Cable length calibration
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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Yo Hal!
On Wed, 29 Jun 2016 12:28:50 -0700
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?
TDR. The GPS already sends a voltage down the cable to power the
antenna. Send a sharp down the cable and see when the reflection comes
back.
Even some of my cheap managed ethernet switches can do that.
RGDS
GARY
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588
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.
We have a smart antenna where the receiver is in the antenna housing and the link to the timing receiver is digital over coax. On that, we can run a variant of DTI (DocSis Timing Interface) which calibrates the cable delay automagically. Not sure what the original question was but yes, it's possible.
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces+greg.dowd=microsemi.com@febo.com] On Behalf Of Hal Murray
Sent: Wednesday, June 29, 2016 12:29 PM
To: Discussion of precise time and frequency measurement
Cc: hmurray@megapathdsl.net
Subject: [time-nuts] Cable length calibration
EXTERNAL EMAIL
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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Am Wed, 29 Jun 2016 12:28:50 -0700
schrieb Hal Murray hmurray@megapathdsl.net:
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.
Possibly a bit far-fetched, but Time-Domain Reflectometry might work
out. It wouldn't directly yield the physical cable length but rather
twice the propagation time, but as the time delay is what this all
would be about, this seems a fair deal. I'm sure this could in
principle be done, but haven't thought about it for long enough to have
an idea on how to address some more or less obvious obstacles.
Best regards,
Florian
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.
--
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Some of the cheap gigabit network cards can and with surprising accuracy
On 29 Jun 2016 22:02, "Gary E. Miller" gem@rellim.com wrote:
Yo Hal!
On Wed, 29 Jun 2016 12:28:50 -0700
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?
TDR. The GPS already sends a voltage down the cable to power the
antenna. Send a sharp down the cable and see when the reflection comes
back.
Even some of my cheap managed ethernet switches can do that.
RGDS
GARY
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588
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.
Am 29.06.2016 um 22:07 schrieb Gary E. Miller:
TDR. The GPS already sends a voltage down the cable to power the
antenna. Send a sharp down the cable and see when the reflection comes
back.
Even some of my cheap managed ethernet switches can do that.
What reflection? If the antenna preamp has at least a somewhat decent S22
(output return loss) there will be exactly nothing that comes back. And the
cable attenuation counts twice; GPS antennas have high gain to accommodate
a lot of attenuation.
regards, Gerhard
On Wed, 29 Jun 2016 20:18:49 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
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...
That's rather "simple". The impedance of the LNA is anything but
constant over frequency, said capacitor with the choke are one of
those things that make it different than 50 Ohm. You only need to
find one frequency at which the impedance is close to a short
or to an open and then send down a sine at that frequency and
measure the reflection. This is probably easier than doing TDR,
but unfortunately it is still quite involved.
Another way would be to send down a sine at a known frequency,
couple it out at the LNA and inject sharp pulses into the
antenna, at the rate of the sine. This way the whole path
through the antenna and the LNA down to the receiver can
be measured. It even makes it possible to measure the phase
differences between different frequencies (think L1, L2).
But this would require a custom receiver.
Attila Kinali
--
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Any simple idea will be worded in the most complicated way.
Yo Poul-Henning!
On Wed, 29 Jun 2016 20:18:49 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
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...
The TDR would only put the same voltage on the cable that it already
does. So nothing gets fried. If you are worried about the antenna,
disconnect it before doing the TDR.
It works on live ethernet cables, so it can't be too harsh.
A real good TDR can shouw you every tight bend in a cable.
Or buy a tape measure.
RGDS
GARY
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588