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Pls help analyse this fm transmitter circuit.

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holax12

New Member
Hello,

Please kindly explain to me how the components of this fm transmitter works and how they are selected. The circuit diagram is located at a . Thanks.
 

crutschow

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Most Helpful Member
The circuit is a simple (and not particularly stable) one-transistor oscillator which is modulated by the audio input. The audio voltage changes the operating point of the transistor slightly which changes it's apparent collector capacitance. Since the collector capacitance is part of the oscillating tank circuit, the oscillation frequency is changed, giving FM modulation. This is described in more detail in the link you posted.

The component values are selected to bias the transistor at the desired operating point and to oscillate at the desired FM radio frequency.
 

BrownOut

Banned
If I understand FM modulators correctly, it's actually the B-E capacitance that is changed by the input signal. I think Collector cap. is changed very little.
 

holax12

New Member
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks
 

crutschow

Well-Known Member
Most Helpful Member
If I understand FM modulators correctly, it's actually the B-E capacitance that is changed by the input signal. I think Collector cap. is changed very little.
According to the link, the collector-base capacitance is changed due to the change in the base collector voltage which, I believe, is correct. The base-emitter voltage (and consequently its capacitance) is little changed by the audio input voltage.
 

holax12

New Member
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks
 

BrownOut

Banned
According to the link, the collector-base capacitance is changed due to the change in the base collector voltage which, I believe, is correct. The base-emitter voltage (and consequently its capacitance) is little changed by the audio input voltage.
I might have been wrong. Makes sense.
 
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BrownOut

Banned
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks
The feedback is always less than unity. The transistor has to make up for lossed in the feedback network ( C's and L's connected to the transistor Collector and Emitter )
 

crutschow

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Most Helpful Member
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks
What is meant is that the feedback loop gain is greater than unity. The transistor provides the gain for this.
 

unclejed613

Well-Known Member
Most Helpful Member
the feedback is through the 5pf cap to the emitter. the base is at RF ground, so the feedback to the emitter through the 5pf cap provides positive feedback to make the circuit oscillate. the reverse bias between B-C is part of the tuned circuit, and the audio voltage changes the bias, thereby changing the capacitance slightly. as the capacitance changes at an audio rate, you get frequency modulation of the oscillator. you adjust the deviation (the amount of frequency change) by adjusting the audio input level. the 100k resistor provides enough base bias to get oscillation, the .001uf cap is an RF bypass for the base, but is small enough to allow audio on the base
 

marcbarker

New Member
The 5 pF capacitor is only a little puffage, but has a big effect. There's a magical amount of pF here. I've seen this as an adjustable trimmer cap before. For a UHF TV transmitter I did in the early 80's, this cap was about 0.5 pF, it was two short insulated wires twisted together. You can double the transmit power by getting this cap optimised.

The problem with this circuit is that if a fly lands on the antenna, the tuning drifts. If you follow the transistor with a common-collector buffer driving the antenna instead, it's vastly improved. I used to use a dual-gate mosfet, with G2 strapped to the supply rail.
 

Hero999

Banned
Hello,

Please kindly explain to me how the components of this fm transmitter works and how they are selected. The circuit diagram is located at a . Thanks.
What FM transmitter, you've forgotten to attach the schematic.
 

MrAl

Well-Known Member
Most Helpful Member
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks

Hi,


That statement is a general statement that says something about getting
a real world LC oscillator to keep oscillating. In theory, a perfect LC
tank will oscillate indefinitely because there is no energy loss. In the
real world however, the inductor absorbs some energy and the cap does
too, and also we usually want to tap off a little energy to use for the
application. This means we need to keep pumping more energy into
the tank circuit to make sure it keeps oscillating. This is usually done with
an amplifier of some sort. We can not simply make the gain equal to
unity however, because the oscillations would die down as the tank
circuit used up the energy and caused less and less feedback, so we
make the gain slightly higher than unity and that keeps providing enough
energy to feed the LC tank as well as the losses that are ever present.
 
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Hero999

Banned

marcbarker

New Member
I saw it somewhere that for the LC circuit to oscillate it needs feedback and the feedback must be greater than the unity. I want explanation on what this mean and the component responsible for the feedback. Thanks
That 5pF capacitor I'd mentioned earlier is the culprit.

As MrAl says, it's a generalisation. This kind of circuit tends to attract overanalysis too, being so simple.

This is one of those circuits that it's best you just build it and experiment with it, trying things out. It's probably not worth getting too bogged down with the theory of how it works, because of how simple it is.
 
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