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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Ulrich Bangert wrote:
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
Ulrich
This depends on whether the low pass filter has a shunt capacitor at its
input or a series inductor.
With the shunt capacitor the RF is shunted to ground through this capacitor.
With a series inductor the RF sees a relatively high input impedance and
the mixer will not perform well.
Bruce
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Mon, 25 Jun 2007 20:37:29 +1200
Message-ID: 467F7EC9.8090602@xtra.co.nz
Ulrich Bangert wrote:
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
Ulrich
This depends on whether the low pass filter has a shunt capacitor at its
input or a series inductor.
With the shunt capacitor the RF is shunted to ground through this capacitor.
With a series inductor the RF sees a relatively high input impedance and
the mixer will not perform well.
Depends on what you want to acheive. NIST made some experiments and found that
this was indeed what increased the gain in the frequency comparisions they
made. They where infact a bit supprised as this was contruary to what they
beleived. It's in the NIST archive, but I could pull the document number out if
given some chance to dig around.
So, what is normally a good thing in RF may not necessarilly be the best
strategy for frequency comparision, at least when it comes to mixers.
Cheers,
Magnus
Any suggestions on the simplest, semi inexpensive way, to be able to look
at a one Pulse Per Second wave form? It's seems that most oscilloscopes will
not let me look that low in frequency. Basically I want to know, and see if
I have a pure, clean 1 PPS waveform.
thanks,
Raimond
Raimond,
About the only thing you can do is put the scope in normal trigger mode,
looking for a positive going transition. If you set the horizontal sweep at
about 1mS/cm you should see the rising edge of the pulse once per second. If
you have an old scope without any storage capability the image will
naturally fade quickly. This may take some tweaking on the trigger settings
for the scope, but you should be able to find a combination of trigger
settings and horizontal division size that will work.
If your scope's trigger circuitry is stable enough you should be able to
decrease the horizontal division size so that you can get a general idea of
what the leading edge of the 1PPS pulse looks like. Depending on the
receiver it should have a very fast rise-time and be a smooth ramp.
Naturally, you will not be able to make any measurements as to the accuracy
of the pulse, but this will give you a quick health check.
Best regards,
Randy Warner
Senior Applications Engineer
Geodetics, Inc.
858.729.0872
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Raimond Melkers
Sent: Monday, June 25, 2007 8:43 AM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] 1 PPS visual
Any suggestions on the simplest, semi inexpensive way, to be able to look
at a one Pulse Per Second wave form? It's seems that most oscilloscopes will
not let me look that low in frequency. Basically I want to know, and see if
I have a pure, clean 1 PPS waveform.
thanks,
Raimond
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Randy,
and that's what I'm looking for, the actual measurement isn't that
important, the fidelity of the pulse is. The concern is, that if I over
drive the out put amplifiers of the distribution amps, I can make a VERY
ugly picture, much like taking a five MHz sine wave, and turning it into a
"dirty" semi square wave generator.
Raimond
-----Original Message-----
From: Randy Warner [mailto:randy@geodetics.com]
Sent: Monday, June 25, 2007 10:57
To: Raimond.Melkers@L-3Com.com; 'Discussion of precise time and frequency
measurement'
Subject: RE: [time-nuts] 1 PPS visual Test
Raimond,
About the only thing you can do is put the scope in normal trigger mode,
looking for a positive going transition. If you set the horizontal sweep at
about 1mS/cm you should see the rising edge of the pulse once per second. If
you have an old scope without any storage capability the image will
naturally fade quickly. This may take some tweaking on the trigger settings
for the scope, but you should be able to find a combination of trigger
settings and horizontal division size that will work.
If your scope's trigger circuitry is stable enough you should be able to
decrease the horizontal division size so that you can get a general idea of
what the leading edge of the 1PPS pulse looks like. Depending on the
receiver it should have a very fast rise-time and be a smooth ramp.
Naturally, you will not be able to make any measurements as to the accuracy
of the pulse, but this will give you a quick health check.
