> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > >

Without nulling the carrier the dynamic range of your ADC will limit the measurement system phase noise fllor. With a 300KHz beat frequency the post mixer preamp response need not extend to dc. Bruce Stephan Sandenbergh wrote: > Thanks, > > I'm familiar with the designs you posted to measure voltage noise ect. on > you home page. These, with some modification, mainly removing the blocking > caps, seems that it would do the trick. > > Cheers, > > Stephan. > > On 10 March 2011 10:50, Bruce Griffiths<bruce.griffiths@xtra.co.nz> wrote: > > >> For conventional phase noise measurements at offsets in the (10Hz, 20kHz) >> range one can use a sound card with a low noise preaamp. >> Suitable sound card preamps with lower noise floors than Enrico's or >> Wenzel's designs can be built using readily available components. >> Wider bandwidths ( up to 1MHz or so) are not difficult to achieve. >> >> Bruce >> >> >> Stephan Sandenbergh wrote: >> >> >>> Hi, >>> >>> Cross-correlation a very clever idea! Thanks for the reference - Rubiola >>> got >>> some good sources of reference on his home page. >>> >>> One thing though - for a phase-noise kit one will probably need to replace >>> the ZCD with a low-noise amplification stage of around 80dB to be to allow >>> sampling at ADC voltage levels? >>> >>> Cheers, >>> >>> Stephan. >>> >>> On 8 March 2011 22:28, Magnus Danielson<magnus@rubidium.dyndns.org> >>> wrote: >>> >>> >>> >>> >>>> On 03/08/2011 07:46 PM, Stephan Sandenbergh wrote: >>>> >>>> >>>> >>>> >>>>> Hi, >>>>> >>>>> I recently noticed something interesting: The DMTD measurement gives a >>>>> set >>>>> of phase values x(t). From which fractional frequency y(t) is >>>>> calculable. >>>>> So >>>>> now it seems viable to plot the spectrum, Sy(f) and if you scale it >>>>> properly >>>>> you arrive at Sphi(f). If I'm not making a gross error somewhere the >>>>> math >>>>> seems to check out. But, I'm wondering is there a physical reason why >>>>> this >>>>> isn't valid? >>>>> >>>>> I have not seen this being done anywhere - so I assume there is. >>>>> However, >>>>> it >>>>> seems possible to plot Sphi(f) for 1Hz< f<100kHz when having a vbeat = >>>>> 100kHz sampled for 1 second. >>>>> >>>>> I'm familiar with the loose and tight phase-locked methods of measuring >>>>> phase noise, but am quite curious to know if phase noise from a DMTD >>>>> measurement is a valid assumption. >>>>> >>>>> I would guess that if the frequency domain phase noise measurement >>>>> requires >>>>> phase-lock then the time-domain measurement requires as well. However, >>>>> here >>>>> in lies my real interest - two GPSDOs are phase-locked (not to 1Hz, >>>>> something far less I know) so can it be possible to measure GPSDO Adev >>>>> and >>>>> phase-noise using a single DMTD run? Am I making a wrong assumption >>>>> somewhere? >>>>> >>>>> >>>>> >>>>> >>>> An architecture not completely different to the DMTD architecture is used >>>> in phase-noise kits. Instead of having two sources and one intermediary >>>> oscillator is instead there one source and two intermediary oscillators. >>>> The >>>> oscillators is locked to the carrier frequency rather than an offset. The >>>> mixed down signal is then cross-correlated to get the spectrum. >>>> Increasing >>>> the averaging factor and the spectrum can be suppressed below that of the >>>> intermediary oscillators. Since the two intermediary oscillators have >>>> uncorrelated noise, the external noise is what correlates over time. This >>>> technique is simply called cross-correlation. Such a cross-correlation >>>> setup >>>> can run very close to the carrier in terms of offsets. >>>> >>>> In contrast will a DMTD with it's offset frequency be problematic at low >>>> offsets since the positive and negative offsets noise will not occur at >>>> the >>>> same frequency in a DMTD setup. Consider a a DMTD with a 10 Hz offset, >>>> pointing a spectrum analyzer on 100 Hz will measure the down-converted >>>> average of