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" <[email protected]> >>> To: "WarrenS" <[email protected]>; "Discussion of precise time and >>> frequency measurement" <[email protected]> >>> 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" <[email protected]> >>>>> >>>>> >>>>> >>>>> >>>>> >>>> Bruce >>>> >>>> >>>> >>>> >>>> >>> >>> >> > >
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