Best regards,
Randy Warner
Senior Applications Engineer
Geodetics, Inc.
858.729.0872
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Raimond Melkers
Sent: Monday, June 25, 2007 8:43 AM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] 1 PPS visual
Any suggestions on the simplest, semi inexpensive way, to be able to look
at a one Pulse Per Second wave form? It's seems that most oscilloscopes will
not let me look that low in frequency. Basically I want to know, and see if
I have a pure, clean 1 PPS waveform.
thanks,
Raimond
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Bruce,
A few final thoughts.
-
Thanks for the critical view; it does help.
-
Like many time-nuts I have a reasonably good 10MHz source &
sometimes need to check out a newly acquired OXCO to ensure
it can muster 1E9 or 1E10 performance (with 10x headroom).
An SR620 would be ideal, but it's just too many $$ ;even used.
I expect "casual" participants of time-nuts already have a basic,
decent counter e.g. HP5335A & a basic decent synthesizer
e.g. PTS040, Fluke6060(?), HP3335x or 6x. Also, I assumed a
coaxial level 7 mixer & suitable lowpass filter would be available. -
I read the JPL paper (more than once) & developed the first three
stages (modified for 1KHz bandpass) per their process. At that point
the measured jitter was well under 1ns rms; which was enough to enable
1E12 resolution for 10MHz sources. I deliberately choose the ADA4899-1
opamp since it's characterized for 5V operation, low noise, fast & cheap
enough ($4.30/ea). It was apparent that even with 2 stages the ZCD
was still under 1ns jitter; the risetime wasn't blazing, but it was
obviously
good enough. -
Without PCB capability (at home & now retired) even this simple
circuit is tough to build; each part adds significantly to the effort when
doing 1-up. So I examined the need for every part in an effort to
minimize parts count, but retain jitter performance. I found that the
opamp overload recovery was more than fast enough to discard the
limiting without measurable deterioration in jitter. Lots of parts went
away; construction became easy. -
I went TOO FAR. The opamps I had exhibited such low offset that I
DC coupled without thinking about it. WRONG answer (as you noted),
Rookie mistake. I have shown the AC coupling & 2nd stage feedback
resistor in the revised circuit. -
The ZCD costs <$20 for parts & about 2 hours to build/check out.
It performs well enough to look at stable sources to 2 parts in 1E12
in 50 to 100 seconds and be confident in the data. The noise floor is
easy to measure & verifies functionality.
Is it "well designed" ? NO. Could it be (much) better? Certainly.
Does it work well for it's intended purpose? Yes.
My assumptions about equipment may be out of line. In my case, eBay
supplied everything, except the mixer, filter & ADA4899-1s, so this
effort didn't require much in the way of extra $$. It does what I wanted.
As previously observed, the mixer should have a diplexer between it and
the filter for the mixer higher order products to be terminated properly.
I examined the filter input Z, as terminated, and found it to be from 150
ohms inductive to 1200 ohms inductive from 10 to 30 MHz. This suggests
the use of a feedthrough termination of around 100 ohms as a first order
fix. Using a 93 ohm feedthrough, no improvement, or degradation in results
was noted. This could use more study.
From your earlier response, I suspect you have a cheaper, better method
in mind to achieve the same results. Would you detail it?
Regards,
Pete Rawson
Pete wrote:
Bruce,
A few final thoughts.
-
Thanks for the critical view; it does help.
-
Like many time-nuts I have a reasonably good 10MHz source &
sometimes need to check out a newly acquired OXCO to ensure
it can muster 1E9 or 1E10 performance (with 10x headroom).
An SR620 would be ideal, but it's just too many $$ ;even used.
