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 > [email protected] > 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 _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
