Discussion:
How can I measure time-delay of a cable with HP 5370B time-interval counter?
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Dr. David Kirkby
2018-10-29 00:49:31 UTC
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Permalink
I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.

I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.

Loading Image...

I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.

The switch position on the counter are as shown here

Loading Image...

So the main settings are

* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)

With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of

time = distance / velocity = 0.686 / 3e8
= 2.29 ns.

I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator

1 kHz - unstable readings, around 100~300 us.
10 kHz -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns

The numbers look believable with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were

1 MHz -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns

Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.

I must be missing something, but I'm not sure what it is.
--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100
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Bob Bownes
2018-10-29 01:31:34 UTC
Reply
Permalink
Have you tried with just a single pulse?
Post by Dr. David Kirkby
I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.
I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/Path-is-signal-generator-to-start-then-stop.jpg
I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.
The switch position on the counter are as shown here
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/switch-postitions.jpg
So the main settings are
* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)
With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of
time = distance / velocity = 0.686 / 3e8
= 2.29 ns.
I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator
1 kHz - unstable readings, around 100~300 us.
10 kHz -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns
The numbers look believable with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were
1 MHz -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns
Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.
I must be missing something, but I'm not sure what it is.
--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100
_______________________________________________
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John Ackermann. N8UR
2018-10-29 01:32:21 UTC
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Permalink
I suspect it's triggering or aliasing issues if you're using high frequency sine waves.  The canonical way to do that measurement is with a fast-rise-time edge at PPS sorts of rates.  And you'd normally use a common reference to reduce the number of variables.

John
Post by Dr. David Kirkby
I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.
I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/Path-is-signal-generator-to-start-then-stop.jpg
I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.
The switch position on the counter are as shown here
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/switch-postitions.jpg
So the main settings are
* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)
With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of
time = distance / velocity = 0.686 / 3e8
= 2.29 ns.
I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator
1 kHz - unstable readings, around 100~300 us.
10 kHz -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns
The numbers look believable with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were
1 MHz -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns
Note, the function generator and counter do not share a common
frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.
I must be missing something, but I'm not sure what it is.
--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100
_______________________________________________
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
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Tom Van Baak
2018-10-29 09:42:28 UTC
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David,

Just to see if your setup is working:

1) Set the pulse generator to as fast a risetime as possible; ns or less. Use a low pulse rate (100 Hz is fine).
2) Use a BNC tee at the generator, into two equal 2 meter cables, each one into a 5370B input.
3) Set manual trigger, 50R, 1.0 V, DC
4) Now collect time interval data in block/stats mode. You should see a mean of under +/-1 ns and a stdev in low ps.

/tvb


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Dr. David Kirkby
2018-10-29 15:23:49 UTC
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Post by Tom Van Baak
David,
1) Set the pulse generator to as fast a risetime as possible; ns or less.
Use a low pulse rate (100 Hz is fine).
Unfortunately, I don't have such a pulse generator, so I can't run that
test. But it is clear the system is sensitive to the trigger levels, so I
guess is the problem. I have done all the confidence checks in the manual
on this TI counter before and it was fine.

Also, I am interested in the delay of the cable at low frequencies, as I
suspect that might depart significantly from the usual figure based on the
"velocity factor". Certainly the impedance of coax rise at low frequencies
because the normal formula

Zo = sqrt(L/C)
is not valid before a few MHz. A more accurate formula is

Zo = sqrt ( (R + j w L )/ ) / (G + j w C))

where R = Resistance per unit length
L = inductance per unit length
G = Conductance per unit length
C = Capacitance per unit length.

So feeding in short pulses brings the validity of such a test into
question.
Post by Tom Van Baak
2) Use a BNC tee at the generator, into two equal 2 meter cables, each one
into a 5370B input.
3) Set manual trigger, 50R, 1.0 V, DC
4) Now collect time interval data in block/stats mode. You should see a
mean of under +/-1 ns and a stdev in low ps.
/tvb
Thank you. I will look for a pulse generator. I would use the 1 pps from
the GPS receiver, but my HP 58503A GPS receiver has decided to pack up. I
need to have a look at that, but it is not the highest priority task just
now.

This possible trigger issue metioned by Hal Murray is probably a result of
the knobs not exactly lining up with the positions they are in. It is
fairly clear that the marker on one of them is vertical at 0 V, but the
other is not. I need to try to get the knob back on the shaft in a slightly
different position. But they were both set to preset, but it is clear that
the reading is sensitive to the trigger points.

I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don't have
anything really fast.

