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.
