[time-nuts] FTS 1200

[Bob Camp]

Hi

Normally when you forward bias a varicap, nothing strange happens. The capacitance goes way up as it goes forward and then it pretty much turns into an RF short circuit. The series resistance of the device drops considerably as this all takes place. If it's in series with the crystal, the frequency will just go way low. In this case way low might be 5 or 10 Hz. The drive level should go up, as opposed to stopping oscillation. 

Of course there are a lot of weird circuits out there. This one may be set up strange. 

Bob

On Aug 10, 2012, at 2:09 PM, EWKehren at aol.com wrote:

> Ed
> I think so too. I hope if it is a shorted capacitor it stays but looking at 
> for instance at the bias of a 10811. There would be no capacitor right at 
> the  diode. I have the tuning network for a 1130 and it shows a 20 K to an 
> internal  +12 V
> Bert
> 
> 
> In a message dated 8/10/2012 2:01:22 P.M. Eastern Daylight Time,  
> ed_palmer at sasktel.net writes:
> 
> Bert,
> 
> I wasn't suggesting amps of current.  A normal  reverse-biased diode 
> would give nanoamps of current flowing out.   Microamps (or more) of 
> current flowing in would show that the internal  biasing of the varactor 
> was messed up.  A bad diode could also show  current flowing in, but your 
> success with negative voltage biasing  suggests that the diode could be 
> okay.
> 
> Ed
> 
> On 8/10/2012 11:31 AM,  EWKehren at aol.com wrote:
>> Ed
>> 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.
>> 
>> Ed
>> 
>> 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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