On 9/4/17 1:18 PM, Wes wrote:
If these are COTS attenuators, their own return loss is unlikely to be
40 dB. In fact grabbing an old HP catalog off my bookshelf (I'm dating
myself) I see a typical type N attenuator specified as 1.2 VSWR (~21 dB
RL). I went on a quick "shopping" trip looking for an L-band, type N
bias tee. I'll spare you the links, but typically they are also rated
at 1.2 VSWR.
Be careful, that's the "catalog spec" which means "what we can
inexpensively measure"..
It's like SMA connectors, which are specified at 1.05:1 or 1.1:1 and
<0.3dB loss.
https://www.amphenolrf.com/connectors/sma.html
In reality, they are a LOT better, it's just that measuring that in a
production environment is tough.
I'd not want to set up a manufacturing test set that measured loss with
an uncertainty of 0.01 dB.
I'll point folks to:
Jesch's paper in 1976
http://ieeexplore.ieee.org/document/6312234/
and then Maury
https://www.maurymw.com/pdf/datasheets/5A-021.pdf
Someone at Maury did a paper which I can't find right now where they
measured a bunch of SMA connectors over hundreds if not thousands of
mate/demate cycles.
RF cafe has a nice summary
http://www.rfcafe.com/references/articles/Joe-Cahak/rf-connectors-cables-joe-cahak-6-2014.htm
Perhaps NIST, with an unlimited supply of tax money, splurged and
manufactured bias tees with >40 dB RL. Maybe they did the same with the
attenuators. We'll never know because they didn't provide an equipment
list or a measurement procedure. They said nothing about the cable
either, other than they started with RG-58 and replaced it with "better"
cable.
But you could probably send an email to the author and ask. NIST, like
JPL, is one of those places where people work forever. Tom Otoshi, who
wrote a report on N connectors in 1963 cited by Maury, above, still
works at JPL (I think.. I confess I haven't seen him recently, he might
have retired, but he was certainly around in the last 10 years), and
given the span of years, that N connector work was probably when he was
a just out of school engineer.
A few words about cable, since that is what this discussion is all
about. Cable, regardless of type and manufacturer, has its own RL, also
known in that business as Structural Return Loss (SRL) See:
https://www.belden.com/docs/upload/hdcarltp.pdf and
http://www.keysight.com/upload/cmc_upload/All/E206COMPTEST_METHOD.pdf.
At least the authors admit, "Thus far we have seen little difference in
the data."
Wes
On 9/3/2017 3:02 PM, Bill Byrom wrote:
For precision timing measurements, I would think that there would be
concern about the double reflections of a badly mismatched low loss
transmission line (such as using 75 ohm line in a 50 ohm environment).
The re-reflected signal will act similar to multipath (as a delayed
aggressor) on all satellite signals equally. The impedance mismatch
delayed reflection aggressor could aggravate timing errors due to
changes in temperature or stress in the cable. Whether this is important
for you depends on how time-nutty you want to get.
See these papers:
Effects of Antenna Cables on GPS Timing Receivers:
http://tf.boulder.nist.gov/general/pdf/1384.pdf
Absolute Calibration of a Geodetic Time Transfer System:
http://xenon.colorado.edu/paperIrevise2.pdf
--
Bill Byrom N5BB
_______________________________________________
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.
_______________________________________________
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.