I have a VNA which can make measurements of phase difference down to 300
kHz, but don't trust those because of the fact one calibrates with a 50 ohm
load, 50 ohm calibration standards, yet I know the impedance will rise well
above 50 ohms at low frequencies.

I was looking for a different approach than a VNA, to make comparisons with
a VNA.

Dave
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Mark Goldberg
2018-10-29 16:29:07 UTC
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On Mon, Oct 29, 2018 at 8:25 AM Dr. David Kirkby <
Post by Dr. David Kirkby
I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don't have
anything really fast.
I have a VNA which can make measurements of phase difference down to 300
kHz, but don't trust those because of the fact one calibrates with a 50 ohm
load, 50 ohm calibration standards, yet I know the impedance will rise well
above 50 ohms at low frequencies.
I was looking for a different approach than a VNA, to make comparisons with
a VNA.
Dave
My MDO3024 (a modern scope) can measure delay and phase between two inputs,
using the measured zero crossings, including statistics of the measurement.
If you can obtain a similar scope, you could use two identical 1 Meg probes
and a separate termination.

Regards,

Mark
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Bill Byrom
2018-11-01 06:31:50 UTC
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Sorry for the delayed response, David. It took me a while to do a bit of research and I have some clues to why you measured those confusing results. Please refer to the HP5370B operation and service manual at the following link.
The specifications are on pages 1-3 and 1-4 (pages 12-13 of the PDF).
The input amplifier schematic diagram is on part three of page 8-85 fold out (page 256 of the PDF).

https://literature.cdn.keysight.com/litweb/pdf/05370-90031.pdf?id=713250

You have the following connections, following the cable from the source to the termination:

(1) Sinewave generator: If you want to measure the low frequency characteristics of the cable, you will need to use a sine waveform. If you use a fast risetime pulse or square wave you will measure the characteristics at frequencies related to the risetime of the pulse, not the period or width of the pulse.

(2) BNC TEE connector at the Start input of the HP5370B: You have the switches for the Start channel set to 1 M ohm, divide by 1, AC coupling, and rising edge. The input impedance of the Start input is 1 M ohm in parallel with <50 pF, and the BNC TEE adds a few more pF and also a few mm of propagation distance between the center of the Tee and the reference plane at the Start connector. At low frequencies these effects can be safely ignored, but I'm guessing that at 30 MHz you might see an affect if you are measuring with 3 digit resolution. The AC coupling capacitor and 1 Mohm termination produce a highpass RC filter with a corner frequency of about 16 Hz. At 10 MHz this is inconsequential, but at 1 kHz this will cause some noticeable phase errors.

(3) BNC termination at the Stop input of the HP5370B: You have the switches for the Stop channel set to 50 ohm, divide by 1, AC coupling, and rising edge. The combination of AC coupling and 50 ohm termination used at low frequencies is one of your main problems. The combination of the 10 nF (0.01 uF) series AC coupling capacitor and 50 ohm termination after that coupling capacitor produces a high pass RC filter with a corner frequency of about 318 kHz. At the corner frequency the Stop input amplifier amplitude is 0.707 of the voltage at the BNC TEE, and at 10 kHz the counter voltage comparator has a very small input swing due to the highpass characteristics. The phase is affected greatly, and at the corner frequency of 318 kHz the counter sees a 45 degree phase error due to the AC coupling highpass filter. That's a 393 time delay error due to the AC coupling at that frequency, and the effect gets worse as the frequency is reduced.

Skin effect below 100 MHz: In addition to the methodology (wrong measurement tool and setup), I question the premise behind the test. I believe that the low frequency (below 30 MHz) dielectric properties of common RG-58 and similar coaxial cable are reasonably stable versus frequency. But the skin depth depends on frequency, and this will change the resistance and more importantly the internal inductance of the center conductor. At very low frequencies the AC fields penetrate throughout the center copper wire conductor and this causes internal inductance. As the frequency is increased the skin depth produces a significant reduction in the cross sectional area where conduction can occur, so the inductance is lowered.

The crossover frequency where the AC resistance (due to skin effect) equals the DC resistance of a copper conductor depends on the wire diameter as shown in figure 4 of this informal paper:
http://www.ve2azx.net/technical/CoaxialCableDelay.pdf

Figure 7 in that paper shows a graph of the calculated delay (ns per meter of cable length) of RG-58 coax between 1 MHz and 1 GHz on a logarithmic frequency scale. You can see that the calculated change in the propagation delay is about:
1 MHz: 1.630 ns per meter
10 MHz: 1.573 ns per meter
100 MHz: 1.555 ns per meter
1 GHz: 1.550 ns per meter

