[time-nuts] FTS 1200

Ed Palmer ed_palmer at sasktel.net
Fri Aug 10 18:00:46 UTC 2012


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.


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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