I expect "casual" participants of time-nuts already have a basic,
decent counter e.g. HP5335A & a basic decent synthesizer
e.g. PTS040, Fluke6060(?), HP3335x or 6x. Also, I assumed a
coaxial level 7 mixer & suitable lowpass filter would be available. -
I read the JPL paper (more than once) & developed the first three
stages (modified for 1KHz bandpass) per their process. At that point
the measured jitter was well under 1ns rms; which was enough to enable
1E12 resolution for 10MHz sources. I deliberately choose the ADA4899-1
opamp since it's characterized for 5V operation, low noise, fast & cheap
enough ($4.30/ea). It was apparent that even with 2 stages the ZCD
was still under 1ns jitter; the risetime wasn't blazing, but it was
obviously
good enough. -
Without PCB capability (at home & now retired) even this simple
circuit is tough to build; each part adds significantly to the effort
when
doing 1-up. So I examined the need for every part in an effort to
minimize parts count, but retain jitter performance. I found that the
opamp overload recovery was more than fast enough to discard the
limiting without measurable deterioration in jitter. Lots of parts went
away; construction became easy. -
I went TOO FAR. The opamps I had exhibited such low offset that I
DC coupled without thinking about it. WRONG answer (as you noted),
Rookie mistake. I have shown the AC coupling & 2nd stage feedback
resistor in the revised circuit. -
The ZCD costs <$20 for parts & about 2 hours to build/check out.
It performs well enough to look at stable sources to 2 parts in 1E12
in 50 to 100 seconds and be confident in the data. The noise floor is
easy to measure & verifies functionality.
Is it "well designed" ? NO. Could it be (much) better? Certainly.
Does it work well for it's intended purpose? Yes.
My assumptions about equipment may be out of line. In my case, eBay
supplied everything, except the mixer, filter & ADA4899-1s, so this
effort didn't require much in the way of extra $$. It does what I wanted.
As previously observed, the mixer should have a diplexer between it and
the filter for the mixer higher order products to be terminated properly.
I examined the filter input Z, as terminated, and found it to be from 150
ohms inductive to 1200 ohms inductive from 10 to 30 MHz. This suggests
the use of a feedthrough termination of around 100 ohms as a first order
fix. Using a 93 ohm feedthrough, no improvement, or degradation in
results
was noted. This could use more study.
From your earlier response, I suspect you have a cheaper, better method
in mind to achieve the same results. Would you detail it?
Regards,
Pete Rawson
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Pete
Try connecting the input stage inductor and capacitor in parallel with
the 6190 ohm feedback resistor, but before you do this replace the first
opamp with a lower gain bandwidth (audio??) device that is unity gain
stable. This will produce a first gain stage that amplifies the signal
of interest as well as the noise within the tuned circuit bandwidth
without unduly amplifying the noise not within the tuned circuit bandpass.
The other thing you could do since you've chosen a 1kHz beat frequency
is to use an audio transformer to step up the output of the mixer before
amplifying it. NB dont forget to connect the transformer to ground
through a capacitor that has a low impedance at 1kHz (this ensures that
the dc load current at the mixer IF port is low)..
The mixer IF port should be terminated with a 10nF capacitor and a
simple low pass filter consisting of say a 100uH inductor and a 1nF
capacitor substituted for the 1.9MHz bandpass filter.
This, as shown by the NIST paper alluded to by Magnus, will increase the
mixer sensitivity considerably. You should also run the mixer with both
the RF and LO ports saturated ie more than 7dBm for both of these ports.
The mixer output noise at the 1KHz beat frequency will be somewhere in
the vicinity of 100nV/rtHz, so if you have say a 1V peak output then the
inherent jitter due to mixer noise will be around 160ps rms for a tuned
circuit noise bandwidth of 100Hz. With a suitable amplifier choice you
shouldn't degrade this by more than 5% or so. Achieving a resolution of
better than 1E-13 in 1 second with a 10MHz input and a suitable counter
is easy, provided you dont rely on the counters input circuitry to
trigger on the amplified mixer output you need a signal chain like that
used by JPL. With a suitable comparator and well designed limiting
stages one can easily achieve this resolution.
