A sound-card back end has always seemed like a pretty reasonable approach to me, if you're inclined to go the DMTD route. I wouldn't send a 'baseband' signal to the sound card, though -- I'd upconvert it to a few kHz to get away from the numerous bad things that sound cards do near DC.
-- john, KE5FX > Hi > > My main concern with the low frequency pole in the sound card is > the quality of the R/C used. You can certainly model what ever > you have. If they used an aluminum electrolytic for the "C" it > may not be the same next time you check it .... > > On a 10 Hz system, a 1 Hz pole is probably not an issue. It might > get in the way with a 1 Hz beat note. > > Another thing I have only seen in passing: "Sigma Delta's have > poor low frequency noise characteristics". I haven't dug into it > to see if that's really true or not. If you buy your own ADC's, > you certainly would not be restricted to a Sigma Delta. > > Even with a cheap pre-built FPGA board, you could look into > higher sample rates than a conventional sound card. You would > drop back to 16 bits, but it might be worth it. > > Bob > > > On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: > > > Even better is to toss out the mixers and sample the RF signals > directly. > > However suitable ADCs cost $US100 or more each. > > To which one has to add an FPGA and an interface to a PC with > sufficient throughput to handle the down converted I + Q samples. > > > > Bob Camp wrote: > >> Hi > >> > >> You probably could put a couple of cheap DAC's > > > > (ADCs are preferable as it avoids having to implement the > conversion logic plus comparator required when using a DAC.) > > > >> on a board with a FPGA and reduce the data on the fly. I'd > guess that would be be in the same $100 range as a half way > decent sound card. Clock the DAC's off of a 10 MHz reference and > eliminate the cal issue. > >> > >> If you are down around 10 Hz or worse yet 1 Hz, the AC > coupling of the sound card will get in the way, even with a > bandpass approach. You really don't know what they may have in > there at the low end. Build it yourself and that stuff's not an issue. > >> > >> Bob > >> > >> > > My sound card has a 1Hz cutoff RC high pass input filter plus > an internal high pass digital filter. > > Its not too difficult to measure the sound card frequency > response using a white noise source for example. > > > > Bruce > >> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: > >> > >> > >>> If one has a high end sound card then it could be used to > implement the bandpass filter and replace the zero crossing detector. > >>> It may be necessary to insert a pilot tone to calibrate the > sound card sampling clock frequency. > >>> A noise floor of about 1E-13/Tau should be achievable. > >>> This simplifies the DMTD system by replacing the zero > crossing detector with a low gain linear preamp. > >>> > >>> If one analyses the resultant data off line then one can also > try out different techniques such as a Costas receiver rather > than a simple bandpass filter plus zero crossing detector. > >>> However 1000 seconds of data for 2 channels of 24 bit samples > at 192KSPS will result in a file with a size of at least 1.15GB. > >>> > >>> Bruce > >>> > >>> > >>> Bruce Griffiths wrote: > >>> > >>>> If one were to use a bandpass filter with a Q of 10 to > filter the beat frequency output of the mixer, then if the input > frequency is 10MHz and the filter component tempco is 100ppm/C > then the resultant phase shift tempco is about 16ps/C referred to > the mixer input frequency. > >>>> > >>>> This phase shift tempco is certainly low enough not to have > significant impact when measuring the frequency stability of a > typical 10811A if the temperature fluctuations are kept small > enough during the run. > >>>> > >>>> The effect of using a bandpass filter with too narrow a > bandwidth is to artificially reduce ADEV for small Tau, so it may > be prudent to use a higher beat frequency that 1Hz or even 10Hz > and not calculate ADEV for Tau less than say 10(??) times the > beat frequency period. A trade off between this and the effect of > aliasing is required. > >>>> > >>>> Bruce > >>>> > >>>> Bob Camp wrote: > >>>> > >>>>> Hi > >>>>> > >>>>> With most 10811 range