I would like to see more in one place about implementing these ideas. The bits are scattered in different places and I'm not sure where to find them all.
One resource for getting the signals sampled at SOTA performance for a reasonable price is this demo board from TI: PCM4222EVM http://focus.ti.com/lit/ug/sbau124/sbau124.pdf $149 from TI https://estore.ti.com/Search.aspx?k=pcm4222evm It supports differential input at 192 KHz sampling with spdif/aes outputs. It has on board crystal oscillators from Pletronics. You can also run it from an external oscillator. I'm not sure how far from the standard frequencies you can go and still have the system get good data. You would need a sound card that has an SPDIF input that runs at 192 KHz. Some motherboards may be able to do this. The board is a very good implementation of their best audio frequency ADC. I don't think you could build it anywhere near the price of the demo board. You would need a sound card that has a native ASIO driver to keep Windows from mucking around with the signals ideally. I would not recommend a USB audio interface at this time. They are linked to the PC through some ugly mechanisms if accuracy is important (adaptive sample??). Better solutions are happening for USB but they are few and expensive for now. You can get PCI cards from ESI, M-audio, EMU that would do the job pretty well with drivers that work. For Linux I would suggest the ESI Juli@ since I know it works well in Linux. It may be possible to create a plug in for Audacity that will do the math and conversion to get the phase noise plots etc from the system. Audacity will do the job of collecting the data. http://audacity.sourceforge.net/ --Demian Date: Fri, 11 Mar 2011 14:21:55 -0800 From: "John Miles" <jmi...@pop.net> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Subject: Re: [time-nuts] Plot phase noise spectrum from DMTD measurement? Message-ID: <hmebkhfeacmnlmhiafdnoeaeaeah.jmi...@pop.net> Content-Type: text/plain; charset="us-ascii" > 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 ------------------------------ _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts End of time-nuts Digest, Vol 80, Issue 44 ***************************************** _______________________________________________ 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.
