[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 6 23:12:22 UTC 2010

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 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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