Are you really distributing low frequency sinewave signals, or pulses with a low repetition frequency?
--
Bill Byrom N5BB
Post by Dr. David Kirkby
Post by Tom Van Baak
David,
1) Set the pulse generator to as fast a risetime as possible; ns or less.
Use a low pulse rate (100 Hz is fine).
Unfortunately, I don't have such a pulse generator, so I can't run that
test. But it is clear the system is sensitive to the trigger levels, so I
guess is the problem. I have done all the confidence checks in the manual
on this TI counter before and it was fine.
Also, I am interested in the delay of the cable at low frequencies, as I
suspect that might depart significantly from the usual figure based on the
"velocity factor". Certainly the impedance of coax rise at low frequencies
because the normal formula
Zo = sqrt(L/C)
is not valid before a few MHz. A more accurate formula is
Zo = sqrt ( (R + j w L )/ ) / (G + j w C))
where R = Resistance per unit length
L = inductance per unit length
G = Conductance per unit length
C = Capacitance per unit length.
So feeding in short pulses brings the validity of such a test into
question.
Post by Tom Van Baak
2) Use a BNC tee at the generator, into two equal 2 meter cables, each one
into a 5370B input.
3) Set manual trigger, 50R, 1.0 V, DC
4) Now collect time interval data in block/stats mode. You should see a
mean of under +/-1 ns and a stdev in low ps.
/tvb
Thank you. I will look for a pulse generator. I would use the 1 pps from
the GPS receiver, but my HP 58503A GPS receiver has decided to pack up. I
need to have a look at that, but it is not the highest priority task just
now.
This possible trigger issue metioned by Hal Murray is probably a result of
the knobs not exactly lining up with the positions they are in. It is
fairly clear that the marker on one of them is vertical at 0 V, but the
other is not. I need to try to get the knob back on the shaft in a slightly
different position. But they were both set to preset, but it is clear that
the reading is sensitive to the trigger points.
I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don't have
anything really fast.
I have a VNA which can make measurements of phase difference down to 300
kHz, but don't trust those because of the fact one calibrates with a 50 ohm
load, 50 ohm calibration standards, yet I know the impedance will rise well
above 50 ohms at low frequencies.
I was looking for a different approach than a VNA, to make comparisons with
a VNA.
Dave
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David C. Partridge
2018-10-29 10:38:52 UTC
Reply
Permalink
If you have a Tektronix 7000 series 'scope, then a 7S12 equipped with a S52
pulse generator and an S6 sampler head will talk all the pain out of
measuring cable length and will also show you any impedance mismatches.
This way you don't need to suspect a bad cable, you can prove it's bad.

If you need a cable checked, I can do it as I have 7S12 plugins.
Dave

-----Original Message-----
From: time-nuts [mailto:time-nuts-***@lists.febo.com] On Behalf Of Dr.
David Kirkby
Sent: 29 October 2018 00:50
To: time-***@lists.febo.com
Subject: [time-nuts] How can I measure time-delay of a cable with HP 5370B
time-interval counter?

I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.

I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.

https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/Path-is-signal-g
enerator-to-start-then-stop.jpg

I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.

The switch position on the counter are as shown here

https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/switch-postition
s.jpg

So the main settings are

* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)

With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of

time = distance / velocity = 0.686 / 3e8
= 2.29 ns.

I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator

1 kHz - unstable readings, around 100~300 us.
10 kHz -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns

The numbers look believable with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were

1 MHz -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns

Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.

I must be missing something, but I'm not sure what it is.
--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100
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time-nuts mailing list -- time-***@lists.febo.com
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Artek Manuals
2018-10-29 11:57:00 UTC
Reply
Permalink
David

First let me say that I have never used a 5370B

Ignoring the lower frequency stuff for the moment, Can you measure (
accurately) the trigger levels of both the start and stop gates? Slight
differences in the trigger points at each end will will obviously add
error in the measurement.

The next flag for thought is your comment "assuming a velocity factor of
.7" What if the velocity factor is really .66 ? This would account for
almost half of the error.

Rerun you data with larger and smaller output levels from the function
generator. Increasing error" with lower signal levels (or vice versa
lower erros with increased voltage ) would implicate a trigger
differences as the source of error as well

Dave
***@artekmanuals.com

David on another note I tried replying to you directly and the email
bounced ?

