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, [email protected] 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,
[email protected] 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, [email protected] 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,
[email protected] 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,
[email protected] 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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