# dc voltage controlled oscillator, input voltage 0-1000mV

Discussion in 'The Projects Forum' started by gauravkalra, Jun 21, 2010.

1. ### gauravkalra Thread Starter New Member

Jun 21, 2010
3
0
hello everyone
i want to make wireless FM temperature sensor
sensor has dc otput voltage range 0 to 1000mV
oscillator frequency required is near 100 MHz

2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Here is something you can start with. See the attached .pdf document, look at the last page.

You will need to use a faster IC, as the 74HC is limited to around 50MHz.

You'll need to design a VCO (voltage controlled oscillator) for the input portion, as the existing design is controlled by a sensor with variable capacitance.

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3. ### Markd77 Senior Member

Sep 7, 2009
2,803
596
100MHz is in the band of FM radio transmission. Best check if you can legally transmit on that frequency in your country and how much power you can put out.

4. ### gauravkalra Thread Starter New Member

Jun 21, 2010
3
0
ya in our region it is okay

5. ### gauravkalra Thread Starter New Member

Jun 21, 2010
3
0
i saw pdf and analyze the circuit
i have some questions in my mind
1 it is doing amplitude modulation or frequency modulation?
2 while calculating R2*C2 time constant we get carrier frequency
now if take C2 10pF and r2 is 1k then i get frequency 100 MHz but it written for only 10 to 50 MHz
3 can fm band is possible?
4 how to use enable pin inthe starting?
5 it will give square wave output so can a square wave output is decoded at the fm reciever?

6. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
I thought you said you analyzed the circuit?
You forgot about L1 and L2. Use Wheeler's formula to get the approximate inductance
As I mentioned originally, you will need a faster IC.

Sorry, but your sentence structure reminded me of that image.
As I mentioned before, you would need to use a faster IC.

A logic 1 enables.
A logic 0 disables.
The L2/C4/C3/L1 network outputs a sine wave.

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7. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
See the attached for a rough simulation of the circuit.

I decreased C1 by 90% just to make the simulation time manageable.

The output is a gated 50MHz signal. This will be easier to construct and decode than an FM signal, as the base frequency would have to be extremely accurate.

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