On 10/29/2018 6:38 AM, David C. Partridge wrote:

<SNIP>
Post by Dr. David Kirkby
If
I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.
I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/Path-is-signal-g
enerator-to-start-then-stop.jpg
I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.
The switch position on the counter are as shown here
https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/switch-postition
s.jpg
So the main settings are
* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)
With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of
time = distance / velocity = 0.686 / 3e8
= 2.29 ns.
I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator
1 kHz - unstable readings, around 100~300 us.
10 kHz -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns
The numbers look believable with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were
1 MHz -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns
Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.
I must be missing something, but I'm not sure what it is.
--
Dave
***@ArtekManuals.com
www.ArtekManuals.com


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Chris Caudle
2018-10-29 15:01:32 UTC
Reply
Permalink
Post by Artek Manuals
The next flag for thought is your comment "assuming a velocity factor of
.7" What if the velocity factor is really .66 ? This would account for
almost half of the error.
Propagation velocity has an inverse dependence on permittivity, and
permittivity changes with frequency. Electrical delay time will not be
constant with frequency because of that.

In addition to the fundamental physics at play, there are instrumentation
difficulties. The rise time at low frequencies is long enough that any
50Hz/60Hz interference from power line related current flow can modify the
trigger point and influence the measurement. The fast rise time signals
proposed for evaluating the measurement setup get around that, but then of
course you are measuring a wideband signal, which rather misses the point
of the original goal of measuring vs. frequency, so at some point after
verifying the setup basics you will have to go back to narrow band
signals.

A 5370 is a somewhat coarse instrument for this type of measurement, a VNA
which has a suitable lower measurement frequency would probably be more
suitable.
--
Chris Caudle





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and follow the instructions there.
s***@wp.pl
2018-10-29 22:01:36 UTC
Reply
Permalink
Hello, Maybe this method will help: 1. connect splitter to generator ,any two coaxes to splitter and then connect them to start and stop channel of TIC (CAB I and CAB II) 2. Generate any time interval (period, pulse width etc) 3. Measured time interval is zero - level, reference level(TI0) 4. Connect coax under test to CAB II and then TIC channel 5. Generate the same time interval as in p.2 6. Measured TI is TI1 7. Replace CAB II and coax under test, generate the same time interval as in p.2 8. Measured TI is TI2 9. Calculate delay: T = TI0 - (TI1+TI2)/2 For best result, use the same timebase for generator and TIC




Dnia 29 października 2018 17:00 time-nuts-***@lists.febo.c napisał(a):


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Today&#39;s Topics:

 1. Re: How can I measure time-delay of a cable with HP 5370B
    time-interval counter? (David C. Partridge)
 2. Re: How can I measure time-delay of a cable with HP 5370B
    time-interval counter? (Artek Manuals)
 3. Re: Question about noisetypes and ADEV (Attila Kinali)
 4. Re: Question about noisetypes and ADEV (Magnus Danielson)
 5. Re: How can I measure time-delay of a cable with HP 5370B
    time-interval counter? (Chris Caudle)
 6. Re: How can I measure time-delay of a cable with HP 5370B
    time-interval counter? (Dr. David Kirkby)


------------------------------

Message: 1
Date: Mon, 29 Oct 2018 10:38:52 -0000
From: &#34;David C. Partridge&#34; &lt;***@perdrix.co.uk
To: &#34;&#39;Discussion of precise time and frequency measurement&#39;&#34;
&lt;time-***@lists.febo.com&gt;
Subject: Re: [time-nuts] How can I measure time-delay of a cable with
HP 5370B time-interval counter?
Message-ID: &lt;004801d46f73$96e047b0$c4a0d71
Content-Type: text/plain; charset=&#34;us-ascii&#34;

If you have a Tektronix 7000 series &#39;scope, then a 7S12 equipped with a S52
pulse generator and an S6 sampler head will talk all the pain out of
measuring cable length and will also show you any impedance mismatches.
This way you don&#39;t need to suspect a bad cable, you can prove it&#39;s bad.

If you need a cable checked, I can do it as I have 7S12 plugins.
Dave

-----Original Message-----
From: time-nuts [mailto:time-nuts-***@list On Behalf Of Dr.
David Kirkby
Sent: 29 October 2018 00:50
To: time-***@lists.febo.com
Subject: [time-nuts] How can I measure time-delay of a cable with HP 5370B
time-interval counter?

I&#39;m trying to do something which would seem conceptually easy, but I&#39;m
getting results I can&#39;t understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.

I&#39;m feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here&#39;s
a photo of the complete setup.

www.kirkbymicrowave.co.uk www.kirkbymicrowave.co.uk
enerator-to-start-then-stop.jp

I&#39;ve set the 5370B&#39;s START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.

The switch position on the counter are as shown here

www.kirkbymicrowave.co.uk www.kirkbymicrowave.co.uk
s.jpg

So the main settings are

* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)

With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of

time =  distance / velocity = 0.686 / 3e8
= 2.29 ns.

