using a2 astable multis to build a simple fm comm system

[bhargava]

[glow=red,2,300]hi,[/glow]
I just got an idea to use 2 astable multis to build a simple fm communication system

this uses 1st multi as voltage to freqency converter to convert the input from mic into corresponding frequency of pulses then transmit it.

then at receivers end we use another multi to convert the pulses from antenna to
vo;tages by applying the pulses to the collector of the transistor of 2nd astable multi.(ie., using the bilateral property of the astable multi used as frequency to voltage converter) the feeding this analog voltage to a speaker via amplipfiers.

this makes a simple fm communication set. believe it works .,!!!!
am I right!!!!

 

[audioguru2]

Hi Bhargava,
Welcome to our forum. ;D
Your idea is a variation of using the vco of a common and cheap Phase-Locked-Loop IC as the FM transmitter then another PLL as the FM decoder.

 

[bhargava]

[glow=red,2,300]Hi,[/glow]
AudioGuru
I basically meant to ask if the astablemulti could be used as a bilaterral element when it is used as voltage to frequency converter.
Please clear this doubt

 

[audioguru2]

I basically meant to ask if the astablemulti could be used as a bilaterral element when it is used as voltage to frequency converter.
Please clear this doubt

You want to re-invent the wheel?
If you are talking about a two-transistor simple multivibrator, then how are you going to make it operate at a high enough frequency? How are you going to vary its frequency with a voltage?

The antenna doesn't receive pulses. It receives a continuous high frequency carrier, and its frequency changes slightly with the modulation. I don't know how you can make a multivibrator lock onto the frequency and perform as an f to V converter.

A simple FM transmitter has a VHF oscillator that is tuned to its carrier frequency by an LC tank. It is frequency-modulated by voltage-controlling the value of the C in the tank. You can do it with a varicap diode in parallel with the tank, or by amplitude-modulating the oscillator transistor which varies its capacitance.

An FM receiver is complicated. The signal strength from the antenna could be a few microvolts so needs amplification. The RF amplifier needs to be tuned and have AGC so that it isn't overloaded by strong local stations on nearby frequencies. Since a very high amount of amplification is required, the RF frequency is converted to a lower "IF" frequency with a tuned local oscillator and mixer. Then most of the amplification and filtering is done with an IF amplifier/limiter IC that also contains an FM detector.
You could detect the FM modulated IF signal yourself if you want with a mono-stable or with a phase-detector part of a PLL IC. ;D

 

[bhargava]

Hi audioguru,
Thanks I got my doubt clarified

 

[bhargava]

to vary the frequency with voltage I apply the signal voltage to the collector resistors of both the transistors apart from thenormal power supply.
this induces change in frequency of the pulses at the collectors.thus i use an astable multi as vto f converter

 

[audioguru2]

What is the highest frequency that your two-transistor multivibrator oscillate? 200kHz?
What will you do to get as high as 108MHz? That's 540 times higher. ;D