How Do You Calculate Component Values in an FM Transmitter Circuit?

[alexi0313]

Homework Statement

Hi I am weak at circuit, but I need to analyse how this circuit works and to determine the values of C4, C5 ,C6,C7, and L1.
I need help for the understanding function of the whole component and determine the values of these component

Homework Equations

f=1/2√LC

The Attempt at a Solution

Basically, from what I've tried to research over the internet, I make a speculation of the circuit.

First of all, the circuit starts with the electret microphone picks up sounds and produces a certain millivolt signal. The c1 couples the microphone to the input of the first stage which is the oscillator circuit form by a transistor ,R2 and R3.The R3 and R2 bias signal to the transistor and being amplified here. C1 also together with C2 form an Ac coupling to block the DC. The c2 passes the audio signal to the RF oscillator(Q2 transistor) meanwhile the R6 limits the input to the base of Q2.

Q2 ,C5 and L1 form tank circuit which act as tuning to govern the frequency of operation.
The c6 on the emitter taps off a small portion of the signal and passes it to the output stage. From here, we can determine the value of C5 and L1 by using the equation mentioned above.
Let say I want to tune the frequency at 101MHz, and the value of the inductor is set to 3.6uH, I will be getting capacitor value of 0.68pF.

So far ,this is what I have got, nothing much.
Please correct me if I analyse the circuit wrongly.
and help me with the values of the rest of the components.
Thanks.

 

[NascentOxygen]

Hi alexi0313. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

the first stage which is the oscillator circuit form by a transistor ,R2 and R3

Q1 is an audio amplifier. There is no tuned circuit here, so no obvious components to cause it to be an oscillator. Otherwise your description seems accurate. I´m not sure how to calculate the necessary value for C6, sorry.

 

[alexi0313]

Thanks NascentOxygen.

There is no tuned circuit here, so no obvious components to cause it to be an oscillator...I´m not sure how to calculate the necessary value for C6

But any chance you know what its function as well as the C4 and C7?

 

[NascentOxygen]

I´d say C6 might be the feedback that causes that stage to oscillate; does it look like positive feedback? C4 looks like it will make sure the base is at ground for RF; though it may also form a low-pass filter (with R6//R8) to limit the audio spectrum and thus limit the bandwidth of the RF output. Which is its real purpose, I don´t know. I´ve not had much to do with designing oscillators, sorry.

Your stage 2 appears to have a topology similar to that of the Hartley in Fig 6-1 (c) here, if you swap emitter with collector: http://dev.emcelettronica.com/fundamentals-transistors-transistor-oscillators

 

[rezeew]

Hi there,

It seems like you have a good understanding of the basic function of the circuit. The FM transmitter circuit is used to convert an audio signal into an FM radio signal, which can then be picked up by an FM receiver. Let's break down the function of each component in the circuit:

1. Electret microphone - This component picks up sound waves and converts them into an electrical signal.

2. C1 - This capacitor couples the microphone to the input of the first stage of the circuit. It blocks any DC signals and allows only the AC audio signal to pass through.

3. R2, R3 - These resistors are used to bias the first transistor (Q1) and provide a stable operating point for the transistor.

4. Q1 - This is the first stage of the circuit, also known as the audio amplifier stage. The audio signal from the microphone is amplified here and passed on to the next stage.

5. C2 - This capacitor is used for AC coupling between the first and second stages of the circuit. It blocks any DC signals and allows only the AC audio signal to pass through.

6. Q2 - This is the second stage of the circuit, also known as the RF oscillator stage. It takes the amplified audio signal from the first stage and converts it into an FM radio signal.

7. R6 - This resistor limits the input to the base of Q2, ensuring that the output signal is not overdriven.

8. C5, L1 - These components, along with Q2, form a tank circuit which acts as a tuning circuit to govern the frequency of operation. The values of C5 and L1 can be calculated using the equation you provided (f=1/2√LC). The values will vary depending on the desired frequency of operation.

9. C6 - This capacitor taps off a small portion of the output signal from Q2 and passes it to the final stage.

10. C4, C7 - These capacitors are used for filtering and smoothing the output signal.

In order to determine the values of C4, C5, C6, C7, and L1, you will need to know the desired frequency of operation and the specifications of the other components in the circuit. You can also use online calculators or simulation software to help you determine the values. It is important to note that the values may vary depending on the specific components you are using and the accuracy of the calculations

 

[DeeNos]

Hello, it seems like you have a good understanding of the circuit so far. The first step would be to determine the frequency range that this FM transmitter is designed to operate at. This will give you an idea of the values of C5, C6, C7, and L1. Once you have that information, you can use the equation you mentioned to calculate the values of these components. However, keep in mind that the values may need to be adjusted for practical purposes, such as component availability and stability. It may also be helpful to consult a textbook or reference material on FM transmitter circuits to get a better understanding of how these components work together. I hope this helps and good luck with your analysis!