[time-nuts] ? phase comparison or other device

[Dr Bruce Griffiths]

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