# Understanding a circuit (local oscillator)

Discussion in 'General Electronics Chat' started by cam0, Apr 30, 2010.

1. ### cam0 Thread Starter New Member

Mar 23, 2010
25
0
Hi,

I ordered a soft rock, but I confess that I am a bit confused trying to understand exactly the circuit.

Here is the circuit:

This is a "local oscillator". X1 is a crystal oscillator at 28.224 MHz. C10 = 100pF, C11 is not used here.
There are many things I don't understand here, essentially on Q1:

• Why do we have R14 ? Removing it would give a better result, wouldn't it ? (why ?)
• Why C12 ? This is apparently a coupling capacitor, but again, why 22pF and why not removing it ? Besides, why taking the signal on the Emitter and not on the collector ?
• It sounds weird to me we already have an oscillator when it is supposed to be a colpitt oscillator. So why using X1 instead of an inductor ?
• I've seen that we can use transistors for amplification, but usually, the collector is somehow connected to the base (with resistors in between). But here, we have the base that is connected to the emitter... I did all the maths to solve this, but it doesn't really talk to me. Actually, if we wouldn't have C10 & C11 (and the shunt wire), it would have been easier to understand , so what's the goal of this part ?
• The second stage (Q2) is just used for amplification, it has a more "common" design I can understand.
I'm completely lost :S and I would really appreciate your help. Btw don't take it bad, I already asked these questions on the softrock group, but I am not getting the answers I am looking for and I know that here are very skilled people.

Thanks a lot for your help.

Apr 5, 2008
18,898
3,716
Hello,

Where did you find the kit?

Bertus

3. ### cam0 Thread Starter New Member

Mar 23, 2010
25
0

Apr 5, 2008
18,898
3,716
Hello,

The oscillator used is a colpitts oscillator.
The colpitts oscillator is descibed at figure 7.

Bertus

• ###### Crystal_oscillators.pdf
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5. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
If you think about the input and output waveforms on a common-emitter transistor stage, the output is inverted relative to the input.
Coupling fron the collector back to the base would give negative feedback.

An emitter follower stage has the output from the emitter _in_ phase with the input at the base, though there is no voltage gain.

The combination of crystal and feedback caps allow it to work as an oscillator.

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