carrier+(100-10) Hz and carrier-(100+10) Hz, thus carrier+90 >>>> Hz >>>> and carrier-110 Hz. >>>> >>>> Creating a mixed-mode setup for phase-noise/DMTD will however be >>>> possible. >>>> >>>> So, DMTD as such is relatively limited, but add an RF switch and another >>>> oscillator and you get a cross-correlation phase-noise kit. >>>> >>>> To turbo-charge the phase-noise kit use a quadrature combiner and >>>> amplitude >>>> adjustment to create a interferometric mixdown, working around part of >>>> the >>>> mixer limitations. Enrico Rubiola has writen about this approach. >>>> >>>> Cheers, >>>> Magnus >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>>> >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >>> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > >

S/N drops spectacular! Any probs to cut the noise-floor here? The text looks like art. - Henry -- ehydra.dyndns.info Bruce Griffiths schrieb: > Stephan Sandenbergh wrote: >> Thanks, >> >> I'm familiar with the designs you posted to measure voltage noise ect. on >> you home page. These, with some modification, mainly removing the >> blocking >> caps, seems that it would do the trick. >> >> Cheers, >> >> Stephan. >> >> On 10 March 2011 10:50, Bruce Griffiths<bruce.griffiths@xtra.co.nz> >> wrote: >> >> >>> For conventional phase noise measurements at offsets in the (10Hz, >>> 20kHz) >>> range one can use a sound card with a low noise preaamp. >>> Suitable sound card preamps with lower noise floors than Enrico's or >>> Wenzel's designs can be built using readily available components. >>> Wider bandwidths ( up to 1MHz or so) are not difficult to achieve. >>> >>> Bruce >>> >>> >>> Stephan Sandenbergh wrote: >>> >>> >>>> Hi, >>>> >>>> Cross-correlation a very clever idea! Thanks for the reference - >>>> Rubiola >>>> got >>>> some good sources of reference on his home page. >>>> >>>> One thing though - for a phase-noise kit one will probably need to >>>> replace >>>> the ZCD with a low-noise amplification stage of around 80dB to be to >>>> allow >>>> sampling at ADC voltage levels? >>>> >>>> Cheers, >>>> >>>> Stephan. >>>> >>>> On 8 March 2011 22:28, Magnus Danielson<magnus@rubidium.dyndns.org> >>>> wrote: >>>> >>>> >>>> >>>> >>>>> On 03/08/2011 07:46 PM, Stephan Sandenbergh wrote: >>>>> >>>>> >>>>> >>>>> >>>>>> Hi, >>>>>> >>>>>> I recently noticed something interesting: The DMTD measurement >>>>>> gives a >>>>>> set >>>>>> of phase values x(t). From which fractional frequency y(t) is >>>>>> calculable. >>>>>> So >>>>>> now it seems viable to plot the spectrum, Sy(f) and if you scale it >>>>>> properly >>>>>> you arrive at Sphi(f). If I'm not making a gross error somewhere the >>>>>> math >>>>>> seems to check out. But, I'm wondering is there a physical reason why >>>>>> this >>>>>> isn't valid? >>>>>> >>>>>> I have not seen this being done anywhere - so I assume there is. >>>>>> However, >>>>>> it >>>>>> seems possible to plot Sphi(f) for 1Hz< f<100kHz when having a >>>>>> vbeat = >>>>>> 100kHz sampled for 1 second. >>>>>> >>>>>> I'm familiar with the loose and tight phase-locked methods of >>>>>> measuring >>>>>> phase noise, but am quite curious to know if phase noise from a DMTD >>>>>> measurement is a valid assumption. >>>>>> >>>>>> I would guess that if the frequency domain phase noise measurement >>>>>> requires >>>>>> phase-lock then the time-domain measurement requires as well. >>>>>> However, >>>>>> here >>>>>> in lies my real interest - two GPSDOs are phase-locked (not to 1Hz, >>>>>> something far less I know) so can it be possible to measure GPSDO >>>>>> Adev >>>>>> and >>>>>> phase-noise using a single DMTD run? Am I making a wrong assumption >>>>>> somewhere? >>>>>> >>>>>> >>>>>> >>>>>> >>>>> An architecture not completely different to the DMTD architecture >>>>> is used >>>>> in phase-noise kits. Instead of having two sources and one >>>>> intermediary >>>>> oscillator is instead there one source and two intermediary >>>>> oscillators. >>>>> The >>>>> oscillators is locked to the carrier frequency