[time-nuts] FTS 1200

EWKehren at aol.com EWKehren at aol.com
Fri Aug 10 17:31:56 UTC 2012

I did not see any current, but that is also due to the fact that there is  
most likely at least 10 K between pin 2 and the varactor. Most likely if it 
was  biased for instance + 12 Volt, there would also be a resistor before 
the  diode. I suspect John is right that with my + voltage the diode was 
forward  biased and blocked oscillation.  Maybe this one was strictly - tuning  
voltage, hope so. I  will run it for a time and observe stability. May be  
similar to HP 10811 bias.
Thanks   Bert
In a message dated 8/10/2012 11:28:46 A.M. Eastern Daylight Time,  
ed_palmer at sasktel.net writes:

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 
> 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:
>  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  
> 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  
>> 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    
>>> 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 
>> That's a   suspicious-sounding voltage.  Are you  sure you're  not
>>   forward-biasing the varicap?  Maybe some of  these  OCXOs were  
>> for use with negative EFC  voltage.
>>   If so, then  driving the  diode with a negative voltage should raise  
>>  operating  frequency (which is what you   want.)
>> -- john,  KE5FX
>>    www.miles.io

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