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"<[email protected]> >>>>>>>> To: "Discussion of precise time and frequency >>>>>>>> measurement"<[email protected]> >>>>>>>> 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"<[email protected]> >>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>>> To: "Discussion of precise time and frequency >>>>>>>>> measurement"<[email protected]> >>>>>>>>> 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
