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Modulate a transmitter with a signal generator? Answered

I'm doing some experiments with an fm transmitter. Now, instead of transmitting and modulating audio, I wanna try to feed the base of transistor using a ramp generated by a NE555. I ended up with this schematic http://s30.postimg.org/pbe0oyu4x/base_sawtooth_modulation.png , would be ok to pass the signal on the base using a 220 ohm resistor and a 10nF capacitor? I suspect that I have to increase the value of the resistor R5.

kind regards.


Don't see this is a problem - try it.

Thank you for your concise answer.
Which behaviour can I expect modulating the transistor with the sawtooth signal? The ramp will also change the output frequency?

Kind regards.

Not exactly - The carrier will have a rather harsh buzz on it (assuming your modulating at audio frequency). i.e the carrier will be modulated at audio frequency (i assume)

Your FM transmitter operates by having the audio shift the output frequency my the amount of audio input - are you wanting to sweep the carrier frequency over a range?

Yes: I attempt to sweep the signal over a wide range of frequencies.

I choosed a sawtooth signal to drive the transistor because I know that a sawtooth wave is rich of even and odd harmonics.

The NE555 out will provide a sawtooth signal which will have a high frequency (look at R4 in the schematic).

I just want to make a circuit able to show a great range of frequencies on the spectrum analyzer, nothing of suspect, although in my country we can transmit few milliwatts with fm transmitter.

Kind regards.

Ah no your doing this wrong. The carrier signal, the base frequency is set by l1 and C6 the audio input only swings this frequency either side of the base by an amount equal to the audio input so a few hz to a few 1000 hz. You need to look for a different design if you want to sweep a larger range.

Yes, I know that I could use a varactor diode to achieve the wideband sweep. However for now I will do some experiment on the transistor's base.

I accept your answer because is encouraging, however.
Thank you.