AFC in an FM Oscillator - Page 1
Forum section: electronics / general

author: Scr1bbl3
author_rank: Contributor
timestamp: March 15, 2021, 04:21:20 am
content: Alright all you boffins, here's a circuit I'm just not sure about. This is a10.7MHz oscillator and a varactor for automatic frequency control. As we go above 10.7MHz, the voltage to the varactor's cathode drops. This increases capacitance. How will this cause the frequency to change accordingly. i.e. cause the oscillator to return to 10.7MHz.? There is a corresponding drop on the varactor anode - is a decrease in drive? Thanks for the assist.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2021, 05:09:39 am
content: Presumably, all the connected capacitors are forming a resonant network with L3? So, as that total capacitance increases, resonant frequency drops.This fact is...very basic, but it seems to be the only thing missing from your observation? -- if you're very new to this, might I suggest, say, some of the classic ARRL Handbooks? Lots of good information in there.Tim

author: Scr1bbl3
author_rank: Contributor
timestamp: March 15, 2021, 06:23:47 pm
content: Thanks, I guess I didn't explain myself particularly well. I know L3 and the variable caps (along with C10) form a resonant network. C9 provides positive feedback for the oscillator. The oscillator is on the collector while the varactor is on the emitter. How does this emitter capacitance change effect the oscillator tank circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2021, 06:42:44 pm
content: Ah. I don't know what tradeoffs they were going for, why they needed so many capacitors, or why they placed the varactor there specifically.It's not like the emitter is an ideal output. At these frequencies, everything has reactance, everything interacts. So, in general we should expect that connecting the varactor pretty much anywhere there, will have some effect.You've also cropped out the surrounding circuit, and component values (or they're on a completely separate sheet, I hate so much when they do that), so I can't tell which capacitors are bypass or anything. Even bypass capacitors can have some effect, not so much at 10MHz, but at the ~100MHz in what I assume is the RF/LO front end, elsewhere, even mere lead inductance has noticeable effect.Tim

author: emece67
author_rank: Frequent Contributor
timestamp: March 15, 2021, 06:54:31 pm
content: .

author: retrolefty
author_rank: Super Contributor
timestamp: March 15, 2021, 07:04:11 pm
content: I think we are missing the complete picture without showing how the DC varactor control voltage is developed to allow for a closed loop AFC function. This function allows the 10.7 mhz oscillator to deviate +/- to optimize IF performance.

author: Scr1bbl3
author_rank: Contributor
timestamp: March 15, 2021, 07:12:08 pm
content: The voltage comes from a ratio detector whose output is about 7.4V. The voltage at the junction of the varactor and R30 is about 3.4V. As the we drift up above 10.7MHz, the voltage at the junction decreases a few mV. Below 10.7MHz, the voltage goes up a few mV. The ratio detector acts as the sensing portion of the feedback.

author: Scr1bbl3
author_rank: Contributor
timestamp: March 15, 2021, 10:58:29 pm
content: The voltage comes from a ratio detector whose output is about 7.4V. The voltage at the junction of the varactor and R30 is about 3.4V. As the we drift up above 10.7MHz, the voltage at the junction decreases a few mV. Below 10.7MHz, the voltage goes up a few mV. The ratio detector acts as the sensing portion of the feedback.