oscillators the impact of a simple > bandpass filter is low enough to not be a major issue. That's for > normal lab temperatures with the circuitry in a conventional die > cast box. No guarantee if you open the window and let the fresh > air blow in during the run. > >>>>> > >>>>> That's true with a heterodyne. I can see no obvious reason > it would not be true on DMTD. > >>>>> > >>>>> Bob > >>>>> > >>>>> > >>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: > >>>>> > >>>>> > >>>>>> The only major issue with DMTD systems is that they > undersample the phase fluctuations and hence are subject to > aliasing effects. > >>>>>> The low pass filter has to have a bandwidth of the same > order as the beat frequency or the beat frequency signal will be > significantly attenuated. > >>>>>> Since the phase is only sampled once per beat frequency > period the phase fluctuations are undersampled. > >>>>>> Various attempts to use both zero crossings have not been > successful. > >>>>>> > >>>>>> In principle if one can overcome the increased phase shift > tempco associated with a bandpass filter, using a bandpass filter > can in principle ensure that the phase fluctuations are oversampled. > >>>>>> > >>>>>> > >>>>>> Bruce > >>>>>> > >>>>>> Bob Camp wrote: > >>>>>> > >>>>>>> Hi > >>>>>>> > >>>>>>> A straight heterodyne system will get you to the floor of > most 10811's with a very simple (2 stage) limiter. As with the > DMTD, the counter requirements aren't really all that severe. > >>>>>>> > >>>>>>> Bob > >>>>>>> > >>>>>>> > >>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>>> > >>>>>>>>> "It's possible / likely for injection lock ... to be a > problem ..." > >>>>>>>>> > >>>>>>>>> > >>>>>>>> Something I certainly worried about and tested for. > >>>>>>>> What I found (for MY case) is that injection lock is NOT > a problem. > >>>>>>>> The reason being is that unlike most other ways, where > the two OSC have to be completely independent, > >>>>>>>> The tight loop approach forces the Two Osc to "Lock with > something like 60 + db gain, > >>>>>>>> so a little stray -80db injection lock coupling that > would very much limit other systems has > >>>>>>>> no measurable effect at e-13. Just one of the neat > little side effects that make the tight loop approach so simple. > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>>> "then a part in 10^14 is going to be at the 100 of > nanovolts level." > >>>>>>>>> > >>>>>>>>> > >>>>>>>> For that example, just need to put a simple discrete 100 > to 1 resistor divider > >>>>>>>> in-between the control voltage and the EFC and now you > have a nice workable 10uv. > >>>>>>>> BUT the bigger point is, probable not needed, cause you > are NOT going to do any better than the stability of the OSC with > a grounded shorted EFC input. > >>>>>>>> > >>>>>>>> as you said and I agree is so true: > >>>>>>>> > >>>>>>>> > >>>>>>>>> "There is no perfect way to do any of this, only a lot > of compromises ... you need to watch out for". > >>>>>>>>> > >>>>>>>>> > >>>>>>>> But you did not offer any easier way to do it, which is > what the original request was for and my answer addressed. > >>>>>>>> This is the cheapest easiest way BY FAR to get high > performance, at low tau, ADEV numbers that I've seen. > >>>>>>>> > >>>>>>>> ws > >>>>>>>> *************** > >>>>>>>> > >>>>>>>> ----- Original Message ----- From: "Bob Camp"<li...@cq.nu> > >>>>>>>> To: "Discussion of precise time and frequency > measurement"<time-nuts@febo.com> > >>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM > >>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>>> Hi > >>>>>>>>> > >>>>>>>>> It's possible / likely to injection lock with the tight > loop approach and get data that's much better than reality. A lot > depends on the specific oscillators under test and the buffers > (if any) between the oscillators and mixer. > >>>>>>>>> > >>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt > then a part in 10^14 is going to be at the 100 of nanovolts > level. Certainly not impossible, but it does present it's own set > of issues. Lab gear to do it is available, but not all that > common. DC offsets and their temperature coefficients along with > thermocouple effects could make things exciting. > >>>>>>>>> > >>>>>>>>> There is no perfect way to do any of this, only a lot > of compromises here or there. Each approach has stuff you need to > watch out for. > >>>>>>>>> > >>>>>>>>> Bob > >>>>>>>>> > >>>>>>>>> -------------------------------------------------- > >>>>>>>>> From: "WarrenS"<warrensjmail-...