[time-nuts] Data collection cycle -- multiple sources to one counter?

John Ackermann N8UR jra at febo.com
Sun Jun 19 08:44:53 EDT 2005

Brooke Clarke wrote:

> Hi John:
> If you have Time RAIM turned on then using GPS as the start pulse is
> good, that way if the RAIM shuts off the GPS no measurement will be
> started.  Using GPS as the stop pulse would result in an extra long
> measurement.
Interesting idea -- though I thought that the convention was to put the
reference source on STOP and the unknown on START.

> I'm using 1000 seconds per reading on the SR620.  I like it in that
> you get new readings fairly often so if something changes you don't
> have to wait forever.  The sigma is almost always in the 9 ns area,
> which is the saw tooth size for the M12T+.

Makes sense; my idea of 15 minutes (900 seconds) is close to that.  I
guess one consideration is what gives you a convenient tau at the output
-- with three sources, 20 minutes would give you a tau of 1 hour, and
reduce the amount of switching.

> I moved an HP 59307A Dual 4-way HP-IB coax switch here to do just what
> you describe, but am not happy with the performance of the FTS4060. 
> The longer you measure each source the fewer relay cycles the switch
> needs to do, making it last longer.

That's the same box I'm using.  I got a bunch of them, and some of the
matching open-frame relay boxes, from a guy a couple of years ago and
they do come in handy.

> When working with the HP 4380S Balanced Network Analyzer system I
> found that using flexible cables resulted in an unstable system.  My
> first improvement was to use 0.141" semi rigid coax (it's much more on
> the rigid side) and got a mandrel to bend it.  Later I discovered you
> can get flexible 0.141" coax that's not "rigid" and it worked great,
> no mandrel needed.  The point is that you probably will need to have
> good control on all the coax lines.  The lines will change length with
> temperature, so the longer they are the more temperature variation
> there will be.

That's a really good point.  I don't think it's feasible to use rigid or
semi-rigid cable in the setup I'm doing now, but that certainly would be
the best answer for maximum stability.  The good news is that all the
cables are relatively short (6 feet or less).


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