rather than an >>>>> offset. The >>>>> mixed down signal is then cross-correlated to get the spectrum. >>>>> Increasing >>>>> the averaging factor and the spectrum can be suppressed below that >>>>> of the >>>>> intermediary oscillators. Since the two intermediary oscillators have >>>>> uncorrelated noise, the external noise is what correlates over >>>>> time. This >>>>> technique is simply called cross-correlation. Such a cross-correlation >>>>> setup >>>>> can run very close to the carrier in terms of offsets. >>>>> >>>>> In contrast will a DMTD with it's offset frequency be problematic >>>>> at low >>>>> offsets since the positive and negative offsets noise will not >>>>> occur at >>>>> the >>>>> same frequency in a DMTD setup. Consider a a DMTD with a 10 Hz offset, >>>>> pointing a spectrum analyzer on 100 Hz will measure the down-converted >>>>> average of carrier+(100-10) Hz and carrier-(100+10) Hz, thus >>>>> carrier+90 >>>>> Hz >>>>> and carrier-110 Hz. >>>>> >>>>> Creating a mixed-mode setup for phase-noise/DMTD will however be >>>>> possible. >>>>> >>>>> So, DMTD as such is relatively limited, but add an RF switch and >>>>> another >>>>> oscillator and you get a cross-correlation phase-noise kit. >>>>> >>>>> To turbo-charge the phase-noise kit use a quadrature combiner and >>>>> amplitude >>>>> adjustment to create a interferometric mixdown, working around part of >>>>> the >>>>> mixer limitations. Enrico Rubiola has writen about this approach. >>>>> >>>>> Cheers, >>>>> Magnus >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>>> >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>>> >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

Hi, Ok some cool advice - this thread is an interesting thought exercise. I'm going to think about it a some more, but it seems, in comparison at least, the loose phase-lock technique remains the simplest. Provided you have a low-frequency spectrum analyser handy. The sound card idea is clever as well - however, I'd assume one needs to measure the ADCs clocking oscillator offset, since that will be apparent when plotting the beat frequency phase (what I mean is that sampling will then look like another mixing process). What I usually due is to clock the sampling system off a clock that's correlated to the clock under test. This resolves that issue. However, I'd like to experiment with the cross-correlation idea, since I've got a setup that will lend itself perfect to that. Maybe I could save myself some time, with clever post-processing. Can anyone recommend a fundamental text on the cross-correlation technique? Regards, Stephan. On 10 March 2011 20:53, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote: > Without nulling the carrier the dynamic range of your ADC will limit the > measurement system phase noise fllor. > > With a 300KHz beat frequency the post mixer preamp response need not extend > to dc. > > Bruce > > > Stephan Sandenbergh wrote: > >> Thanks, >> >> I'm familiar with the designs you posted to measure voltage noise ect. on >> you home page. These, with some modification, mainly removing the blocking >> caps, seems that it would do the trick. >> >> Cheers, >> >> Stephan. >> >> On 10 March 2011 10:50, Bruce Griffiths<bruce.griffiths@xtra.co.nz> >> wrote: >> >> >> >>> For conventional phase noise measurements at offsets in the (10Hz, 20kHz) >>> range one can use a sound card with a low noise preaamp. >>> Suitable sound card preamps with lower noise floors than Enrico's or >>> Wenzel's designs can be built using readily available components. >>> Wider bandwidths ( up to 1MHz or so) are not difficult to achieve. >>> >>> Bruce >>> >>> >>> Stephan Sandenbergh wrote: >>> >>> >>> >>>> Hi, >>>> >>>> Cross-correlation a very clever idea! Thanks for the reference - Rubiola >>>> got >>>> some good sources of reference on his home page. >>>> >>>> One thing though - for a phase-noise kit one will probably need to >>>> replace >>>> the ZCD with a low-noise amplification stage of around 80dB to be to >>>> allow >>>> sampling at ADC voltage levels? >>>> >>>> Cheers, >>>> >>>> Stephan. >>>> >>>> On 8 March 2011 22:28, Magnus Danielson<magnus@rubidium.dyndns.org> >>>> wrote: >>>> >>>> >>>> >>>> >>>> >>>>> On 03/08/2011 07:46 PM, Stephan Sandenbergh wrote: >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> Hi, >>>>>> >>>>>> I recently noticed something interesting: The DMTD measurement gives a >>>>>> set >>>>>> of phase values x(t). From which fractional frequency y(t) is >>>>>> calculable. >>>>>> So >>>>>> now it seems viable to plot the spectrum, Sy(f) and if you scale it >>>>>> properly >>>>>> you arrive at Sphi(f). If I'm not making a gross error somewhere the >>>>>> math >>>>>> seems to check out. But, I'm wondering is there a physical reason why >>>>>> this >>>>>> isn't valid? >>>>>> >>>>>> I have not seen this being done anywhere - so I assume there is. >>>>>> However, >>>>>> it >>>>>> seems possible to plot Sphi(f) for 1Hz< f<100kHz when having a >>>>>> vbeat = >>>>>> 100kHz sampled for 1 second. >>>>>> >>>>>> I'm familiar with the loose and tight phase-locked methods of >>>>>> measuring >>>>>> phase noise, but am quite curious to know if phase noise from a DMTD >>>>>> measurement is a valid assumption. >>>>>> >>>>>> I would guess that if the frequency domain phase noise measurement >>>>>> requires >>>>>> phase-lock then the time-domain measurement requires as well. However, >>>>>> here >>>>>> in lies my real interest - two GPSDOs are phase-locked (not to 1Hz, >>>>>> something far less I know) so can it be possible to measure GPSDO Adev >>>>>> and >>>>>> phase-noise using a single DMTD run? Am I making a wrong assumption >>>>>> somewhere? >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>> An architecture not completely different to the DMTD architecture is >>>>> used >>>>> in phase-noise kits. Instead of having two sources and one intermediary >>>>> oscillator is instead there one source and two intermediary >>>>> oscillators. >>>>> The >>>>> oscillators is locked to the carrier frequency rather than an offset. >>>>> The >>>>> mixed down signal is then cross-correlated to get the spectrum. >>>>> Increasing >>>>> the averaging factor and the spectrum can be suppressed below that of >>>>> the >>>>> intermediary oscillators. Since the two intermediary oscillators have >>>>> uncorrelated noise, the external noise is what correlates over time. >>>>> This >>>>> technique is simply called cross-correlation. Such a cross-correlation >>>>> setup >>>>> can run very close to the carrier in terms of offsets. >>>>> >>>>> In contrast will a DMTD with it's offset frequency be problematic at >>>>> low >>>>> offsets since the positive and negative offsets noise will not occur at >>>>> the >>>>> same frequency in a DMTD setup. Consider a a DMTD with a 10 Hz offset, >>>>> pointing a spectrum analyzer on 100 Hz will measure the down-converted >>>>> average of carrier+(100-10) Hz and carrier-(100+10) Hz, thus carrier+90 >>>>> Hz >>>>> and carrier-110 Hz. >>>>> >>>>> Creating a mixed-mode setup for phase-noise/DMTD will however be >>>>> possible. >>>>> >>>>> So, DMTD as such is relatively limited, but add an RF switch and >>>>> another >>>>> oscillator and you get a cross-correlation phase-noise kit. >>>>> >>>>> To turbo-charge the phase-noise kit use a quadrature combiner and >>>>> amplitude >>>>> adjustment to create a interferometric mixdown, working around part of >>>>> the >>>>> mixer limitations. Enrico Rubiola has writen about this approach. >>>>> >>>>> Cheers, >>>>> Magnus >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>>> >>>>> >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>>> >>>> >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