With a relatively low resolution counter you will get better results if
you use a 1Hz beat frequency like JPL did, then a resolution of around
1E-14 in 1 second with a 10MHz input and a cheap 100ns resolution
counter is easily achieved.
You can do 1E-12 resolution in a minute or so with nothing more advanced
than a high resolution chart recorder (perhaps not a physical one but
use an inexpensive ADC) and a linear phase comparator, you don't even
need a counter.
Ulrich and I are currently working on phase comparator that promises
higher resolution than you have achieved, however the JPL approach is
capable of a resolution several orders of magnitude better than this.
The resolution offered by the JPL approach is required when you want to
compare a couple of hydrogen masers or other sources with equivalent or
better stability. For less stable sources a simple phase comparator
should suffice for most purposes.
A project to produce an equivalent of the JPL zero crossing detector is
underway.
Inexpensive audio grade opamps will be used, wideband opamps are
unnecessary.
The final stage will be a relatively slow comparator (AD790).
The largest contribution to phase instability will be the temperature
dependent phase shift of the 1Hz low pass filter.
Just as JPL did a mixer with separate isolated grounds for all 3 ports
will be used (eg Minicircuits RMS-1, HP10534B etc)
Standard synthesizers and DDS circuits are far too noisy for generating
the 10MHz -1Hz signal, so an offset generator equivalent to the one JPL
developed will be required for optimum performance.
Bruce
Hi Pete,
- I read the JPL paper (more than once)...
Do you have it available in electronic form (or know a link that I
might download it from)?
Thanks,
Peter Vince (G8ZZR, London)
I have the JPL zero crossing detector paper scanned in.
(John Dick, et al, 1990 PTTI). It is definitely a must
read.
Do you want me to email to you?
Rick Karlquist N6RK
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Peter Vince
Sent: Wednesday, June 27, 2007 11:55 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ? phase comparison or other device
Hi Pete,
- I read the JPL paper (more than once)...
Do you have it available in electronic form (or know a link that I
might download it from)?
Thanks,
Peter Vince (G8ZZR, London)
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Post it to Didier Juges's site?
ftp.ko4bb.com
User: manuals
Password: manuals
-- john, KE5FX
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Richard (Rick) Karlquist
Sent: Wednesday, June 27, 2007 7:37 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ? phase comparison or other device
I have the JPL zero crossing detector paper scanned in.
(John Dick, et al, 1990 PTTI). It is definitely a must
read.
Do you want me to email to you?
Rick Karlquist N6RK
That will be welcome of course.
Please note that I will be out of the country for a week starting
tomorrow (June 28), so anything uploaded will have to wait until my
return to be found at the usual place.
However, anything uploaded can be immediately downloaded using ftp and
the same login account as for uploading.
http://www.ko4bb.com/ham_radio/Manuals/1_Upload_Instructions.html
Didier KO4BB
John Miles wrote:
Post it to Didier Juges's site?
ftp.ko4bb.com
User: manuals
Password: manuals
-- john, KE5FX
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Richard (Rick) Karlquist
Sent: Wednesday, June 27, 2007 7:37 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ? phase comparison or other device
I have the JPL zero crossing detector paper scanned in.
(John Dick, et al, 1990 PTTI). It is definitely a must
read.
Do you want me to email to you?
Rick Karlquist N6RK
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Rick, I'd love to have it, too... I've seen references for quite a
while, but never been able to get the actual document.
Thanks,
John
Richard (Rick) Karlquist wrote:
I have the JPL zero crossing detector paper scanned in.
(John Dick, et al, 1990 PTTI). It is definitely a must
read.
Do you want me to email to you?
Rick Karlquist N6RK
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Peter Vince
Sent: Wednesday, June 27, 2007 11:55 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ? phase comparison or other device
Hi Pete,
- I read the JPL paper (more than once)...
Do you have it available in electronic form (or know a link that I
might download it from)?
