Cable length calibration
The 1503 was Tek's long-line (10km) TDR analyzer. IIRC, it put
haversine pulses of about 5v onto the line under test (~10v open
circuit). The 1502 (~600m line length) put pulses of about 200mV onto
the line under test (~400mV open circuit).
Best regards,
Charles
Scott wrote:
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
David had written:
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".
Scott had written:
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 6/30/16 5:52 PM, Tim Shoppa wrote:
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.
or rig it as a pendulum, and correct for the gravitational pull of the
moon and sun<grin>
Le 29 juin 2016 à 22:18, Poul-Henning Kamp phk@phk.freebsd.dk a écrit :
In message 20160629192850.19C2940605C@ip-64-139-1-69.sjc.megapath.net, Hal Mu
rray writes:
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?
TDR ?
If it wasn't behind a choke, the inrush current to the antenna
preamp power filtering capacitor could be measured, but the choke
ruins that.
The trouble is how to do it without frying the antenna preamp...
Seriously...
GPS antennas and receivers are cheap, I would just use two GPS antennas
with a known difference in cable-length.
Sounds simple, but even after a days reflection I don’t see how you find the complete path delay. You would get the cable delay (OPs concern) provided they were the same antenna/cable type combinations, but not delay induced by the antenna electronics. From another post that delay seems to be non-negligable. I find it curious that antenna manafacturers don’t seem to give this parameter. I looked at some datasheets on Trimble and Leica sites but they don’t have it.
As for me, I measure cable delay by injecting a 1PPS into it through a T and for the RG174 attached to the patch antennas, just sacrificed one by cutting the head off and measuring that.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Hi
On Jul 1, 2016, at 3:40 AM, Mike Cook michael.cook@sfr.fr wrote:
Le 29 juin 2016 à 22:18, Poul-Henning Kamp phk@phk.freebsd.dk a écrit :
In message 20160629192850.19C2940605C@ip-64-139-1-69.sjc.megapath.net, Hal Mu
rray writes:
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?
TDR ?
If it wasn't behind a choke, the inrush current to the antenna
preamp power filtering capacitor could be measured, but the choke
ruins that.
The trouble is how to do it without frying the antenna preamp...
Seriously...
GPS antennas and receivers are cheap, I would just use two GPS antennas
with a known difference in cable-length.
Sounds simple, but even after a days reflection I don’t see how you find the complete path delay. You would get the cable delay (OPs concern) provided they were the same antenna/cable type combinations, but not delay induced by the antenna electronics. From another post that delay seems to be non-negligable. I find it curious that antenna manafacturers don’t seem to give this parameter. I looked at some datasheets on Trimble and Leica sites but they don’t have it.
As for me, I measure cable delay by injecting a 1PPS into it through a T and for the RG174 attached to the patch antennas, just sacrificed one by cutting the head off and measuring that.
If you dig into the papers on calibrating the delay of the whole antenna, there are a bunch of things they run into. I suspect
the biggest one is quite simple: nobody but TimeNuts care. The survey guys are happy once they know the phase center. The
NIST category guys are going to calibrate it anyway. The cell phone outfits never did any real GPS time calibration, they did
it all on the other end of the system. Way to many cables in a cell site to measure them all ….
Bob
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Hi Mike:
For quite a while I was heavily into "chirp" transmissions. These are HF ionosphere radio transmissions that sweep from
2 to 30 MHz at 100 kHz/sec.
In order to "tune" the radio to a specific station (you can not tune by frequency) you need to know the start time
schedule for that specific station (time nuts content).
When GPS became popular the transmitters switched to GPS.
http://www.prc68.com/I/RCS-5A.shtml
You can use a pulse of RF to calibrate the time delay through your HF receiver to get a more accurate time of reception
value. That helps because with a GPS synchronized transmitter you can determine it's great circle distance from you.
Under some conditions you can see a transmission going around the Earth 2 or 3 times.
In a similar way if you used a pair non amplified versions of a GPS antenna back to back you could determine the time
delay of the pair and then divide by two.
--
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.
-------- Original Message --------
Le 29 juin 2016 à 22:18, Poul-Henning Kamp phk@phk.freebsd.dk a écrit :
In message 20160629192850.19C2940605C@ip-64-139-1-69.sjc.megapath.net, Hal Mu
rray writes:
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?
TDR ?
If it wasn't behind a choke, the inrush current to the antenna
preamp power filtering capacitor could be measured, but the choke
ruins that.
The trouble is how to do it without frying the antenna preamp...
Seriously...
GPS antennas and receivers are cheap, I would just use two GPS antennas
with a known difference in cable-length.
Sounds simple, but even after a days reflection I don’t see how you find the complete path delay. You would get the cable delay (OPs concern) provided they were the same antenna/cable type combinations, but not delay induced by the antenna electronics. From another post that delay seems to be non-negligable. I find it curious that antenna manafacturers don’t seem to give this parameter. I looked at some datasheets on Trimble and Leica sites but they don’t have it.
As for me, I measure cable delay by injecting a 1PPS into it through a T and for the RG174 attached to the patch antennas, just sacrificed one by cutting the head off and measuring that.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On 7/1/16 9:04 AM, Brooke Clarke wrote:
Hi Mike:
For quite a while I was heavily into "chirp" transmissions. These are
HF ionosphere radio transmissions that sweep from 2 to 30 MHz at 100
kHz/sec.
In order to "tune" the radio to a specific station (you can not tune by
frequency) you need to know the start time schedule for that specific
station (time nuts content).
When GPS became popular the transmitters switched to GPS.
http://www.prc68.com/I/RCS-5A.shtml
I'm building a satellite (actually 2 of them) that is, among other
things, designed to receive these transmissions. It turns out that
accurately measuring the "propagation delay" through the receiver (as in
from "EM wavefront" to "time stamped samples in the output stream" is
non trivial.
There's some phase shift/time delay from the physical interaction with
the antenna and the load impedance presented by the LNA. Then there's
the filters and amplifiers in the analog chain. Finally, there's the ADC
sampling (pipeline delay between voltage at sampling instant to when
bits appear at the output) delay, and the various delays through the
digital signal processing (which is fortunately deterministic, but
non-trivial to actually "measure")
Fortunately, I only claim 10 microsecond timing accuracy so the 100
kHz/second chirp means that the downconverted stream might have a
frequency error of 1 Hz. (that is, if I tell the receiver that the
chirp starts at 12:34:56.001, and I actually start the ramp at
12:34:56.00101, I'll see a 1 Hz error in frequency.. if I have a several
kHz output bandwidth, it will still be in there.
Given that ionospheric delays and propagation delays are substantially
longer than 10 microseconds, this isn't an issue. 1km is 3 microseconds,
1000km is 3 milliseconds (or 300 Hz).