Cable length calibration
In message 20160629151528.543f499a@spidey.rellim.com, "Gary E. Miller" writes
:
It works on live ethernet cables, so it can't be too harsh.
Ethernet is incredibly robust, to the tune of a couple hundred volts
pretty much any way you can connect it.
Most GPS antenna preamps will croak before you get up to 10V.
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On Wed, 29 Jun 2016 22:57:30 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
Most GPS antenna preamps will croak before you get up to 10V.
The Fluke TDR is 4V max:
RGDS
GARY
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588
If the nominal velocity of propagation is known and length is known delay is easily determined mathematically
Time Domain Reflectrometry is the usual technique for finding cable length but even there the cables NVP is an essential parameter if you want to compute length but not essential in time nuts application because we are interested in delay which a TDR reads directly When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker. Also most TDRs like the Tek 1502 can put 100v or more on the cable which will blow most GPS antennas
On Jun 29, 2016, at 4:08 PM, Brooke Clarke brooke@pacific.net wrote:
Hi Hal:
I think the cal process is essentially a time domain reflection measure of cable length. The GPS receiver and the cable cal hardware would be in the antenna unit.
The 1 PPS signal would be aligned at the output of the cable.
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-------- Original Message --------
brooke@pacific.net said:
At one point they were looking into making a GPS time receiver where the
cable length calibration would be built-in.
How would you do that?
The obvious way is to compare the time you get with a known-good time, but if
you had that, why would you want this new GPS with an unknown cable length.
You might be able to do it by measuring the DC drop. Getting enough accuracy
seems tough.
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The Tektronix 1502 uses a tunnel diode pulser to produce a 50
picosecond output step of about 200 millivolts. There is a misprint
in the theory section of the service manual which says "400 V" instead
of "400mV".
On Thu, 30 Jun 2016 04:19:43 -0400, you wrote:
If the nominal velocity of propagation is known and length is known delay is easily determined mathematically
Time Domain Reflectrometry is the usual technique for finding cable length but even there the cables NVP is an essential parameter if you want to compute length but not essential in time nuts application because we are interested in delay which a TDR reads directly When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker. Also most TDRs like the Tek 1502 can put 100v or more on the cable which will blow most GPS antennas
Scott wrote:
When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker.
Someone else previously asked how you're expected to do TDR on a GPS
antenna cable since everything is matched (therefore, presumably, no
reflections).
There are ALWAYS reflections. As one message in this thread noted, TDR
easily shows sharp bends in coax. Plus, a GPS antenna will be "matched"
only at a relatively narrow band of frequencies centered on the
frequency(ies) it receives (L1, L1+L2, etc.). Also note that GPS
antennas specified as having 50 ohm outputs are often (if not usually)
used with 75 ohm cable. (Trimble actually advises this.)
You can do the TDR situationally (when the receiver is powered up, and
whenever an antenna fault is cleared), or repetitively during operation
(e.g., once a minute). I see no reason why either strategy would not
work well on a GPS antenna cable.
There is no need to use signals that would damage a GPS antenna preamp.
Best regards,
Charles
On 30 June 2016 at 09:19, Scott McGrath scmcgrath@gmail.com wrote:
If the nominal velocity of propagation is known and length is known delay
is easily determined mathematically
Except that coax does not have a uniform impedance or velocity factor. Both
will vary as a function of position and frequency. How relevant this is
depends on the accuracy you require, but since it is time-nuts, it is
reasonable to assume that such a simplistic method is not of the standard
expected on time-nuts.
Dave
I'd say it depends on the Time Nut 😀. NVP X Distance will get you close and for a beginning time nut is a worthwhile exercise
To improve delay calculations now you need instrumentation that not all especially beginning time nuts own. I've got a 20 Ghz Agilent scope/TDR along with a 110 Ghz network analyzer and just putting a cable on the network analyzer and handling it you can see the characteristics change So yes you are correct that the simple NVPxDistance is not suitable for advanced time nuttery. In fact the delay will change with temperature and barometric pressure unless you are using hardline and that's subject to humidity unless filled with dry nitrogen under pressure.
Content by Scott
Typos by Siri
On Jun 30, 2016, at 5:03 PM, Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby@kirkbymicrowave.co.uk wrote:
On 30 June 2016 at 09:19, Scott McGrath scmcgrath@gmail.com wrote:
If the nominal velocity of propagation is known and length is known delay
is easily determined mathematically
Except that coax does not have a uniform impedance or velocity factor. Both
will vary as a function of position and frequency. How relevant this is
depends on the accuracy you require, but since it is time-nuts, it is
reasonable to assume that such a simplistic method is not of the standard
expected on time-nuts.
Dave
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As a practical matter, in the lab we seldom need a cable delay
measured to better than +/- 0.5 ns, which we usually do as a time
interval measurement, with a 1 pps into a tee on channel A of a TIC
and then the cable from the tee to channel B.
For cables up to 40 m or so, just measuring the physical length is as
accurate (experimentally determined!).
A while back, EURAMET ran a pilot where a few lengths of cable were
circulated amongst a number of NMIs who then had to measure the
delays. The reported scatter was about +/- 1 ns, as I recall. This
mainly came down to differences in test signals, trigger levels etc.
Cheers
Michael
On Fri, Jul 1, 2016 at 7:03 AM, Dr. David Kirkby (Kirkby Microwave
Ltd) drkirkby@kirkbymicrowave.co.uk wrote:
On 30 June 2016 at 09:19, Scott McGrath scmcgrath@gmail.com wrote:
If the nominal velocity of propagation is known and length is known delay
is easily determined mathematically
Except that coax does not have a uniform impedance or velocity factor. Both
will vary as a function of position and frequency. How relevant this is
depends on the accuracy you require, but since it is time-nuts, it is
reasonable to assume that such a simplistic method is not of the standard
expected on time-nuts.
Dave
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and follow the instructions there.
This is highly dependent on the TDR especially ones designed for long twisted pair runs where a high voltage pulse is used to overcome resistive losses
Content by Scott
Typos by Siri
On Jun 30, 2016, at 10:59 AM, David davidwhess@gmail.com wrote:
The Tektronix 1502 uses a tunnel diode pulser to produce a 50
picosecond output step of about 200 millivolts. There is a misprint
in the theory section of the service manual which says "400 V" instead
of "400mV".
On Thu, 30 Jun 2016 04:19:43 -0400, you wrote:
If the nominal velocity of propagation is known and length is known delay is easily determined mathematically
Time Domain Reflectrometry is the usual technique for finding cable length but even there the cables NVP is an essential parameter if you want to compute length but not essential in time nuts application because we are interested in delay which a TDR reads directly When using a TDR its best if cable is unterminated as the discontinuity at the end is helpful as a marker. Also most TDRs like the Tek 1502 can put 100v or more on the cable which will blow most GPS antennas
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We are all time nuts, so there's an obvious answer: What you do, is raise
the GPS up to a height the same as the cable length. You then drop it,
measure the time until it hits the ground, and use d = 0.5 a * t * t to
calculate d. Then you correct for the velocity factor.
Tim N3QE
On Wed, Jun 29, 2016 at 3:28 PM, Hal Murray hmurray@megapathdsl.net wrote:
brooke@pacific.net said:
At one point they were looking into making a GPS time receiver where the
cable length calibration would be built-in.
How would you do that?
The obvious way is to compare the time you get with a known-good time, but
if
you had that, why would you want this new GPS with an unknown cable length.
You might be able to do it by measuring the DC drop. Getting enough
accuracy
seems tough.
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