Any noise or drift in the "2nd LO", so to speak, would be common-mode between the two channels. It shouldn't be all that critical.
-- john, KE5FX > -----Original Message----- > From: [email protected] [mailto:[email protected]]on > Behalf Of Bob Camp > Sent: Saturday, February 06, 2010 5:52 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] ADEV vs MDEV - using sound card > > > Hi > > Any approach that includes building a low noise synthesizer is > opening up a whole new set of issues. I would much prefer to do > my building at audio. Audio parts are cheap, and performance is > usually a lot easier to check than at RF. > > Bob > > > On Feb 6, 2010, at 8:30 PM, Bruce Griffiths wrote: > > > Which just leaves the minor problem of the offset oscillator. > > > > One option is to use a phase truncation spur free output > frequency from a DDS. > > If one is using the Costas receiver approach the beat frequency > need not be a nice round number like 1.0000KHz. > > > > Another method is to use a crystal whose frequency is offset a > few kHz from 10MHz. > > > > Yet another is the classical method of dividing 10MHz by 100 > and subtracting (using an LSB mixer) the resultant 100KHz from > 10MHz to produce 9.9MHz, then divide the 9.9MHz signal by 100 and > add (using a USB mixer) the resultant 99kHz signal to the 9.99Mhz > signal to produce a 9.999MHz output. > > > > Bruce > > > > John Miles wrote: > >> 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"<[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. _______________________________________________ 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.
