[time-nuts] Low cost alternate to Dual Mixer/DMTD

WarrenS warrensjmail-one at yahoo.com
Sat Oct 3 00:39:22 UTC 2009


ws Reply to Bruce

> You also need to measure the EFC slope at the operating point as the EFC
> transfer characteristic can be highly nonlinear.
Yes there is lots of things that can be done wrong but
Another one of this configuration's many advantages is that the operating 
range of both the EFC and the Phase detector is very small,
typical under a millivolt, so nonlinearly is NOT a problem.
To calibrate end to end so that everything is included, The DUT can be changed by a 
small known offset, of say 1e-8 and measure the voltage change at the DVM/ADC output.
Mine is calibrated for 1 mV per 1e-10 at the EFC, 
That calibration is linear over > than a 1 Hz (1e-7) offset range.


> Also need to ensure that injection locking doesn't occur 
> through injection  via the EFC input.
Yes, One of the reasons for the isolations transformers (and lots of bypass caps). 
If there is ANY ground noise between the Oscillators it can effect the EFC voltage.
Like all low level uV signal measurement and control, a lot of attention HAS to be paid 
to insure there is no added noise or errors. This takes good analog  and digital understanding 
of possible noise sources. Differential input and output amps go a long way to insure 
there is no ground loops, offset voltages or noise coupled errors.

>>Maybe due to the fact that the Osc are locked.
> That is the worst possible case for injection locking.
Possible, but not exactly what I've seen in this configuration.
The effect of coupling between Oscillators  is very phase sensitive, 
and can be positive, neg or null as their relative phase shifts.
By adjusting the phase in the way I noted any changing effects
can be easily seen when there is ANY interaction between Osc.


>> This is verified by adding a slow low level freq modulation on the DUT
> Probably not a good test for injection locking as a small shift in
> frequency from equality rapidly attenuates the effective injection
> locking signal.
Another one of the advantages of this circuit is that there is NO shift in freq 
from equaqlity (If that means what I think it is). In any case,
I have found this to be a good test because ANY coupling of any type between 
Oscillators causes a nonlinear transfer function, as a function of voltage and/or freq. 
So by checking that the transfer function between the DUT EFC input 
and the fast ADC output is linear and freq independent over a wide range of signals, 
non coupling is assured down to the level of the noise.
      Another check I did was to unlock the two Osc and add an  freq offset, 
to see if that caused ANY effect at all on the other Osc.

> Better test is to insert a very high reverse isolation amplifier between
> each ocxo and the mixer and see if that makes any difference.
Maybe so, and I did try to do that per your early suggestion, but the test was unsuccessful 
because of  my poor isolation/buffer amps. It's something I'll get back to when 
I've lower the noise more to see if there is anything below the present noise level.

ws

***********************
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>


> WarrenS wrote:
>>
>> Bruce
>>
>> Yes, there are a few disadvantages using this simple low 
>> cost configuration along with all of its advantages.
>> You can not get everything for nothing, but you can get 
>> higher speed, better resolution and less noise from this.
>>
>>> This can not be used to evaluate the stability of an offset oscillator.
>> Correct,  It can not test osc frequencies that are much different than
>> the ref osc such as an offset Osc,  BUT
>> one of the many advantages of this configuration is that an offset Osc
>> is not needed.  This will give freq offset data down to 
>> under 1 ms (>1 kHz) without Any offset osc.
>>
> You also need to measure the EFC slope at the operating point as the EFC
> transfer characteristic can be highly nonlinear.
> 
>>> You need to ensure that the isolation between the 2 sources is sufficient
>> I have no problem with signal injection at 1e-12 resolution levels, even
>> with 10811s without buffers using 10db attenuators and ground Isolation 
>> transformers at each Osc output. Maybe due to the fact that they are locked.
> 
> That is the worst possible case for injection locking.
> 
>> This is verified by adding a slow low level freq modulation on the DUT
>> osc and plotting the freq change on the reference's EFC.
> 
> Probably not a good test for injection locking as a small shift in
> frequency from equality rapidly attenuates the effective injection
> locking signal.
> Better test is to insert a very high reverse isolation amplifier between
> each ocxo and the mixer and see if that makes any difference.
> Also need to ensure that injection locking doesn't occur through
> injection via the EFC input.
> 
> 
>> The phase error can also be changed by adding large or small offsets
>> at the Loop amp's input to check for any pulling tendency.
>>
>> ws
>> 
> Bruce
> 
********************
> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
>> Warren
>>
>> You need to ensure that the isolation between the 2 sources is
>> sufficient to ensure that locking due to unwanted signal injection
>> doesn't significantly effect the effective VCO EFC to frequency transfer
>> function.
>>
>> This method requires that the one of the 2 sources being compared can be
>> phase locked to the other.
>> This isn't always possible, for example, if one wishes to evaluate the
>> stability of an offset oscillator this technique cant be used.
>>
>> Bruce
>>
> *************
>> WarrenS wrote:
>>> I have been using a simple low cost, high performance alternate solution
>>> to the standard Dual Mixer/DMTD.
>>> The idea is based on an 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 PC data logging DVM,
>>> It was simple enough to be up and running from scratch in minutes,
>>> and the best part, it cost me nothing because I already had the four main
>>> parts that are needed.
>>> When a high resolution data logging DVM is used you don't need the offset
>>> voltage.
>>> To get better performance which seems to exceed most DMTD for low tau
>>> numbers, it takes a little more work
>>> and the use of a higher speed oversampling ADC data logger and a good
>>> offset voltage.
>>>
>>> I'll also add that this is not a popular solution,
>>> but as far as I've been able to determine it is the BEST SIMPLE
>>> configuration,
>>> IF you know analog and have an HP 10811 osc to use for the reference.
>>> I've attached a Basic modified NIST Block Diagram showing what I made:
>>>
>>> The NIST paper sums it up quite nicely:
>>> Using this configuration, it is not difficult to achieve a sensitivity of
>>> a part in 1e-14 per Hz resolution of the frequency counter,
>>> so one has excellent precision capabilities with this system.
>>> (I'm achieving well under  0.1 ps Phase resolution, and 1e-12 at Freq
>>> resolution with 0.1 TC, limited by my noisy reference)
>>>
>>> Note that the logged data is in Frequency and not Phase.
>>> I have found Ulrich's Plotter program great for doing the ADEV graphs
>>>
>>> As always, Negative criticism welcome,
>>> Have fun
>>> ws
>>>





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