packages of two WWVB watches: $20 at www.woot.com while they last <EOM>
I thought that someone was designing a circuit that could be used to compare
two oscillators.
What happened to that project? I now have a HP 5370A so I have
something, but
I would like to make simultaneous measurements on three or four "precision"
clocks.I am not qualified to design a "state of the art" device, so I am
looking for others
to do that.
Thanks
Bill K7NOM
Bill Janssen wrote:
I thought that someone was designing a circuit that could be used to compare
two oscillators.
What happened to that project? I now have a HP 5370A so I have
something, but
I would like to make simultaneous measurements on three or four "precision"
clocks.I am not qualified to design a "state of the art" device, so I am
looking for others
to do that.
Thanks
Bill K7NOM
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Bill
Ulrich and I have designed and Ulrich is currently testing a CPLD
implementation of the improved version of the HP K34-5991A linear phase
detector.
It includes programmable prescalers (1-256) so that frequency like 10MHz
and 5MHz for example can be compared. The maximum input frequency is
about 50MHz.
It has 2 quadrature phase outputs. The prescalers also allow the phase
detector gain to be adjusted. The phase detector has a triangular wave
characteristic with a period of 4 cycles of the input frequency to the
phase detector (ie at the built in prescaler output).
Preliminary results using a very crude kitchen table "breadboard"
indicate that instabilities of a few parts in 1E12 are easily seen
within an hour or so.
Sensitivity is likely to be much better than this but a 10X prescaler
was used on each 10MHz input.
Comparing 3 or 4 standards requires using a set of distribution
amplifiers plus a set of linear phase comparators to achieve the desired
configuration.
This is more flexible than trying to anticipate exactly how many
channels a user may want, it also has less crosstalk than an
implementation with more than 2 input frequencies to a single board or CPLD.
With external prescalers the maximum input frequency can be extended to
100MHz or more.
Bruce
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
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
Bruce,
This idea is NOT intended to rival the JPL results. Instead,
it's intended to be cheap, easy to replicate & allow rather
low cost instruments to be used to compare good sources
to parts in 1E12, quickly. The 1KHz heterodyne frequency
makes life much easier than 1Hz. Noisy components &
ground loops are still of concern, but not so hard to fix.
ADA4899-1 overload recovery is <50ns (per data sheet).
I've attached a rather poor schematic which doesn't show
power supply decoupling or the need to pull the disable
pin high. The ADA4899-1 uses 14mA per part, but it's
quiet & fast. Metal film resistors are fine for this low
noise application & all are low values to keep noise down.
The inductors are easy to wind, but I found materials other
than moly permalloy powder to be too noisy. Even with
MPP material, cores with u>200 are prone to field induced
shifts which are unacceptable.
Regards,
Pete Rawson
Pete wrote:
Bruce,
This idea is NOT intended to rival the JPL results. Instead,
it's intended to be cheap, easy to replicate & allow rather
low cost instruments to be used to compare good sources
to parts in 1E12, quickly. The 1KHz heterodyne frequency
makes life much easier than 1Hz. Noisy components &
ground loops are still of concern, but not so hard to fix.
ADA4899-1 overload recovery is <50ns (per data sheet).
I've attached a rather poor schematic which doesn't show
power supply decoupling or the need to pull the disable pin high. The
ADA4899-1 uses 14mA per part, but it's
quiet & fast. Metal film resistors are fine for this low
noise application & all are low values to keep noise down.
The inductors are easy to wind, but I found materials other
than moly permalloy powder to be too noisy. Even with
MPP material, cores with u>200 are prone to field induced
shifts which are unacceptable.
Regards,
Pete Rawson
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Pete
Even so, it pays to use a well designed circuit instead of something
thrown together with little understanding of what you are doing.
The JPL design is not expensive and doesn't require particularly exotic
wideband components or high resolution counters.
There is still a noise advantage in using a 1Hz beat frequency, suitable
opamps are readily available.
Magnetic shielding of the inductors and/or the entire circuit is
probably advisable for the best performance.
The circuit diagram is sufficient to confirm my suspicions.
The input stage noise gain will be high at frequencies away from the
1kHz frequency of interest.
This is a very poor design.
It is very easy to do much better with the same components.
A 50ns overload recovery will be somewhat problematic when you are
attempting 1ns or less timing jitter.
A well designed and simple feedback bound circuit will be much faster.
Using an inverting amplifier input stage is not optimum for noise.
In fact the input stage doesn't need to use such a wideband opamp, a low
noise opamp with a more modest gain bandwidth configured as a non
inverting stage with gain followed by a bandpass filter will have far
better performance.
Only the final limiting stage needs to be fast.
Also since you are using a 1kHz offset frequency it may be advantageous
to use a transformer to couple the mixer output to the input stage, a
stepup transformer will improve the equivalent input noise significantly
even when using a somewhat noisier slower and cheaper opamp for the
input stage.
A low pass filter with a lower cutoff frequency than the several MHz
of the BLP 1.9 is desirable between the mixer and the input amplifier,
a tuned bandpass filter would be optimum but don't forget to terminate
the mixer IF port in a suitable impedance at frequencies other than the
beat frequency. It should be possible to combine the tuned bandpass
filter and the stepup transformer.
Try reading the JPL article to gain an understanding of how to do it
properly.
Although their design uses cascaded low pass filtered amplifiers with
feedback bound circuits, the same technique can be used with bandpss
filters.
Since you use a 1kHz beat frequency it is advantageous to AC couple the
various stages to reduce the effective output dc offset.
Low frequency earth loops will limit the performance unless a different
mixer with dc isolated RF. LO and IF outputs is used.
Suitable mixers are available.
Your claimed performance is comparable with that which can be achieved
using a linear phase comparator which neither requires a mixer (other
than the implicit mixer built into the phase comparator) nor a high
resolution counter.
Bruce
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
WB6BNQ wrote:
Bruce,
Can you provide a link to the JPL system you reference above ?
Thank you,
Bill....WB6BNQ
Bill
http://ntrs.nasa.gov/index.jsp?method=order&oaiID=19910016462
http://ntrs.nasa.gov/index.jsp?method=order&oaiID=19910016462
There is also, or was, a free to download source for this paper
somewhere, which I cant recall.
Bruce
Pete,
- Mini-circuits BLP-1.9 low pass filter.
terminating the mixer if output with an lowpass/bandpass filter and NOT
with an diplexer is not so good an idea. Where does the rf go?
Best regards
Ulrich Bangert
-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von Pete
Gesendet: Sonntag, 24. Juni 2007 03:38
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] ? phase comparison or other device
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
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