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FM Transmitter

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wakiki

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I'm currently working on a FM transmitter and found this design on the web. I have the necessary equipments such as an osciloscope, function generator and etc in the lab. Can someone tell me how should i start in testing the transmitter as i'm actually builiding it on a protoboard first. Will the oscilloscope show modulated waveforms when i actually put in a let's say a 10mV of AC voltage acting as the input signal?Also, i'm trying to simulate the circuit using Multisim and Pspice 9.2. What analysis should i perform and which point of the circuit should i measure in order to see the modulated waveform on the simulation software?

I'm not even sure whether the design will work on the protoboard as the components is a bit far apart. I hope someone here could really help out.

Thanks :)

**broken link removed**

**broken link removed**
 
It's a crude 'bug' type transmitter, there are LOT'S of other threads on here about them. Being VHF layout is critical, it won't work on a breadboard - and veroboard layout requires care. Audioguru has posted pictures of his version on veroboard a number of times here.

Unless you have a 200+MHz scope you won't see anything, and in any case you won't really see the modulation.
 
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I've done a search and read most of the threads regarding these types of buggy FM transmitters. Yes,most of the replies states that modulation will no be visible on common oscilloscopes. Then i shall straightaway built it on the PCB instead. But if i were to simulate the circuit;

1. Where will the modulated waveform be genereated?Is it at the collector end of the second transistor (at the reactance modulator)?

2. Is the carrier sort of being generated at the oscillator cirucit(at the parallel resonance circuit) and being modulated at the second transistor?

3. For the input signal from the electret mic, can i use a AC signal source? Maybe a 10mV ac signal? Does the frequency matter for this? If yes, what frequency would you suggest?

I would really appreciate it if you could help me out with this. Sorry if I have too many questions. BTW, thanks for replying my thread. :)
 
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wakiki said:
I've done a search and read most of the threads regarding these types of buggy FM transmitters. Yes,most of the replies states that modulation will no be visible on common oscilloscopes. Then i shall straightaway built it on the PCB instead. But if i were to simulate the circuit;

1. Where will the modulated waveform be genereated?Is it at the collector end of the second transistor (at the reactance modulator)?

There are three stages:

1) Mike preamp
2) Oscillator/modulator
3) RF amplifier/buffer

2. Is the carrier sort of being generated at the oscillator cirucit(at the parallel resonance circuit) and being modulated at the second transistor?

The second transistor is an oscillator, with feedback from collector to emitter (common base), at the same time (for lower frequencies) it accepts an audio input in it's base. This produces changes in the internal capacitance which alters it's frequency - it gives a rather crude combination of both AM and FM.

3. For the input signal from the electret mic, can i use a AC signal source? Maybe a 10mV ac signal? Does the frequency matter for this? If yes, what frequency would you suggest?

1KHz is usual for audio testing, and yes you can.
 
Full modulation on the FM band is 150kHz peak to peak. So a 100MHz carrier has its frequency changed a max of only 150k/100M= 0.15% peak to peak and won't be seen on a 'scope.
 
Guys...what modifications should be done if I want to use a headphone jack as the audio input? The idea is I wanna use it with a portable player and hear its audio output through a radio. I've found some designs and they only need to modify the 39k resistor to a higher resistance. Any other modifications need to be done? Well, the audio quality is not that important now as I just hope to get some audio...I think knowing the output voltage at the headphone jack would help a lot...Isit??
 
Buy a commercial product if you're trying to do an ipod transmitter, even the 'bug' you're using is going to have significantly higher output power than is legal. Seeing as how the output frequency shift is limited to the input voltage change the output could be easily overmodulted on accident especially while you're tweaking with component values. If you over or under modulate it badly enough the entire RF output section might oscilate and you could be creating harmonics and noise who knows on what frequency.
 
Sceadwian said:
Buy a commercial product if you're trying to do an ipod transmitter, even the 'bug' you're using is going to have significantly higher output power than is legal. Seeing as how the output frequency shift is limited to the input voltage change the output could be easily overmodulted on accident especially while you're tweaking with component values. If you over or under modulate it badly enough the entire RF output section might oscilate and you could be creating harmonics and noise who knows on what frequency.

I'm not making an ipod transmitter...I'm just experimenting how can i modify the transmitter to receive an input via a headphone jack...Thanks for the info anyway :) Any other people can help?
 
The level from headphone jacks is more than 100mV. It is stereo so both channels need to be converted to mono.
The input for a toy FM transmitter is about 10mV and has a biasing resistor (for the microphone) that must be removed.

Make this circuit that combines the stereo channels and attenuates them:
 

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audioguru said:
The level from headphone jacks is more than 100mV. It is stereo so both channels need to be converted to mono.
The input for a toy FM transmitter is about 10mV and has a biasing resistor (for the microphone) that must be removed.

Make this circuit that combines the stereo channels and attenuates them:

audioguru..thanks so much for the help...so the L and R channels are the wires which split into both the left and right headphone rite??just wanna confirm...so i just need to connect these two wires to the attenuator and then to the transmitter's input rite?
 
The left and right channels have their own wires and do not "split". Connect them to the attenuator. Then connect the output of the attenuator to the input capacitor of the transmitter and remove its resistor that powereed its microphone. Connect the ground of the cable to the ground of the transmitter's circuit.

You will need to adjust the volume of the music source so that the FM transmitter isn't overloaded.

It is a very simple transmitter so its frequency drifts with temperature changes and when its battery voltage runs down. It doesn't have treble boost like FM radio stations have so it will sound like no treble on an FM radio.
It is a simple cheap toy.
 
audioguru said:
The left and right channels have their own wires and do not "split". Connect them to the attenuator. Then connect the output of the attenuator to the input capacitor of the transmitter and remove its resistor that powereed its microphone. Connect the ground of the cable to the ground of the transmitter's circuit.

You will need to adjust the volume of the music source so that the FM transmitter isn't overloaded.

It is a very simple transmitter so its frequency drifts with temperature changes and when its battery voltage runs down. It doesn't have treble boost like FM radio stations have so it will sound like no treble on an FM radio.
It is a simple cheap toy.

What i mean by split is with the L and R wires which connects to the headphones...Thanks anyway...
 
audioguru,the preamplifier is a common emitter amplifier rite..So how does the calculation for the gain vary as i have noticed that there is no voltage divider network present in the base. So does this mean that the output voltage will not be of proportionate value to the input voltage? Does the small signal cirucit equivalent still applies here, by omitting one of the resistors normally available in voltage divider networks of a common emitter amplifiers?
 
The preamp transistor has a single base bias resistor connected to its collector, so it has DC and AC negative feedback. There is a big long formula to determine its gain. If the gain is too high then turn down the level from the source.
 
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