Tone Transmission over FM

[wormgear]

Hi,
I am designing a circuit which I can use to transmit simple tones (sine waves, Vpp is variable) using FM over short distances (<50m) . I have used an op-amp RC oscillator to generate the tone. I want to modulate the carrier (HF range <30MHz. Also to avoid the commercial FM range (85-105MHz)) with these tones, so I can transmit signals over this range. I am facing problems with my RF stage as I cannot get a design that is stable and can be modulated. Can anyone help me with the design /stage? At the receiving end, I intend to demodulate this and use a bandpass filter bank to separate the tones (which is essentially signalling using tones). Any help with the modulation / demodulation theory and/or design will be appreciated.

Thanks
Wormgear

 

[davenn]

for any sort of decent stability you need a crystal oscillator of some descritpion
a free running RC osc will never be stable enough
have a look at Hartley, Colpitts, Butler oscillators Crystal based not free running

here's a hint cct for you .....

cheers
Dave

 

[wormgear]

Dave, Thanks for your quick reply and the circuit. Will test this out. Also, are all capacitor units in pf? And the resistors?
I use the RC circuit to generate my signal tones (from 0.5 to 5 kHz) which will modulate the carrier (a few MHz). So what I meant was that the signal tone generation is not a problem (op-amp based - RC or quadrature (to increase stability)). But I cannot get this tone to modulate my LC tank circuit (LC values chosen to generate around 4.5 MHz). My guess is that the theory is not clear (given that base-collector capacitance plays a role) or I should settle for a different design ( a crystal based one as above). Can I modulate the 4MHz output in your circuit with a simple sine wave? If yes, how do I go about it?

 

[MikeMl]

Look up a varactor diode. Unless you have a ham licence, don't put your transmitter in the ham bands. You are more likely to get caught and fined if you operate on ham frequencies.

 

[unclejed613]

you need a VCXO (voltage controlled crystal oscillator) which uses a varactor to shift a crystal oscillator.

 

[audioguru]

A crystal oscillator has a very stable frequency due to the crystal but it cannot be FM modulated much.
If you use a low frequency crystal oscillator then double, triple or multiply the frequency then the tiny amount of FM at the lower frequency is also multiplied.

 

[unclejed613]

the other method to generate a crystal controlled FM signal is to use a PLL locked to a crystal. the loop filter has a cutoff frequency of about 200hz. the audio (high pass filtered with a cutoff of 250hz) is mixed with the output of the loop filter to provide a control signal to the VCO.