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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 3 12:53:16 UTC 2009


WarrenS wrote:
> ws reply to ">" Bruce's comments
>
>   
>>> That calibration is linear over > than a 1 Hz (1e-7) offset range.
>>>       
>> Whilst that may be true for your OCXO, this is certainly 
>> not  true for every ocxo one may wish to measure.
>>     
> It is not the measured OSC that needs to be linear,  
> but the reference Osc, but I'm sure you knew that.
> And NO not all reference Osc will be that good, 
> but close enough, probable by an order of magnitude 
> when proper end to end calibration is correctly done.
> If the range is small enough, it will be linear enough even 
> for a Very nonlinear  EFC curve.
> This is built to check High precision 10 MHz Osc, 
> If they are off by more a Hz or even 1e-9,  you don't need this kind of performance.
>   
Not true, if the offset is accurately known then a stable oscillator
that has a frequency offset > 1E-9 is just as useful for calibration
purposes as one that has an offset of 1E-11 or less.
> This will easy give 1% absolute difference accuracy at 10 MHz. 
> So the freq difference for a 1e-10 reading could be from 0.99e-10 to 1.01e-10. (plus noise)
> Most would consider it great to be able to take ADEV numbers that are not off more than two to one.
>
>   
>> Since mixer nonlinearity near zero phase isn't an issue with a quiet ocxo
>> pair, a traditional diode double balanced mixer phase detector would be
>> appropriate as it has lower noise than any other mixer/phase detector.
>>     
> With the high negative feedback that this has (2K+), the mixer hardly leaves 'zero' even with a noisy pair.
> I'm using a standard Mini-circuit DB Mixer.
> The mixer is working good enough so as not to be providing any noticeable error or noise.
>   
Have you actually measured the mixer output noise when used as a phase
detector?
> I've tried two other ones and they all give about the same results.
>   
One can either use them with both the RF and LO ports saturated and
suppress AM noise or only saturate the LO port.
The latter mode of operation tends to have a lower mixer phase shift
tempco with a lower phase sensitivity than when both the LO and RF ports
are saturated.
Phase detector IF port termination also affects its gain and noise.
Terminating the IF port in a capacitive load reduces the noise and
increases the phase detector gain.
> The Mixer is inside the Loop with a lot of negative feedback around it, 
> Much of its basic characteristics and error are reduced by the loop gain, 
> including much of its noise.  The system noise is mostly determined by 
> Just the EFC input noise and little else.
> The effective isolation between Oscillators with the mixer output RF shorted and the 10dB attenuators, 
> is good enough not to cause any Osc interaction.
>
>   
I doubt it.
Try estimating the required isolation using Adler's equation.
>> The isolation transformers are not useful for blocking RF, 
>>     
> Right, they pass RF, they don't block it. BUT
> The Isolation transformers are VERY important for a couple of reasons.
> The way these double oven 10811 are made, Their RF output is not isolated,
> but grounded at the inner case. At the end of the well shielded but long RF terminated cable 
> there is a lot of RF noise on the cable shield ground AND a few  mV of DC.
> If this Shield ground is Not isolated but connected to anything, It can cause a lot of errors. 
> These errors can be orders of magnitude worse than other error sources.
> The RF shield ground on this OXCO can not be used in a dual Osc system, 
> to get anywhere near max possible performance. It must be isolated.
>
>   
>> nor in general is an arbitrary bypass capacitor array.
>>     
> The circuit needs bypass caps because of the low level, wide bandwidth, high gain, "DC", feedback loop.
> Even when isolated there is enough RF on the ground shield that it still acts 
> pretty acts like a 40 db down  transmitting antenna.
> That RF needs to be kept out of the uV wide band signals. (And the other Osc)
>
>
>   
>> I'd still prefer to compare results with and without the isolation amplifiers.
>>     
> Yes, and I agree for a good safe universal  GP design, They need to be there.
> I just don't happen to have any yet, and they are low on my priority list of needed improvements.
> With my specific setup, I have tested it well enough that I'm sure that their 
> effect at this time would be small enough as not to be noticed.
>
>   
>> If there is another contributor to phase locking then the full frequency
>> instability wont be reflected by the EFC input fluctuations.
>>     
> Don't get me wrong, in the beginning the oscillators pretty much acted like they were sync locked
>  because of poor ground, common PS, non isolated RF output, etc,etc. That did not go away by luck.
> What I'm saying is they do not now know that each other exist to the level that I can test to, which is about 5e-13.
>
>   

> Something that some seem to of missed is that this is not the standard RF circuit configuration with the standard open loop errors.
> This is a closed loop Neg feedback "PID type" freq control system where the errors inside the loop are reduced by the loop gain.
> The Only significant error outside the loop is the Osc its self and Osc's internal EFC offset. 
> The EFC range being used during a measurement period is typically less than 1/1000 of its range.
>
> ws
>
> ****************
> ----- Original Message ----- 
> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
> To: "WarrenS" <warrensjmail-one at yahoo.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Friday, October 02, 2009 6:00 PM
> Subject: Re: [time-nuts] Low cost alternate to Dual Mixer/DMTD
>
>
>   
>> WarrenS wrote:
>>     
>>> 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.
>>>
>>>       
>> Whilst that may be true for your OCXO, this is certainly not true for
>> every ocxo one may wish to measure.
>> Since mixer nonlinearity near zero phase isnt an issue with a quiet ocxo
>> pair, a traditional diode double balanced mixer phase detector would be
>> appropriate as it hs lower noise than any other mixer/phase detector.
>>     
>>>> 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). 
>>>   
>>>       
>> The isolation tranformers are not useful for blocking RF, nor in general
>> is an arbitrary bypass capacitor array.
>>     
>>> 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.
>>>   
>>>       
>> These measures have little or no effect on RF injection via the EFC input.
>> Effective filtering of the phase detector output and use of a high
>> isolation mixer low noise is required.
>>     
>>>>> 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.
>>>
>>>   
>>>       
>> I'd still prefer to compare results with and without the isolation
>> amplifiers.
>>     
>>>>> 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.
>>>
>>>   
>>>       
>> If there is another contributor to phase locking then the full frequency
>> instability wont be reflected by the EFC input fluctuations.
>>     
>>>> 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>
>>>
>>>
>>>   
>>>       
>> Bruce
>>
>>
>>
>>     
>
>   





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