[time-nuts] ADEV vs MDEV
lists at cq.nu
Sat Feb 6 23:23:41 UTC 2010
Here's my heterodyne vs tight loop logic:
1) With a heterodyne at around 8 Hz your isolation amplifiers are not as critical. Injection locking is a lot less likely at 8 Hz than dead on frequency.
2) You eliminate the need for any form of phase lock.
3) You do need a mixer and a couple of OP-27/37's.
4) Resistors, capacitors, power supplies are likely the same between the two.
5) For time tagging you'll need a picket fence source - not much money there. You could also decide not to time tag.
So far the Hetrodyne is as cheap as the tight lock. Probably a bit cheaper. Most of the parts are the same between the two setups, they simply get wired differently.
The only real question is weather your DVM or my junk counter cost more money. My counter can be pretty bad and still not "get in the way". My old (and long gone) Beckman EPUT meter could probably handle the task. Tough to get data out of a vacuum tube counter though.
Most 10811's are not 1x10^-13 at 1 second. A few might be. The vast majority are up around 0.8 to 2x10^-12 at one second. They were only specified to make 5x10^-12 at one second. Both systems are equally limited by the performance of the reference oscillator.
On Feb 6, 2010, at 5:42 PM, WarrenS wrote:
> 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.
> ----- 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
> 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.
> 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.
>> ----- 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
>>> 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.
>>> 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
>>>> [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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