Thanks,
Peter Vince (G8ZZR, London)
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Dear time-nuts,
there is another article about zero crossing detection,
comes after the very wise article written by my friend J. Dick
Cheers,
Enrico
http://rubiola.org/hidden/collins96comm-zero-crossing-detector.pdf
http://rubiola.org/hidden/dick90ptti-dual-mixer-dc-amplifier.pdf
Enrico Rubiola
professor of electronics
web: http://rubiola.org
e-mail: rubiola@femto-st.fr
FEMTO-ST Institute
32 av. de l'Observatoire
25044 Besancon, FRANCE
voice: +33(0)381.853940 (E.Rubiola)
voice: +33(0)381.853999 (switchboard)
fax: +33(0)381.853998
Peter,
The JPL paper is the second on Enrico Rubiola's posting.
Pete Rawson
Enrico,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
Best regards
Ulrich Bangert
-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von Enrico Rubiola
Gesendet: Donnerstag, 28. Juni 2007 16:25
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] ? phase comparison or other device
Dear time-nuts,
there is another article about zero crossing detection,
comes after the very wise article written by my friend J.
Dick Cheers, Enrico
http://rubiola.org/hidden/collins96comm-zero-crossing-detector.pdf
http://rubiola.org/hidden/dick90ptti-dual-mixer-dc-amplifier.pdf
Enrico Rubiola
professor of electronics
web: http://rubiola.org
e-mail: rubiola@femto-st.fr
FEMTO-ST Institute
32 av. de l'Observatoire
25044 Besancon, FRANCE
voice: +33(0)381.853940 (E.Rubiola)
voice: +33(0)381.853999 (switchboard)
fax: +33(0)381.853998
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Ulrich Bangert wrote:
Enrico,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
Best regards
Ulrich Bangert
Ulrich
Diode clamps are usually essential particularly with opamps as in most
cases their recovery from input overdrive is otherwise too slow to be
useful in such circuits.
The Collins paper also indicates, as I suspected, that the -5v and +5V
clamp levels used in the JPL ZCD are somewhat arbitrary and lower clamp
levels can be used.
Bruce
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Fri, 29 Jun 2007 19:52:22 +1200
Message-ID: 4684BA36.3040401@xtra.co.nz
Ulrich Bangert wrote:
Enrico,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
Best regards
Ulrich Bangert
Ulrich
Diode clamps are usually essential particularly with opamps as in most
cases their recovery from input overdrive is otherwise too slow to be
useful in such circuits.
The Collins paper also indicates, as I suspected, that the -5v and +5V
clamp levels used in the JPL ZCD are somewhat arbitrary and lower clamp
levels can be used.
The clamp levels is gain-wise not very relevant since in the next gainstage
they will be clamped out anyways. The lowpass filtering in each stage will
integrate the clamp levels such that the rise and fall positions will be
shifted from their "true" positions by the clamp level and these shifts will
not have an zero mean. However, if the clamp levels are stable they are less of
a problem. The main clamp level effect will be time-shift and as long as the
levels are stable this time-shift will be stable.
I must have a propper reading of the Collins paper, but I will do that
tomorrow. Refreshing new info from the JPL ZCD paper which confirmed my
initial thoughts while introducing the noise aspect. It did bug me that the
noise may not be continous so the Collins findings in that respect was fairly
obvious. However, it seems like he is not caring about the noise level during
the clamping time, which would make his noise estimates somewhat
over-optimistic. But then again, I haven't given the paper a propper read-
through.
Cheers,
Magnus
From: "Ulrich Bangert" df6jb@ulrich-bangert.de
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Fri, 29 Jun 2007 09:14:02 +0200
Message-ID: 000001c7ba1d$130c2750$03b2fea9@athlon
Enrico,
Ulrich,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
It is actually a specific design-trick on Collins behalf to saturate the
op-amp since this infact acts like a noise-gate. By having the output stage
saturated rather than operating linearly the noise as seen by the output RC
filter is that of only the saturated transistor and not that of the input
gained up. A diode limiter in the feedback path will maintain the op-amp in
the linear operating range and thus cause the noise to continue to polute the
output filter. What you can possibly acheive is the steer how deeply you run
into output saturation tought.
