[time-nuts] Allan deviation -> finally some data!!!
didier at cox.net
Wed Oct 25 19:24:30 EDT 2006
Sorry that I did not specify the setup was the same as when described in
the post about the trigger outputs, as follows:
> >>>> I have a single 10 MHz sine signal fed to the START channel, and
> >>>> the 5370 is set to TI, MEAN, SAMPLE SIZE 1, + TI ONLY, START channel
> >>>> triggers on rise and STOP channel triggers on fall, and START COM is
> >>>> selected. The instrument displays about 60 nS (fairly stable, 150 ps
> >>>> jitter) or so at the moment.
It is obvious I am measuring half the period of the 10 MHz signal, give
or take the differential error due to the triggers not being perfectly
symetrical, instead of measuring the difference between the 2 time bases.
I guess that brings me back to my first question about the need for
proper procedure. Did not realize it would be back so soon :-)
Since I have only one readily accessible spare OCXO at the moment and I
want to get the procedure right before I take anything else apart, I
will feed the output of the 10 MHz reference from the 5370 into the
START channel, and my DUT into the STOP channel and select SEP instead
of START COM.
I believe that once I have the setup correct, I will rapidly find that I
need dividers on both OCXOs... 10 MHz will be too fast and I will be
skipping periods rapidly.
The easiest from there will probably be to hook up the GPS into the
START channel, and a divided down 10 kHz or 1 kHz on the STOP channel.
Maybe I can close the loop using the PC, driving the EFC input of the
OCXO from the serial port. That would allow me to conveniently adjust
the loop parameters.
I will report again when that is straightened. Sorry for the bandwidth.
At least, my logging program works :-)
Dr Bruce Griffiths wrote:
> It would be useful if you describe in some detail your measurement setup.
> Which signals are connected to which inputs on the 5370A?
> How long are the cables? etc.,
> Also given your source selection problems with the 5370A it may be
> instructive to run a simple test.
> Connect a low frequency square wave source to both the START and STOP
> inputs ensuring that the cable length to the STOP input is around 1m or
> so longer than the cable to the START input. Select TI and see if the
> time interval difference is around 5ns (for a 1m cable lenght
> difference) and the noise is substantially less than 100 picosec.
> Select the START and STOP inputs to trigger from the same edge of the
> input waveform.
> The jitter of the square wave source isn't critical.
> A resistive splitter can be used to ensure low reflections when both
> START and STOP inputs are selected for 50 ohm input.
> Otherwise the a BNC T can be used on the START input (select 1 megohm
> input ) with a 1m length of 50 ohm coax running from the BNC T to the
> STOP input which is set for 50 ohm input impedance. Reflections aren't
> a problem if the trigger thresholds are set correctly.
> The square wave source should be capable of producing an output of at
> least 800mV pp into 50 ohms.
> If you have nothing else the PPS output (suitably buffered) from a GPS
> receiver will work just fine.
> Typical measurement jitter for a good 5370 is about 35 picosec.
> If this works then you can have an increased confidence that the 5370 is
> indeed working as it should.
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