[time-nuts] ? phase comparison or other device

[WB6BNQ]

Dr Bruce Griffiths wrote:

> Pete wrote:
> > Here is a scheme that seems to work well for comparing stable frequency
> > sources in the range of 10 to 100 second measurement intervals.
> >
> > Objective - Measure frequency to +/-2E-12 in less than 1 minute.
> >
> > Method - Heterodyne DUT output to 1KHz with a master reference source +
> >                 mixer feeding a tuned zero crossing detector + counter.
> >
> > Equipment - 1. Master reference source at 5 or 10 MHz, e.g. "mature" OXCO or
> >                        GPSDO.
> >                     2. Synthesizer set to DUT - 1KHz, locked to reference
> >                        source. The synthesizer averaged output must settle
> > to
> >                         10uHz in 10 seconds, e.g. HP 3335A or 3336C.
> >                         PTS 040 should work fine, also.
> >                     3. 9 digit/s counter, locked to reference source with
> >                         selectable gate time. An input LPF (100KHz) helps,
> >                         e.g. HP 5335A.
> >                     4. Mini-circuits ZRPD-1 mixer. Other level 7 mixers
> >                          should work, but haven't been tested.
> >                     5. Mini-circuits BLP-1.9 low pass filter. Other filters
> >                         should work, but haven't been tested.
> >                     6. Tuned zero crossing detector, accepts 0 to 5dBm 1KHz
> >                         sinewave input & outputs 1KHz squarewave to counter
> >                         with less than 1nS rms jitter.
> >
> > Setup - DUT set to +7dBm connects to mixer LO port. Synthesizer set to DUT -
> >             1KHz at +4dBm connects to mixer RF port. BLP-1.9 connects to
> >              mixer IF port. ZCD input connects to BLP-1.9. Counter connects
> > to ZCD
> >              output & set for 5 to 10 second gate time. The DUT frequency =
> >              synthesizer setting + counter frequency;
> >              10uHz digit = 1E-12 for 10MHz DUT.
> >
> > The ZCD - Made from 2 Analog devices ADA4899-1, inverting configuration,
> >                   cascaded, using +/- 2.5 volt power supplies. Both amps
> > have their
> >                   non-inverting pins connected (only) to a 100 ohm resistor
> > to ground.
> >                   Both amps have 5uF//5mH to ground on their inverting
> > inputs. The
> >                   input amp has Rin = 422 ohms and Rf = 6190 ohms. The
> > output amp
> >                   has Rin = 562 ohms and Rf = open. The output amp output
> > pin has
> >                   2ea 100 ohm resistors in series to ground. The counter is
> > connected
> >                   to the common point of the 100 ohm resistors. Nominal
> > supply bypassing
> >                   is required. Battery supplies at +/- 3 volts help isolate
> > noise sources.
> >
> > Only 2 ZCD parts aren't junk box items. The Analog Devices ADA4899-1 are in
> > distributor stock as SMT parts only. The 5mH inductors are hand wound on MPP
> > toroid cores. 133 turns on a 55438 core or 114 turns on 2 stacked 55521
> > cores
> > using 22 or 24 AWG wire work fine. Other MPP cores will work, but limit Bmax
> > to 50mT at 1KHz & 0.5V rms. Gapped ferrites are too noisy. The 5uF caps are
> > polypropylene or mylar film types.
> >
> > Noise floor measurements using HP5335A opt 010 as reference source & 1KHz
> > counter + HP3336C synthesizer yielded Favg = 10,000,000.000 001 5 Hz and
> > Fdev = 4.3 uHz for 36 samples at 5.7 second gate time per sample. 10 sample
> > groups are within +/- 2E-12.
> >
> > Pete Rawson
> >
> >
> I am confused the opamp circuitry as described seems to be almost
> exactly the inverse of what is required.
> Please send a schematic so I can check.
>
> Are the MPP cores powdered iron or ferrite?
> The phase stability of the bandpass filters is critical as is any phase
> instability like that exhibited by ferrite cores.
>
> The overdrive recovery characteristics of the ADA4889-1 are not
> specified, how fast does it actually recover from overdrive?
>
> One can do considerably better than this (JPL have a system with a
> resolution of around 1E-15/Tau 1Hz offset, 100MHz input) with lower
> offset frequencies and a well designed amplifier and cascaded limiters,
> however low frequency ground loops are problematic.
> Optical isolation is almost mandatory.
>
> Bruce
>
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Bruce,

Can you provide a link to the JPL system you reference above ?

Thank you,

Bill....WB6BNQ