[time-nuts] Non-impedance matched antenna cables
magnus at rubidium.dyndns.org
Sat Jun 14 04:08:30 EDT 2008
From: "phil" <fortime at bellsouth.net>
Subject: Re: [time-nuts] Non-impedance matched antenna cables
Date: Sat, 14 Jun 2008 02:43:07 -0400
Message-ID: <003501c8cde9$e896a540$8201a8c0 at officemail>
> I can appreciate the implication of all these small variables and the
> wanting to better "every last nanosecond of variation"
> My comment was, at least as I see it, only with a more expensive receiver
> and better timebase will these "smaller" variables be significantly
> relevant. Since the discussion revolved around the Thunderbolt receiver, I
> would think it's internal errors are far greater than the expansion of the
> antenna cable or cable impedance.
While I agree with the general comment that simpler receivers does not require
more advanced error models and concerns to control errors, the Thunderbolt has
a few things going which puts M12M-T to shame. The diciplined OCXO (vs
undiciplined TCXO) controls the timing of the GPS receiver. That removes the
PPS measurement problem (aka sawtooth and sawtooth correction). Infact, it
seems to report deviations with a resolution of 10 ps rather than 1 ns.
These errors comes out of the GPS time solution. The phase stability of the
OCXO is better than the normal TCXO and the cyclic re-alignment is also avoided
as the oscillator nominally tracks GPS time fairly quickly after it has done
The Thunderbolt is still only a single frequency C/A receiver. Multipath is one
of several concerns. I have not experimented with all relevant options yeat.
I run RG-58 cable. Not the best in class by far, but it was lying around
awaiting to be used and I had connectors to it.
Pointer to good phase stable cable with suitable connectors would be
Oh. The electrical length change for many reasons. Physical length change is
only part of it. The dielectrum change properties with temperature, and the
changed wave-equation may change speed significantly larger than the length
extention. I suspect humidity in cable to be part of it too.
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