[time-nuts] ? phase comparison or other device

[Pete]

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