[time-nuts] ADEV vs MDEV - using sound card

[John Miles]

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]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"<lists at cq.nu>
> >>>>>>>>>>> To: "Discussion of precise time and frequency
> >>>>>>>>>>>
> >>> measurement"<time-nuts at 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-one at yahoo.com>
> >>>>>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM
> >>>>>>>>>>>> To: "Discussion of precise time and frequency
> >>>>>>>>>>>>
> >>> measurement"<time-nuts at 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
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >
> >
> >
> > _______________________________________________
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