[time-nuts] Data collection cycle -- multiple sources to one counter?
cfmd at bredband.net
Mon Jun 20 16:49:02 EDT 2005
From: "Dr. David Kirkby" <drkirkby at medphys.ucl.ac.uk>
Subject: Re: [time-nuts] Data collection cycle -- multiple sources to one counter?
Date: Mon, 20 Jun 2005 13:31:35 +0100
Message-ID: <42B6B727.7000207 at medphys.ucl.ac.uk>
> John Ackermann N8UR wrote:
> > I want to monitor all my atomic devices over the long term against GPS.
> > Rather than devoting one counter to each source and gathering a lot more
> > data than is useful, I'm going to use a GPIB-controlled coax switch to
> > route the 1pps from each source into the counter START input. That way
> > I can alternately read each source for X minutes against GPS.
> Just a thought (and probably a stupid one). Is the time delay through a PIN
> diode switch
> sufficiently repeatable to allow such measurements, or would any changes of
> delay with temperature, time etc mess up your measurements? Has anyone
> evaluated PIN diodes for this purpose? They would certainly get around any
> worries of wearing them out, as there are no moving parts.
It should be possible to use some form of solid state switching, but
dependencies to environmental aspects needs to be understood.
One of the things you do want is good isolation from the other input signals,
so the 5 MHz sine from one clock doesn't leak into the 5 MHz sine of another
clock and as these beat against each other (as they tend to do) the beating
appears as a component on the two clocks measurments. The signal-to-noise needs
to be low enought.
Dependencies on junction temperature, rail voltage (control current) and
signal level all comes into play. It would be fund to investigate such a
thing. Hmm... a bunch of 1N4148 should be interesting to test (just for fun).
Should be a good task for the network analyser to chew on. ;O)
Please note that the Mini-Circuits ones have DC-blockers and is listed from
10 MHz and upwards to whatever limit each device is listed for. If you look for
phase-stability you would want the -3 dB corners to be as far away from your
signal as possible as a rule of thumb, we know the phase changes alot at the
corners, but can also change alot in the pass-band. For 1 PPS signals DC levels
should be passed too.
> You can buy a 4-way single throw switch with SMA connectors for under $110
> which has a minimum of 40 dB (typcally 60dB) isolation at 10MHz.
Yes, but can we do better (and cheaper)?
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