[time-nuts] Low cost alternate to Dual Mixer/DMTD
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Oct 3 20:15:54 UTC 2009
WarrenS wrote:
>>> If they are off 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.
>>
> Sorry, an incomplete statement on my part. I should of at least said
> If it is CHANGING by more than a Hz or even 1e-9, ...
> As long as it's nominal freq is consistent and with-in range of the Reference Osc,
> then end to end calibration can be checked at any desired freq
> to minimize calibration errors which I've done by using an accurate
> (and somewhat noise) standard high resolution digital freq synthesizer.
>
>
>
>> Have you actually measured the mixer output noise when used as a phase detector?
>>
> I've done some noise test on it but not what you would consider a standard noise test..
>
Drive the mixer LO and RF ports in quadrature using the same low noise
source.
Low pass filter the IF output and amplify with a low noise high gain
amplifier.
Look at the amplifier output with a spectrum analyser (e.g. sound card
based spectrum analyser).
Calibrate spectrum analyser using the Johnson noise of a resistor.
Calibrate phase detector by adding a known amount of phase noise to one
of the mixer ports (RF or LO).
Simplest way is to use a calibrated RF noise source.
> I've checked its wide band Peak to peak noise both open and closed loop.
> Also did an 'audio' spectrum sweep of the phase detectors output
> to insure that there are no spurs being caused by AM, FM, PS, or ground loops etc.
> Being a control loop person, the most reassuring test that I have done
> is to check the noise of the Reference Osc with and without the
> Feedback connected. This is a standard control loop test to insure that the
> loop is behaving correctly and not adding a lot of its own noise to the system.
>
>
>
>> One can either use them with both the RF and LO ports saturated and
>> suppress AM noise or only saturate the LO port.
>>
> One of the BIGGER advantages of this configuration is that many of the
> standard things one normally has to consider do not directly effect this system.
> Take AM noise, this causes small changes in the loop gain,
> which effect even smaller changes in the loop TC
> which effect even smaller changes in the measured signal noise.
> The loop TC is set way above the alising filter's bandwidth
> of the ADC which is set way above the Tau 0 time.
>
>
>
If both mixer ports are saturated AM noise is suppressed.
>> 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.
>>
> Again Something I'd like to try and get across is many standard things like
> phase Tempco etc have NO significant effect on the measured DUT noise
> with this configuration because they are inside a negative feedback Frequency loop.
> I have verified the limited effect of phase noise by injecting larges amounts of
> signal error at the phase detector output. A one Hz sign wave is down more
> than 60 dB, and anything slower like TC or DC is much less.
> Also standard things that can drive a standard high resolution Phase measurement crazy
> such as touching a cable or connecter have very little or no effect because their effect
> is inside the loop and most all their freq content is outside the measured noise spectrum.
>
>
>
Really?
Phase fluctuations due to temperature fluctuations will modulate the
ocxo EFC input.
Try vibrating the cable instead.
>> 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.
>>
> I put a cap right on its output as the first stage of the LP passive RF filter.
> This being a neg feedback loop where the main effect of the phase detector
> is to effects the loop gain, what I've done is to insure that the loop gain is
> reassuring constant by checking the open loop response of the Phase detector.
>
> If your point is that my setup is not optimized, I totally agree. I know that it can be made much better,
> I have not done all that can be done to optimize the phase detector noise,
> But for now it is low enough and much better that my reference Osc,
> so further improvements are both mostly unnecessary and hard to verify.
>
>
Not if you measure the noise of each component as well (ideally) the
complete setup.
>
>>> The effective isolation between Oscillators with the mixer output RF shorted (by cap)
>>> and the 10dB attenuators, is good enough not to cause (any measurable) Osc interaction
>>>
>> I doubt it, Try estimating the required isolation using Adler's equation.
>>
> I don't know about Adler's equation and a lot of other things that would be nice to know
> But I do know how to test for cause and effect.
> One of the early test I did was to connect the two Oscillators outputs together thru a variable
> attenuator to see how much isolation I'd need and to see the effects when there is not enough isolation.
> That is when I found to my surprise the need to keep the RF ground shields isolated thru ground isolation transformers.
> I'll stand by my statement and can verify with test, that for the current configuration that I am using,
> The two OSC have NO interaction which is measurable above the noise level,
> which for now is good enough for now. I do plan to make it better in the future
>
>
I am not yet convinced of that.
> I find these exchanges interesting and somewhat reassuring that I have considered,
> tested and eliminate at least the standard typical type of problems.
>
> Note all the issues that you have raised up to now are general and apply to standard types of configurations.
> Can you point out some specific issues that I need to be concerned with and may of missed
> that are unique to this configuration?
>
>
Have you measured the frequency response of the ocxo EFC input?
> As always, Thanks for your the comments and feedback
> ws
>
> ***********************
>
>
>
>> 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
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>
>
>
More information about the time-nuts
mailing list