[time-nuts] E1938A oscillator pinning
bruce.griffiths at xtra.co.nz
Mon Aug 27 20:35:29 EDT 2007
SAIDJACK at aol.com wrote:
> Hi Bruce,
> I got it to work, it even locks to -5Hz within 5 minutes and the Amber LED
> goes off.
> The problem was that I had only applied GND to the shield of the connector.
> I added a wire to the EFC pin to EFC return, and connected the three GND pins
> of the connector, and magically it started working.
> Seems one of those "GND" pins is actually a "heater on" type of pin.
> Now I will do some experiments, it seems EFC should be 2.5V for the output
> to be on frequency (not 0V).
> Another interesting observation: my unit has 2.532V on the reference output.
> I could probably try to locate the reference, and replace it with something
> more modern and stable.
> Lastly, the unit seems to have a slow 2-4 second sinusoidial drift in parts
> to the -11 when measuring it's output on my 53132A after about 10 minutes
> warmup. Wonder if that's the oven heater control loop...
There are 3 LT1121I 2.5V references on the board, one located near the
ADC, one near the devices with heatsinks and yet another near the 4 pin
header at the rear of the board (end nearest the D - connector).
There may also be one inside the oven - the osc.pdf paper indicates this.
As long as the voltage is stable the exact value is unimportant.
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