@yahoo.com> > >>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM > >>>>>>>>> To: "Discussion of precise time and frequency > measurement"<time-nuts@febo.com> > >>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> > >>>>>>>>>> Peat said: > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>>> I would appreciate any comments or observations on > the topic of apparatus with demonstrated stability measurements. > >>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for > making stability measurements at the 1E-13 in 1s performance level. > >>>>>>>>>>> > >>>>>>>>>>> > >>>>>>>>>> If you accept that the measurement is going to limited > by the Reference Osc, > >>>>>>>>>> for Low COST and SIMPLE, with the ability to measure > ADEVs at that level, > >>>>>>>>>> Can't beat a simple analog version of NIST's "Tight > Phase-Lock Loop Method of measuring Freq stability". > >>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> By replacing the "Voltage to freq converter, Freq > counter& Printer with a Radio shack type PC data logging DVM, > >>>>>>>>>> It can be up and running from scratch in under an Hr, > with no high end test equipment needed. > >>>>>>>>>> If you want performance that exceeds the best of most > DMTD at low Tau it takes a little more work > >>>>>>>>>> and a higher speed oversampling ADC data logger and a > good offset voltage. > >>>>>>>>>> > >>>>>>>>>> I must add this is not a popular solution (Or a > general Purpose one) but > >>>>>>>>>> IF you know analog and have a GOOD osc with EFC to > use for the reference, > >>>>>>>>>> as far as I've been able to determine it is the BEST > SIMPLE answer that allows High performance. > >>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than > 1e-12 in 0.1 sec (at 30 Hz Bandwidth) > >>>>>>>>>> > >>>>>>>>>> Basic modified NIST Block Diag attached: > >>>>>>>>>> The NIST paper sums it up quite nicely: > >>>>>>>>>> 'It is not difficult to achieve a sensitivity of a > part in e14 per Hz resolution > >>>>>>>>>> so one has excellent precision capabilities with this system.' > >>>>>>>>>> > >>>>>>>>>> This does not address your other question of ADEV vs MDEV, > >>>>>>>>>> What I've described is just a simple way to get the > Low cost, GOOD Raw data. > >>>>>>>>>> What you then do with that Data is a different subject. > >>>>>>>>>> > >>>>>>>>>> You can run the raw data thru one of the many ADEV > programs out there, 'Plotter' being my choice. > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> Have fun > >>>>>>>>>> ws > >>>>>>>>>> > >>>>>>>>>> ************* > >>>>>>>>>> > >>>>>>>>>> [time-nuts] ADEV vs MDEV > >>>>>>>>>> Pete Rawson peterawson at earthlink.net > >>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 > >>>>>>>>>> > >>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with > >>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It > seems that > >>>>>>>>>> existing TI counters can reach this goal in 10s. > (using MDEV estimate > >>>>>>>>>> or 100+s. using ADEV estimate). The question is; does > the MDEV tool > >>>>>>>>>> provide an appropriate measure of stability in this > time range, or is > >>>>>>>>>> the ADEV estimate a more correct answer? > >>>>>>>>>> > >>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, > single shot TI, > >>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I > have data > >>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the > >>>>>>>>>> other counters behave similarly. > >>>>>>>>>> > >>>>>>>>>> I would appreciate any comments or observations on this topic. > >>>>>>>>>> My motivation is to discover the simplest scheme for making > >>>>>>>>>> stability measurements at this performance level; this is NOT > >>>>>>>>>> even close to the state-of-the-art, but can still be useful. > >>>>>>>>>> > >>>>>>>>>> Pete Rawson > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> > >>> > > > > > > > > _______________________________________________ > > 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.
