Re: [time-nuts] Best phase detector / mixer for 100MHz?
Bill, One way to avoid the problem in the 1/f noise region is to avoid it. Mix down to a few kHz and analyze you signal This is a good point! Using my SW (www.hpw-works.com) simple set the x-scaling to a center frequency bandwidth of your choice with enough space in the lower region. This is implemented out of the box. It's like a measurement using the d-jitter but with a costume center frequency. See also the pictures on my homepage with zooming into the 0.005 Hz region using a large sample size. Required is that the source oscillators are stable... Hanspeter -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Bill Fuqua Sent: Donnerstag, 29. November 2012 18:02 To: time-nuts@febo.com Subject: Re: [time-nuts] Best phase detector / mixer for 100MHz? At 03:56 PM 11/29/2012 +, you wrote: By theory, yes... but we use a sound card with a lot of flicker noise on the lower end, also we have the 10...20Hz low freq. cutoff due the usage of a servo / single 5V power. Also the raising noise below 100Hz (ADC serve noise on the ADC power / input circuit) limits the performance. One way to avoid the problem in the 1/f noise region is to avoid it. Mix down to a few kHz and analyze you signal or if you must mix down to DC then chop it to bring it up to a kHz or so and analyze it using a program such as spectrum lab or in software to internally mix back down to DC using software digital mixer and then analyze. You can use a MOSFET analog switch to chop the signal. When you chop it you will have an carrier with upper and lower sidebands. You can simply take the FFT data and either shift the bins making the carrier (chopping frequency) the Zero or DC bin or even better yet add the upper and lower sidebands so that you get a 3db improvement in S/N. I will be db since the sidebands are identical mirror of each other, ie voltage doubles when they are added increasing the signal by 6db but noise introduced by electronics on upper and lower sidebands are not coherent and only increases by 3db. 73 Bill wa4lav ___ 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.
Re: [time-nuts] Best phase detector / mixer for 100MHz?
Hi Bruce, The equivalent phase noise (measured in dBc/Hz) is essentially independent of the sample size. The dBc/Hz normalization is based on this (also the sample rate). While the sample rate sample size = FFT Bin size (resolution or filter band width) is used to get the required correction factor of the spectrum to get the dBc / (1Hz) Y scaling back. So increasing the sample size isnt particularly useful for reducing the phase noise floor. By theory, yes... but we use a sound card with a lot of flicker noise on the lower end, also we have the 10...20Hz low freq. cutoff due the usage of a servo / single 5V power. Also the raising noise below 100Hz (ADC serve noise on the ADC power / input circuit) limits the performance. In my simple test increase of the sample size reduced the noise floor in better way than just using sample size with 1-2K and large averaging cycles. Keep in mind, Bin resolution is sample rate / sample size: Example: - 32khz / 32678 = about 1Hz - 32khz / 1024 = about 31 Hz However with a sound card plus a mixer a somewhat lower number of samples should suffice since there is no carrier. Ideally the input circuit ADC of the 3562A would be nice but with much larger RAM buffer and ASIO interface O:) Hanspeter -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Bruce Griffiths Sent: Dienstag, 27. November 2012 12:31 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Best phase detector / mixer for 100MHz? Support HpW-Works.com wrote: Bruce, There's no evidence of a cross power spectrum function in this suite. One needs to be able to average at least 10,000 cross power spectra for some applications. In the PSD (power density) PSP (power spectrum) there are cross power and cross power complex average implemented (selectable using the spectrum channel mixer)! Additional to this you may apply / add averaging of the resulting spectrum or use additional peak hold. Average of 10'000 cross points is a large count and often seen on 1-4k sample size. Better in my opinion is to use a higher sample size 32k-64k and then less averaging is required. The equivalent phase noise (measured in dBc/Hz) is essentially independent of the sample size. So increasing the sample size isnt particularly useful for reducing the phase noise floor. The increased frequency resolution achieved by increasing the sample size is only useful for measuring spurs. In the direct digital method of measuring phase noise a few terasamples (a few gigasamples at baseband) need to be processed to achieve a sufficiently low instrument noise floor. However with a sound card plus a mixer a somewhat lower number of samples should suffice since there is no carrier. Just download the evaluation version with fully feature set. HpW Bruce ___ 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.
Re: [time-nuts] Best phase detector / mixer for 100MHz?
Bruce, There's no evidence of a cross power spectrum function in this suite. One needs to be able to average at least 10,000 cross power spectra for some applications. In the PSD (power density) PSP (power spectrum) there are cross power and cross power complex average implemented (selectable using the spectrum channel mixer)! Additional to this you may apply / add averaging of the resulting spectrum or use additional peak hold. Average of 10'000 cross points is a large count and often seen on 1-4k sample size. Better in my opinion is to use a higher sample size 32k-64k and then less averaging is required. Just download the evaluation version with fully feature set. HpW ___ 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.
