[time-nuts] ? phase comparison or other device
WB6BNQ
wb6bnq at cox.net
Sun Jun 24 20:36:30 EDT 2007
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
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Bruce,
Can you provide a link to the JPL system you reference above ?
Thank you,
Bill....WB6BNQ
More information about the time-nuts
mailing list