[time-nuts] FTS 1200
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:
> 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:
>> 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:
>> Good that you got the EFC working! But I would be a bit suspicious of
>> needing -13V.
>> It seems from:
>> 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.
>>> 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.
>>> In a message dated 8/9/2012 9:13:29 P.M. Eastern Daylight Time,
>>> jmiles at pop.net writes:
>>>> 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
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