> Ok some cool advice - this thread is an interesting thought exercise. I'm > going to think about it a some more, but it seems, in comparison at least, > the loose phase-lock technique remains the simplest. Provided you have a > low-frequency spectrum analyser handy. It would be a better idea from the standpoint of VCO modulation bandwidth as well. If you measure PN by looking at the tuning voltage in the tight-PLL configuration, you'll probably be limited to offsets of a few kHz before the response rolls off. > The sound card idea is clever as well - however, I'd assume one needs to > measure the ADCs clocking oscillator offset, since that will be apparent > when plotting the beat frequency phase (what I mean is that sampling will > then look like another mixing process). What I usually due is to > clock the > sampling system off a clock that's correlated to the clock under > test. This > resolves that issue. Sound cards will usually end up running within 1 Hz of the desired sampling rate, but it's important to pick a sampling rate that's native to the hardware, or the driver will resample the data. On Windows, many drivers for popular sound cards rely on some imprecise resampling code that apparently was distributed by Microsoft in the DDK. Stick with 44100 or 48000 Hz, or you'll be lucky to land within a few dozen Hz in some cases. Warren's been getting some really nice ADEV plots from a tight PLL sampled with a USB sound card, running a quick and dirty command-line utility I put together to acquire the data and downsample it. I'll post the next build on my web page if anyone else is interested in playing with it. > However, I'd like to experiment with the cross-correlation idea, > since I've > got a setup that will lend itself perfect to that. Maybe I could > save myself > some time, with clever post-processing. > > Can anyone recommend a fundamental text on the cross-correlation > technique? The easy way out is to look for a dual-channel FFT analyzer or one of its successors. The HP 3562A and 3563A models are pretty affordable these days, and they've been used in a number of papers on cross-correlation measurements of various types of noise. SRS also sells some nice multichannel analyzers. If you're looking to write your own processing code, you should search for information on 'cross spectrum' as well as 'cross correlation' techniques, because the former is the correct term for the scenario where the sampled data from both channels is already time-aligned. For its part, the cross spectrum is just a $5 mathematical buzzword for the vector length between the corresponding output bins of two FFTs, obtained by multiplying one array by the complex conjugate of the other. When averaged over time, the real component of the cross spectrum will converge to the common signal at the ADC inputs. Unlike the DUT signal, the ADCs' noise contribution is randomly distributed in phase space. It will converge to zero when averaged and fall out of the measurement... at least to the extent that the channels are truly uncorrelated. Unwanted channel correlation is one of several reasons to use a lab-grade FFT analyzer instead of a sound card for multichannel measurements. At a minimum it would be better to use two USB sound cards and run them from a common clock. Enrico Rubiola's "The cross-spectrum experimental method" is a good survey of the basic principles (http://arxiv.org/abs/1003.0113); also see http://tycho.usno.navy.mil/ptti/ptti2001/paper42.pdf and Walls's original "Cross-correlation phase noise measurements" paper, as well as the various white papers at aglient.com and symmetricom.com. Rubiola's material is the most helpful I've found, really. I've been doing a lot of R&D in this area lately, and I've found that most DSP textbooks are too far removed from real applications (no pun intended) to be of much use. The math is not rocket surgery, but you couldn't tell that from a survey of the academic literature. -- john, KE5FX

On 03/10/2011 09:42 AM, Stephan Sandenbergh wrote: > Hi, > > Cross-correlation a very clever idea! Thanks for the reference - Rubiola got > some good sources of reference on his home page. > > One thing though - for a phase-noise kit one will probably need to replace > the ZCD with a low-noise amplification stage of around 80dB to be to allow > sampling at ADC voltage levels? Yes. You can tap in just after first level amplification but prior to any clipping if you want to use a shared setup. Cheers, Magnus

