# Sinusoidal Variable Frequency Oscillator in a small Circuit? Is there a simple way?

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#### thatsoutherly

Joined Apr 5, 2017
2
Hi Guys,

I'm a Mechanical Engineering student with quite limited electrical knowledge trying to design a circuit.
That's probably my first problem..

Anyway, I am making a simple FM transmitter circuit. But instead of having one oscillating frequency, i need it to produce a oscillating carrier wave that sweeps from 88Mhz - 110 Mhz about once per second.

My goal is to produce a simple FM transmitter circuit that will sound a 'blip' to ALL the available FM frequencies every second.

I have been thinking maybe a sweep generator is needed or frequency synthesiser?? I have no idea. I just figured someone must have already made some sort of oscillator that could to this haha.

Any help would be incredible,
Tom

#### Bordodynov

Joined May 20, 2015
3,179
You can use the oscillator tunable by varicaps. The control voltage must have a sawtooth shape.

#### atferrari

Joined Jan 6, 2004
4,765
How wide is the range?

#### thatsoutherly

Joined Apr 5, 2017
2

#### Alec_t

Joined Sep 17, 2013
14,310
Do you have a licence to transmit in that frequency band? A sweep like that will interfere with other people's radios and be detectable by the authorities!

#### AnalogKid

Joined Aug 1, 2013
11,021
A sine wave will produce longer blips at the low and high ends of the band and shorter ones in the middle. To put the same energy into each broadcast FM channel, you ideally want a triangle wave rather than a sine wave. One LM358 dual opamp can do this; it is the core of many simple function generator circuits. One opamp is an integrator and one is a comparator.

Note that a varicap's response to a control current is non-linear, as is its effect on a tuned circuit's resonant frequency. But its best to start with the "correct" modulating waveform and go from there.

ak

#### DickCappels

Joined Aug 21, 2008
10,164
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