I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I&#39;m getting the following readings, for
different frequencies of the function generator

1 kHz - unstable readings, around 100~300 us.
10 kHz  -&gt; -21.3 us
50 kHz -&gt; -4.27 us
100 kHz -&gt; -1.90 us
250 kHz -&gt; - 528 ns
500 kHz -&gt; 1.837 us
1 MHz -&gt; 956 ns
2 MHz -&gt; 490 ns
3 MHz -&gt; -2.6 ns
4 MHz -&gt; -0.33 ns
5 MHz -&gt; 0.90 ns
6 MHz -&gt; 1.50 ns
7 MHz -&gt; 1.93 ns
8 MHz -&gt; 2.15 ns
9 MHz -&gt; 2.38 ns
10 MHz -&gt; 2.52 ns
11 MHz -&gt; 2.60 ns
20 MHz -&gt; 2.85 ns
30 MHz -&gt; 2.80 ns

The numbers look believable  with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were

1 MHz  -&gt; -26.51 ns
5 MHz -&gt; 9.70 ns
10 MHz -&gt; 9.70 ns
15 MHz -&gt; -57.81 ns
20 MHz -&gt; -41.64 ns
30 MHz -&gt; 7.13 ns

Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.

I must be missing something, but I&#39;m not sure what it is.

--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
www.kirkbymicrowave.co.uk www.kirkbymicrowave.co.uk
Tel 01621-680100 / +44 1621-680100
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------------------------------

Message: 2
Date: Mon, 29 Oct 2018 07:57:00 -0400
From: Artek Manuals &lt;***@ArtekManuals.com&gt;
To: time-***@lists.febo.com
Subject: Re: [time-nuts] How can I measure time-delay of a cable with
HP 5370B time-interval counter?
Message-ID: &lt;b0ac7b03-c460-7e6d-1d5d-163fd
Content-Type: text/plain; charset=windows-1252; format=flowed

David

First let me say that I have never used a 5370B

Ignoring the lower frequency stuff for the moment, Can you measure (
accurately) the trigger levels of both the start and stop gates? Slight
differences in the trigger points at each end will will obviously add
error in the measurement.

The next flag for thought is your comment &#34;assuming a velocity factor of
.7&#34; What if the velocity factor is really .66 ? This would account for
almost half of the error.

Rerun you data with larger and smaller output levels from the function
generator. Increasing error&#34; with lower signal levels (or vice versa
lower erros with increased voltage ) would implicate a trigger
differences as the source of error as well

Dave
***@artekmanuals.com

David on another note I tried replying to you directly and the email
bounced ?

On 10/29/2018 6:38 AM, David C. Partridge wrote:

&lt;SNIP&gt;

If

I&#39;m trying to do something which would seem conceptually easy, but I&#39;m
getting results I can&#39;t understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.

I&#39;m feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here&#39;s
a photo of the complete setup.

www.kirkbymicrowave.co.uk www.kirkbymicrowave.co.uk
enerator-to-start-then-stop.jp

I&#39;ve set the 5370B&#39;s START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.

The switch position on the counter are as shown here

www.kirkbymicrowave.co.uk www.kirkbymicrowave.co.uk
s.jpg

So the main settings are

* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)

With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of

time =  distance / velocity = 0.686 / 3e8
= 2.29 ns.

I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I&#39;m getting the following readings, for
different frequencies of the function generator

1 kHz - unstable readings, around 100~300 us.
10 kHz  -&gt; -21.3 us
50 kHz -&gt; -4.27 us
100 kHz -&gt; -1.90 us
250 kHz -&gt; - 528 ns
500 kHz -&gt; 1.837 us
1 MHz -&gt; 956 ns
2 MHz -&gt; 490 ns
3 MHz -&gt; -2.6 ns
4 MHz -&gt; -0.33 ns
5 MHz -&gt; 0.90 ns
6 MHz -&gt; 1.50 ns
7 MHz -&gt; 1.93 ns
8 MHz -&gt; 2.15 ns
9 MHz -&gt; 2.38 ns
10 MHz -&gt; 2.52 ns
11 MHz -&gt; 2.60 ns
20 MHz -&gt; 2.85 ns
30 MHz -&gt; 2.80 ns

The numbers look believable  with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were

1 MHz  -&gt; -26.51 ns
5 MHz -&gt; 9.70 ns
10 MHz -&gt; 9.70 ns
15 MHz -&gt; -57.81 ns
20 MHz -&gt; -41.64 ns
30 MHz -&gt; 7.13 ns

Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.

I must be missing something, but I&#39;m not sure what it is.



