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

[John Miles]

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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