[time-nuts] FE 5680A Warning

ws at Yahoo warrensjmail-one at yahoo.com
Sat Nov 12 13:22:11 EST 2011


Bert posted:
> "I am looking at ways to go  directly to the C field to disciplin the Rb. 
> Does any one have info that would  help to do so.

I don't know if this applies to your Rb but this is pretty typical of the 
cheapie Rb.
In the LPRO the C field is created from a low resistance coil that is 
grounded on one side and its current is the DC sum from a few resistors 
going to various fixed and variable voltages.
I just added one more resistor to supply additional current to the C field 
using a 12K 1% Ohm resistor connected to the High side of the C field coil.
This gives an additional  low sensitivity freq EFC input of about 2.3Hz / 
KVolt  with a nominal open circuit voltage near zero (120 mv). A 1e-12 freq 
change = 4.4 mv applied at the open end of the 12K resistor.
I then connect this EFC resistor to the DAC out of a TBolt that is set up to 
discipline an external osc.

The TBolt setup:
I use a times 100 extended range TC. The Tbolt gain is set to .230 Hz / V 
(x100 multiplier), damping set to 8.00 (sqr of 100 divisor).
This gives an effective TC value which is 100 times the TBolt's TC setting 
and a damping of 1/10 thus allowing useful time constants up to 100K sec.
I find a TC setting of 50 sec works best for my LPRO, giving an actual 
disciplined TC of 5000 seconds, and damping of 0.8.

On more advanced C field generators that use AC to cancel out magnetic 
fields, You would of course have to connect your "Summing" resistor to a 
different point in the circuit, but same Idea.
Just need to change the nominal C field Current a little using an external 
summing resistor connected to the right point.

ws

***************
EWKehren at aol.com EWKehren at aol.com
Sat Nov 12 13:54:35 UTC 2011

The FE 5680A's advertised as NEW are not NEW!  A  friend  that  made me
aware of the sale did notice clear signs of use, so I did a  closer 
inspection
under a Microscope and found clear indications that the unit  has been
bolted down in the past. Does work fine and I am looking at ways to go 
directly
to the C field to disciplin the Rb. Does any one have info that would  help
to do so.
Bert Kehren  Miami





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