[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 6 22:48:03 UTC 2010


A round robin test where a couple of well characterised oscillators are 
passed around for intercomparison is perhaps the ideal method of 
evaluating such effects.

Bruce

WarrenS wrote:
> Bob
>
> So, if your point is that there are other ways to do it.  ...We Agree
> (And the reason for the advanced methods is so that the counter 
> resolution is not the limiting factor)
>
> Or are you saying a Tight Phase-Lock Loop" is not the simplest and 
> cheapest way to get 1e13 resolution at 1  sec?
> That I'd have to see something new to believe it.
>
> Just so things do not get too far off the original topic, here is a 
> reminder:
>>>>> "I would appreciate any comments or observations on the SIMPLEST 
>>>>> scheme for making stability measurements at 1e-13 in one sec."
> ws Answer)  Try the "Tight Phase-Lock Loop Method"
>
> May want to compare the blocks and equipment needed for A straight 
> heterodyne system, or a DMTD, compared to the Analog "Tight Phase-Lock 
> Loop" Method,  AND then see what added problems there are because of 
> injection locking, Osc coupling, Phase noise, ETC, ETC.
>
>
> 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 2:03 PM
> Subject: Re: [time-nuts] ADEV vs MDEV
>
>
> 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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