# FM Transmitter Colpitt Oscillator

#### jewdai

##### New Member
I'm having trouble with the analysis of this circuit

I understand (going from left to right) C1 takes the DC decoupled voltage from the Microphone, amplifies this via Q1, C2 decouples it again, then R6 and C3 Low Pass Filter it, but once we get to Q2, I am completly lost.

My question is: How does this circuit act as a colpitt FM oscillator. Most of them I've seen are directly attached to the base.

Secondly how does this circut actually accomplish frequency modulation: Please use math on me!!! (I know that it should change the delta F term in the equation cos(2*pi*fc*t + δF) to achieve FM, or more correctly: cos(2*pi*fc*t + δf*∫(messageSignal))

How can I determine the Frequency deviation of this circuit?

Please use Engineering terminology and Mathematical equations in your analysis: Im a Senior year EE.

#### colin55

##### Well-Known Member
You may want a mathematical analysis of the circuit but can you explain the purpose of the 4p7 and 10n capacitors?

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#### mneary

##### New Member
Even without the math, it's a horrible circuit.

Frequency deviation is accomplished by changes to the base voltage. This affects the Base-Collector capacitance. The tuned circuit is also changed when anything gets near the antenna and of course it drifts as the battery wears down.

#### jewdai

##### New Member
You may want a mathematical analysis of the circuit but can you explain the purpose of the 4p7 and 10n capacitors?
the 10n is prolly part of a LPF and the 4p7 is prolly part of the colpitt oscillator at the end.

I realize this is a horrible design becuase the Qpoints are terrible (biasing Q1 at .9 volts doesnt allow for much negative voltage)

Can you explain how it changes the capacitance, my semiconductor theory about BJTs is a bit rusty.

#### jewdai

##### New Member
My goal is just to build a simple transistor level FM transmitter for my own amusement. More of an exercise on what I've learned as an engineer because I want my masters degree to be in Communications Systems and Signal Processing

#### BrownOut

##### Banned
I don't consider this a colpitts oscillator. Please see this thread for more information. There is a link to a pretty good analysis in that thread.

Edit: My bad, we were discussing a different transmitter in that thread, but some of the discussion might still be useful. I have an article on your transmitter, but I can't find it right now. If I run across it, I'll post.

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#### JimB

##### Super Moderator
My goal is just to build a simple transistor level FM transmitter for my own amusement. More of an exercise on what I've learned as an engineer because I want my masters degree to be in Communications Systems and Signal Processing
You are an engineer, and you are doing a Masters Degree, and you are bothering to analyse a toy circuit like that?
The circuit has so many shortcomings that is is does not come up to the level of "bad".

Do your self a favour and analyse the circuit of a REAL FM transmitter, one which has a defined, stable frequency, one which has sensible coupling to a defined antenna and harmonic supression filters.
One which has an output power of more than a milliwatt or so.
One which has defined modulation characteristics, adjustable deviation, a deviation limiter and appropriate AF frequency response.

JimB

#### jewdai

##### New Member
I would also like to actually build it, for fairly cheap. So I'm looking at what I can get.

Could you provide me with places that have such clearly explained and defined schematics? It becomes rather difficult to find such high level designs. I would really rather work at the transistor level (i.e., no OPAMPs or ICs, including multipliers)

##### New Member
As u know that u can provide arbitory signal from microphone, when this signal reaches to the base of Q1, it triggers your output which you will get at its collector and the thus the voltage at the base of Q2 will change and according to this voltage variation Q2 will generate the output at its collector.

finally you can see as your base voltage varies your fm frequency varies and hence you get the frequency deviation.....

To see it clearly you should go for its simulation...

#### mneary

##### New Member
I don't think modeling this circuit is practical. It depends on the changing output admittance (real and imaginary) of a transistor as its base voltage changes. I don't expect you will find such a detailed model of the 2N3904.

Somebody probably built this circuit either intentionally or by accident discovered the frequency varied as the base voltage.

If you want an inexpensive circuit that you can model and understand, build a real Colpitts oscillator and use a small varactor in parallel with its main tuning cap.

I found this real example in this presentation: http://www.technology.heartland.edu/faculty/chrism/data%20comm/Power%20Points/ECS%20PPTs/ch06.pps

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#### jewdai

##### New Member
Thats actually a really good presentation.

Some stuff i read up in a book today was about using a crystal oscilator in conjunction with the LC Colpitts or Hartley Oscillator

I wish I wasnt on break and I could access my Universty's library.

##### New Member
i think there is one more problem in this circuit, transistor Q1 2N3904 works in MHz range and this circuit utilizing a microphone that can not provide frequency in MHz range..... so one replace this transistor with any transistor that can work in audio frequency range...

datasheet of 2N3904 is given below...

View attachment 2n3904.pdf

#### mneary

##### New Member
i think there is one more problem in this circuit, transistor Q1 2N3904 works in MHz range and this circuit utilizing a microphone that can not provide frequency in MHz range..... so one replace this transistor with any transistor that can work in audio frequency range...
Sorry, couldn't be more wrong. The 2N3904 transistor works perfectly in the audio range, and it is extremely popular as a low power DC switch.