On 03/11/2011 09:33 PM, Stephan Sandenbergh wrote: > Hi, > > Ok some cool advice - this thread is an interesting thought exercise. I'm > going to think about it a some more, but it seems, in comparison at least, > the loose phase-lock technique remains the simplest. Provided you have a > low-frequency spectrum analyser handy. > > The sound card idea is clever as well - however, I'd assume one needs to > measure the ADCs clocking oscillator offset, since that will be apparent > when plotting the beat frequency phase (what I mean is that sampling will > then look like another mixing process). What I usually due is to clock the > sampling system off a clock that's correlated to the clock under test. This > resolves that issue. > > However, I'd like to experiment with the cross-correlation idea, since I've > got a setup that will lend itself perfect to that. Maybe I could save myself > some time, with clever post-processing. > > Can anyone recommend a fundamental text on the cross-correlation technique? Cross-correlation processing doing the hard way would involve N^2 multiplications but you can process it. However, this is now done using FFT so what you do is... Grab the two signals x(t) and y(t) from the two arms of the cross-correlation receiver... (you want them to be sampled at the same time) You then FFT transform them into their frequency variants X(f) and Y(f) To get the cross-correlation spectrum you now do (for all f) C(f) = X(f) * Y(f)* The Y(f)* is the Y(f) with the imaginary term inverted. If you have use for the cross-correlation time-series (usually not needed for phase-noise) then using an IFFT to transform C(f) into c(t) can be done. This processing takes a few tens of lines C-code using FFTW as a FFT library (recommended). You can then average a number of these cross-correlation spectrums to further suppress noise. Remember that you will need to scale the result for proper dBC/sqrt(Hz) which is connected to the carrier strength, sample rate, and FFT length. Cheers, Magnus

Hi, I have a question for you guys. I am currently fixing a 5370B and when I check the internal noise I got 23.2pS I think it is a bit too much. Could you check yours noise? 1) Connect a cable between the rear panel FREQ STD OUT to front panel START input connector 2) Set the input impedances at 50 ohms 3) Attenuator switches at /1 4) AC/DC switches to DC 5) Input slope switches to positive (high) 6) Level controls to preset 7) START COM 8) display rate fully clockwise 9) SAMPLE SIZE 10K 10) STATISTICS STD DEV I got 23.2ps with 2ps unstability. Is that OK? Thanks for your help. Bye, Jean-Louis

John Miles schrieb: > Sound cards will usually end up running within 1 Hz of the desired sampling > rate, but it's important to pick a sampling rate that's native to the > hardware, or the driver will resample the data. On Windows, many drivers > for popular sound cards rely on some imprecise resampling code that > apparently was distributed by Microsoft in the DDK. Stick with 44100 or > 48000 Hz, or you'll be lucky to land within a few dozen Hz in some cases. > One can this circumvent by using ASIO driver or a wrapper like asio4all. This removed the frequency resampling distortion and at the same time the low-pass filtering above 20KHz for my sound-card. > Warren's been getting some really nice ADEV plots from a tight PLL sampled > with a USB sound card, running a quick and dirty command-line utility I put > together to acquire the data and downsample it. I'll post the next build on > my web page if anyone else is interested in playing with it. > Where? - Henry -- ehydra.dyndns.info

At 05:26 AM 3/12/2011, Jean-Louis Noel wrote... >I am currently fixing a 5370B and when I check the internal noise I >got >23.2pS I think it is a bit too much. > >Could you check yours noise? The manual says to expect jitter of 35 ps typical, 100 max. One of mine is ~30, the other ~20.