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, [email protected] 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, > [email protected] 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, [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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