So, in this case op-amp saturation is a key trick to increased performance.
Cheers,
Magnus
Magnus Danielson wrote:
Ulrich,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
It is actually a specific design-trick on Collins behalf to saturate the
op-amp since this infact acts like a noise-gate. By having the output stage
saturated rather than operating linearly the noise as seen by the output RC
filter is that of only the saturated transistor and not that of the input
gained up. A diode limiter in the feedback path will maintain the op-amp in
the linear operating range and thus cause the noise to continue to polute the
output filter. What you can possibly acheive is the steer how deeply you run
into output saturation tought.
So, in this case op-amp saturation is a key trick to increased performance.
Not true, there's nothing magic about amplifier saturation, any means
that limits the amplifier output whilst dropping the small signal gain
to a low value will have exactly the same effect.
In most cases recovery from saturation will be too slow for the later
stages of the ZCD.
Those amplifiers that have fast recovery from saturation usually employ
internal diode clamps.
A diode clamp in the feedback path will cut the noise gain to 1 when
either diode turns on. The following diode clamp across the filter
capacitor will reduce the noise gain to a very small value when it turns on.
Both diode clamps and internal saturation will still produce some output
noise although not from the amplifier input stages.
Cheers,
Magnus
Bruce.
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] ? phase comparison or other device
Date: Sun, 01 Jul 2007 02:15:55 +1200
Message-ID: 4686659B.8000205@xtra.co.nz
Magnus Danielson wrote:
Ulrich,
you are right: Both of these articles should be read with Collins's
perhaps the better (and newer!) one.
There is however one question remaining for me: When I learned
electronics it was generally considered bad design to let an amplifier
run into limiting due to supply limitations. If limiting was needed, so
was the rule, then it should be accomplished by planned feedback, say an
pair of antiparallel diodes in the feedback path. Can you give some
comments on whether this also applies to ZCDs or if really supply based
limiting is necessary?
It is actually a specific design-trick on Collins behalf to saturate the
op-amp since this infact acts like a noise-gate. By having the output stage
saturated rather than operating linearly the noise as seen by the output RC
filter is that of only the saturated transistor and not that of the input
gained up. A diode limiter in the feedback path will maintain the op-amp in
the linear operating range and thus cause the noise to continue to polute the
output filter. What you can possibly acheive is the steer how deeply you run
into output saturation tought.
So, in this case op-amp saturation is a key trick to increased performance.
Not true, there's nothing magic about amplifier saturation, any means
that limits the amplifier output whilst dropping the small signal gain
to a low value will have exactly the same effect.
So you don't think the input-to-output gain is greatly affected when in
saturation? That is usually what happneds IMHO.
The gain in saturation will be less than 1. Thus, early noise sources will be
dampend rather than gained. In a diode clamp they will have unity gain rather
than gained. There's the difference.
In most cases recovery from saturation will be too slow for the later
stages of the ZCD.
Those amplifiers that have fast recovery from saturation usually employ
internal diode clamps.
A diode clamp in the feedback path will cut the noise gain to 1 when
either diode turns on. The following diode clamp across the filter
capacitor will reduce the noise gain to a very small value when it turns on.
Both diode clamps and internal saturation will still produce some output
noise although not from the amplifier input stages.
Well, this is true. But it again shifts the noise levels from the ideal zero.
The full gain is when the output saturates, but this has the drawback in
recovery time. I think the output saturation may be better for the first stage
where as a clamped variant is better for the following stages. For the first
stage you also have the noise out of the mixer to consider.
I might buy your argument better if you'd show me that the noise of the
saturated output stage is worse than the gain-of-1 alternative. If the noise
is just slightlty worse with the clamping, then its benefits outperforms the
loss in noise margin.
Cheers,
Magnus