--
Dave
***@ArtekManuals.com
www.ArtekManuals.com


---
This email has been checked for viruses by Avast antivirus software.
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------------------------------

Message: 3
Date: Mon, 29 Oct 2018 14:59:08 +0100
From: Attila Kinali &lt;***@kinali.ch&gt;
To: Discussion of precise time and frequency measurement
&lt;time-***@lists.febo.com&gt;
Subject: Re: [time-nuts] Question about noisetypes and ADEV
Message-ID: &lt;20181029145908.f7fde144af2bc1
Content-Type: text/plain; charset=ISO-8859-1

Moin,

I&#39;m bunching a few mails together, to not clutter the mailinglist too much

On Sat, 27 Oct 2018 23:25:30 +0200
Magnus Danielson &lt;***@rubidium.dyndns.org&gt; wrote:


The integration is very important aspect, as a number of assumptions
becomes embedded into it, such as the f_H frequency which is the Nyquist
frequency for counters, so sampling interval is also a relevant
parameter for expected level.


An important thing to note here is that Gaussian white noise is,
as it is defined, non-continuous (by any continuity measure).
Ie if you take two samples, no matter how close they are time-wise,
their difference in value can be arbitrary large. If you are integrating
over (time-continuous) Gaussian white noise, you have to argue
carefully, why this integral is defined (meaning why calculating it
leads to a single, well defined value). In our case, it&#39;s usually
enough to assume that there is a finite cut-off frequency at which
point the signal falls off with at least 1/f^2 (or &gt;=40dB/dec) to
ensure 1) continuity and 2) convergence of the integral.

For more details, see a textbook on Ito-calculus, e.g. [1]

On Sat, 27 Oct 2018 23:43:33 +0200
Magnus Danielson &lt;***@rubidium.dyndns.org&gt; wrote:


A simple trick to transform uniform distribution to normal distribution
like shape is to take 12 samples and add them together. A special trick
is to take them pair-wise and subtract them and then add 6 differences,
to avoid DC bias of typical uniform distribution generation (as typical
pseudo-noise generators does not have all 0 state in them). The result
of this subtract-add trick is a normal distribution like thing with the
standard distribution of 1. More or fewer sample-pairs can be added if
the product is scaled appropriately.

The Box-Mueller algorithm is another way to convert uniform distribution
to normal distribution.


Please, plase, please, do not use &#34;just 12 samples and add them together&#34;
as an approach for generating normal distributed values! Even if it will
get you something that looks like a normal distribution, it&#39;s quite far
from it. It is also a very slow method and uses up a lot of randomnes.

Box-M?ller is a usable alternative, though I would recommend using
the Ziggurat Method[2], which is very fast and leads to a very good
approximation. When I replaced the &#34;take 30 samples and add them&#34; of
Fran?ois Vernotte&#39;s Sigmatheta package[3] and used the Ziggurat Method,
combined with xorshift1024*[4] for random number generation, I got
a total speed up of a factor of more than 2 (including the FFT and
everything)[5] (yes, I know that xorshift1024* does have some problems
in the quality of random numbers generated, but they shouldn&#39;t be
relevant for the application at hand).


Attila Kinali

[1] &#34;Stochastic Differential Equations&#34;, by Bernt ?ksendal, 2013 (6th ed)

[2] &#34;The Ziggurat Method for Generating Random Variables&#34;,
by Marsaglia and Tsang, 2000
dx.doi.org dx.doi.org

[3] theta.obs-besancon.fr theta.obs-besancon.fr

[4] xoshiro.di.unimi.it xoshiro.di.unimi.it
or more specifically: xoroshiro.di.unimi.it xoroshiro.di.unimi.it

[5] git.kinali.ch git.kinali.ch
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
                Miss Matheson, The Diamond Age, Neal Stephenson



------------------------------

Message: 4
Date: Mon, 29 Oct 2018 15:28:28 +0100
From: Magnus Danielson &lt;***@rubidium.dyndns.org&gt;
To: time-***@lists.febo.com
Cc: ***@rubidium.se
Subject: Re: [time-nuts] Question about noisetypes and ADEV
Message-ID: &lt;2abfba15-f9f7-2ff6-bff8-0ec46
Content-Type: text/plain; charset=utf-8

Hi Attila,

On 10/29/18 2:59 PM, Attila Kinali wrote:

Moin,

I&#39;m bunching a few mails together, to not clutter the mailinglist too much

On Sat, 27 Oct 2018 23:25:30 +0200
Magnus Danielson &lt;***@rubidium.dyndns.org&gt; wrote:


The integration is very important aspect, as a number of assumptions
becomes embedded into it, such as the f_H frequency which is the Nyquist
frequency for counters, so sampling interval is also a relevant
parameter for expected level.


