[time-nuts] FTS 1200

Ed Palmer ed_palmer at sasktel.net
Fri Aug 10 15:27:28 UTC 2012

Hi Bert,

I see on the data sheet that the tuning voltage is supposed to be -10 to 
+10 volts and that the supply voltage is +22 to +30 volts.  I suspect 
that one side of the varactor is supposed to be biased at one half of 
the supply voltage.  But in your case, it looks like that bias is now at 
zero volts due to an internal short - either a short between traces or, 
more likely, a capacitor failure.  When you apply a positive voltage, 
you're changing the DC voltages in the oscillator circuit which disrupts 
the oscillator.

When you apply a positive voltage to the EFC lead, is there a current 
flow into the lead?  Since a varactor is always supposed to be 
reverse-biased there shouldn't be any current.


On 8/10/2012 4:46 AM, EWKehren at aol.com wrote:
> Bjoern
> Thank you for the link I am able to change the frequency 4 Hz from - 2Hz (
> 0V)  to + 2 Hz (-12.2V) using pin 2. Reading the info that you got me
> probably explains the slot next to the connector, but I experience a much wider
> tuning range on pin 2 and John is right any positive voltage forward biases
> the  diode cutting off oscillation. Will do some testing.
> Thanks again Bert
> In a message dated 8/10/2012 6:28:17 A.M. Eastern Daylight Time,
> bg at lysator.liu.se writes:
> Bert,
> Good that you got the EFC working!   But I  would be a bit suspicious of
> needing -13V.
> It seems from:
> http://www.ece.gatech.edu/academic/courses/ece4007/08fall/ece4007l01/al4/dat
> asheets/symmetricon_oscillator_instructionsheet.pdf
> that  the default EFC configuration is (0 to +10)V with a range of 4e-7
> (2Hz).  From the same document there are a lot of other EFC configurations,
> but  none that goes outside of +-10V.
> My 1200 has about 3.5Hz tuning range  on (0,10)V. Se attached jpg. I did
> not check behavior on negative EFC  voltage.
> --
> Björn
>> John,
>>    that did the trick I can tune it with a negative voltage, minus 13 gives me plus 2 Hz but this unit came out of a FTS 5000 and it had a  positive
>> tuning voltage.
>> Bert
>> In a  message dated 8/9/2012 9:13:29 P.M. Eastern Daylight Time,
>>   jmiles at pop.net writes:
>>>   John
>>> Oven did  reduce in current and I can not imagine that it would be   that
>>> close with an overheated oven. At 0 V it is within .5 Hz of  what  they normally are. Ground has no effect but  even 0.8 V on pin 2  stops oscillation
>> That's a  suspicious-sounding voltage.  Are you  sure you're not
>>   forward-biasing the varicap?  Maybe some of these  OCXOs were  specified
>> for use with negative EFC voltage.
>>   If so, then  driving the diode with a negative voltage should raise  the
>> operating  frequency (which is what you  want.)
>> -- john,  KE5FX
>>   www.miles.io

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