Am I going crazy??? - Page 1
Forum section: electronics / projects

author: Mr J
author_rank: Regular Contributor
timestamp: August 24, 2016, 12:22:25 pm
content: Hi everyone,I was just going over some class materials for the Boyscout Radio Merit Badge and found an oscillator schematic on page 32 that looks like it will not work. I am going crazy or is there no feedback loop to keep the oscillator going?

author: Whales
author_rank: Super Contributor
timestamp: August 24, 2016, 12:33:37 pm
content: Hmm.Current entering the base will pull the top of the crystal down. Then eventually the voltage at the top of the 10K resistor drops, prohibiting the transistor from conducting. After the inductor collapses the top of the 10K resistor charges up again, also pulling the top of the crystal and base up. Repeat.

author: Mr J
author_rank: Regular Contributor
timestamp: August 24, 2016, 12:36:16 pm
content: here is a close one that I breadboard and does work, look like the one on the top is missing a cap between Base and Emitter?

author: Andy Watson
author_rank: Super Contributor
timestamp: August 24, 2016, 12:40:29 pm
content: Capacitance between the collector and base provide the feedback.

author: JimRemington
author_rank: Regular Contributor
timestamp: August 24, 2016, 05:51:56 pm
content: Quote from: Mr J on August 24, 2016, 12:36:16 pmTry breadboarding the original. I'll bet it works too.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 24, 2016, 06:06:21 pm
content: Quote from: JimRemington on August 24, 2016, 05:51:56 pmThe breadboard will take care of the feedback for you.

author: StillTrying
author_rank: Super Contributor
timestamp: August 24, 2016, 07:47:07 pm
content: Quote from: Andy Watson on August 24, 2016, 12:40:29 pmIs that positive or negative feedback?

author: Andy Watson
author_rank: Super Contributor
timestamp: August 24, 2016, 09:17:04 pm
content: Quote from: StillTrying on August 24, 2016, 07:47:07 pm"Feedback" was probably a poor choice of word. The collector-base capacitance couples the output tuned-circuit back to the input tuned-circuit, i.e. the crystal. Try searching for "tuned plate tuned grid oscillator".

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 25, 2016, 12:59:06 am
content: It's a Miller crystal oscillator.

author: StillTrying
author_rank: Super Contributor
timestamp: August 25, 2016, 02:32:13 am
content: Quote from: Andy Watson on August 24, 2016, 09:17:04 pmAfter looking at 100s of oscillator circuits I've not been able to convince myself that the effect between a valve's anode and grid is the same as that between a transistor's collector an base."Therefore, if the plate is slightly more positive, the voltage induced on the grid (the opposite plate of this small "capacitor) will be negative."http://electronicstheory.com/COURSES/ELECTRONICS/e101-58.htm

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 25, 2016, 02:58:12 am
content: There is only one way to find out, datasheet time!EDIT: Emitter-Base = 25pFCollector-Base = 8pF

author: Mr J
author_rank: Regular Contributor
timestamp: August 25, 2016, 12:50:46 pm
content: Quote from: Cyberdragon on August 25, 2016, 12:59:06 amThats it!!!! thanks Cyber