An important thing to note here is that Gaussian white noise is,
as it is defined, non-continuous (by any continuity measure).
Ie if you take two samples, no matter how close they are time-wise,
their difference in value can be arbitrary large. If you are integrating
over (time-continuous) Gaussian white noise, you have to argue
carefully, why this integral is defined (meaning why calculating it
leads to a single, well defined value). In our case, it&#39;s usually
enough to assume that there is a finite cut-off frequency at which
point the signal falls off with at least 1/f^2 (or &gt;=40dB/dec) to
ensure 1) continuity and 2) convergence of the integral.


There is aspects of noise which is more or less important depending on
what you do. As we leave WPM it is no longer gaussian anyway. For ADEV
and friends the shape of the PDF isn&#39;t as important as for other things.
The slope of the frequency range is however the important one. It is
only when we do the confidence intervals where the Gaussian shape
becomes relevant for the Chi-square bounds, but those are usually not
precise enough that even rough Gaussian shape is relevant. Even for
noises with none-Gaussian properties, the Chi-square seems to be valid
enough.

For other measures, like bit-error simulations, proper Gaussian shape is
much more important, but only to a certain point. For higher BER values,
the details of the outer part of the shape isn&#39;t all that important,
it&#39;s only as you push into lower BER numbers you need to care.


For more details, see a textbook on Ito-calculus, e.g. [1]

On Sat, 27 Oct 2018 23:43:33 +0200
Magnus Danielson &lt;***@rubidium.dyndns.org&gt; wrote:


A simple trick to transform uniform distribution to normal distribution
like shape is to take 12 samples and add them together. A special trick
is to take them pair-wise and subtract them and then add 6 differences,
to avoid DC bias of typical uniform distribution generation (as typical
pseudo-noise generators does not have all 0 state in them). The result
of this subtract-add trick is a normal distribution like thing with the
standard distribution of 1. More or fewer sample-pairs can be added if
the product is scaled appropriately.

The Box-Mueller algorithm is another way to convert uniform distribution
to normal distribution.


Please, plase, please, do not use &#34;just 12 samples and add them together&#34;
as an approach for generating normal distributed values! Even if it will
get you something that looks like a normal distribution, it&#39;s quite far
from it. It is also a very slow method and uses up a lot of randomnes.


Actually, for many simulations you do not need better &#34;shape&#34;.
There is some simulations where shape comes in, but others where it has
little to no consequence.


Box-M?ller is a usable alternative, though I would recommend using
the Ziggurat Method[2], which is very fast and leads to a very good
approximation. When I replaced the &#34;take 30 samples and add them&#34; of
Fran?ois Vernotte&#39;s Sigmatheta package[3] and used the Ziggurat Method,
combined with xorshift1024*[4] for random number generation, I got
a total speed up of a factor of more than 2 (including the FFT and
everything)[5] (yes, I know that xorshift1024* does have some problems
in the quality of random numbers generated, but they shouldn&#39;t be
relevant for the application at hand).


Getting suitable PRNG polynomials isn&#39;t all that hard, if the length of
the &#34;random&#34; sequence is of concern compared to the length of the
sequence used. It&#39;s a solved problem.

Never the less, thanks for the many references. Will read up on them
eventually.

Cheers,
Magnus



Attila Kinali

[1] &#34;Stochastic Differential Equations&#34;, by Bernt ?ksendal, 2013 (6th ed)

[2] &#34;The Ziggurat Method for Generating Random Variables&#34;,
by Marsaglia and Tsang, 2000
dx.doi.org dx.doi.org

[3] theta.obs-besancon.fr theta.obs-besancon.fr

[4] xoshiro.di.unimi.it xoshiro.di.unimi.it
or more specifically: xoroshiro.di.unimi.it xoroshiro.di.unimi.it

[5] git.kinali.ch git.kinali.ch





------------------------------

Message: 5
Date: Mon, 29 Oct 2018 10:01:32 -0500
From: &#34;Chris Caudle&#34; &lt;***@chriscaudle.org&gt;
To: time-***@lists.febo.com
Subject: Re: [time-nuts] How can I measure time-delay of a cable with
HP 5370B time-interval counter?
Message-ID:
&lt;4c942efd15520e6d47bdb068bf3b
Content-Type: text/plain;charset=iso-8859-1

On Mon, October 29, 2018 6:57 am, Artek Manuals wrote:

The next flag for thought is your comment &#34;assuming a velocity factor of
.7&#34; What if the velocity factor is really .66 ? This would account for
almost half of the error.


Propagation velocity has an inverse dependence on permittivity, and
permittivity changes with frequency.  Electrical delay time will not be
constant with frequency because of that.

In addition to the fundamental physics at play, there are instrumentation
difficulties.  The rise time at low frequencies is long enough that any
50Hz/60Hz interference from power line related current flow can modify the
trigger point and influence the measurement.  The fast rise time signals
proposed for evaluating the measurement setup get around that, but then of
course you are measuring a wideband signal, which rather misses the point
of the original goal of measuring vs. frequency, so at some point after
verifying the setup basics you will have to go back to narrow band
signals.

A 5370 is a somewhat coarse instrument for this type of measurement, a VNA
which has a suitable lower measurement frequency would probably be more
suitable.

--
Chris Caudle







------------------------------

Message: 6
Date: Mon, 29 Oct 2018 15:23:49 +0000
From: &#34;Dr. David Kirkby&#34; &lt;***@kirkbymicrowave.co.u
To: Tom Van Baak &lt;***@leapsecond.com&gt;, time-***@lists.febo.com
Subject: Re: [time-nuts] How can I measure time-delay of a cable with
HP 5370B time-interval counter?
Message-ID:
&lt;CANX10hDi1yY82fXJZvYDVomnnAX
Content-Type: text/plain; charset=&#34;UTF-8&#34;

On Mon, 29 Oct 2018 at 09:43, Tom Van Baak &lt;***@leapsecond.com&gt; wrote:


David,

Just to see if your setup is working:

1) Set the pulse generator to as fast a risetime as possible; ns or less.
Use a low pulse rate (100 Hz is fine).



Unfortunately, I don&#39;t have such a pulse generator, so I can&#39;t run that
test. But it is clear the system is sensitive to the trigger levels, so I
guess is the problem. I have done all the confidence checks in the manual
on this TI counter before and it was fine.

Also, I am interested in the delay of the cable at low frequencies, as I
suspect that might depart significantly from the usual figure based on the
&#34;velocity factor&#34;. Certainly the impedance of coax  rise at low frequencies
because the normal formula

Zo = sqrt(L/C)
is not valid before a few MHz. A more accurate formula is

Zo = sqrt ( (R + j w L )/ ) / (G + j w C))

where R = Resistance per unit length
L = inductance per unit length
G = Conductance per unit length
C = Capacitance per unit length.

So feeding in short pulses brings the validity of such a test into
question.



2) Use a BNC tee at the generator, into two equal 2 meter cables, each one
into a 5370B input.
3) Set manual trigger, 50R, 1.0 V, DC
4) Now collect time interval data in block/stats mode. You should see a
mean of under +/-1 ns and a stdev in low ps.

/tvb



Thank you. I will look for a pulse generator. I would use the 1 pps from
the GPS receiver, but my HP 58503A GPS receiver has decided to pack up. I
need to have a look at that, but it is not the highest priority task just
now.

This possible trigger issue metioned by Hal Murray is probably a result of
the knobs not exactly lining up with the positions they are in. It is
fairly clear that the marker on one of them is vertical at 0  V, but the
other is not. I need to try to get the knob back on the shaft in a slightly
different position. But they were both set to preset, but it is clear that
the reading is sensitive to the trigger points.

I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don&#39;t have
anything really fast.

I have a VNA which can make measurements of phase difference down to 300
kHz, but don&#39;t trust those because of the fact one calibrates with a 50 ohm
load, 50 ohm calibration standards, yet I know the impedance will rise well
above 50 ohms at low frequencies.

I was looking for a different approach than a VNA, to make comparisons with
a VNA.

Dave


------------------------------

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End of time-nuts Digest, Vol 171, Issue 38
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Dan Kemppainen
2018-10-30 14:49:01 UTC
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Post by Mark Goldberg
My MDO3024 (a modern scope) can measure delay and phase between two inputs,
using the measured zero crossings, including statistics of the measurement.
If you can obtain a similar scope, you could use two identical 1 Meg probes
and a separate termination.
Regards,
Mark
Hi,

The problem with this is typically the sample rate of the scopes fall
when looking at the lower frequencies, and you loose time resolution.
With an MDOxxxx series scope setting the record length to 10M would help
to keep the sample rate high. Then post processing the data external to
the scope to best fit a sine may result in something you could use to
calculate a good zero crossing.

I just tried a 10M length on the MDO3104, and that limits the sample
rate to 2.5GS/s, about 400pS per sample with a few cycles of the sine
available to look at. So it may be possible.

Also, Many scopes allow channels to be deskewed relative to each other.
This would probably be something one would want to do before attempting
a test like this. The MDO3104 allows deskew to a few pS, not sure about
the MDO3024. If I get time I'll check the MDO3024 later.

Using a longer cable may help get this into a range where the
measurement may be able to made with a good modern scope and a little
post processing.

Dan

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