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Explanation needed..

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chinsoon

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This is a transmitter circuit that i found from the internet... and i do not really understand

I don really understand how the circuit works, especially for the local oscillator and the amplifier. isnt that a class C amplifier? i have gone through some books but none of them seems to provide explanation on this (especially the local oscillator configuration).

So can some one just explain how it works? basically i have built the circuit, I have all the readings but they are totally out from what i expect, so i doubt that what i know is totally out.

if you have any online tutorials on this, please tell me as well.
Thanks for the help.
 

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It's just a standard circuit, a crystal oscillator (might be a colpitts?, it's years since I've done them), followed by a simple RF power amplifier, I would suggest that the 470K bias resistor makes it more class B than class C.
 
Not just a power amplifier but a a tuned amplifier that selects a harmonic of the crystals fundermental.
 
Hero999 said:
Not just a power amplifier but a a tuned amplifier that selects a harmonic of the crystals fundermental.

Depends on what the crystal is? - you can get 27MHz fundamental crystals for radio control, which are probably more common than overtone ones.
 
starting from the left side:

The BC547 is configured to oscillate at the crystal frequency. The 47K ohm base resistor and 680 ohm emitter resistor bias the transistor to a reasonable Qpoint so that it can provide current gain. The voltage "generated" at the crystal is amplified by the transistor. Emitter voltage is fed back to the base through the 39pf capacitor, scaled by the 8.2 pF capacitor resulting in a net positive feedback at high frequency. The feedback is most effective at the resonant frequency of the crystal since the crystal presents a high impedance at that frequency. Once the oscillator begins oscillating, operation is sustained by this positive feedback.

Some of the emitter current at the crystal frequency is fed through the 10 nF capacitor to the BC337 which is configured in common emitter to provide current gain. The 470K resistor feeds DC bias to the base and raises the base voltage up to about 0.7 volts. The gain of the transistor insures that the emitter current is about 1 mA in this configuration. The high frequency signal applied to the base varies the current at the emitter and so also at the collector, and therefore varies the voltage at the collector. L2 and the 39 pF capacitor are tuned to form a resonant tank circuit that offers a relatively high impedance at only the crystal frequency, so the amount of voltage at the collector at this frequency is maximised. The antenna is connected directly to the collector of the BC337 and so has the collector voltage applied to its input. As a result, it accepts some RF power from the amplifier and radiates that into space.

Although it is not entirely clear, the NE555 is probably used to turn the oscillator and amplifier on and off by applying pulses of 9V to their power supply rail.
 
Nigel Goodwin said:
Depends on what the crystal is? - you can get 27MHz fundamental crystals for radio control, which are probably more common than overtone ones.
You have a point, it probably is a fundermental oscillator.
 
well, thanks for the reply first of all.

though still blur about your conversation, at least i got something new..

but why isnt the crystal producing waveforms at 27MHz? I have measured the waveform from the crystal to ground and the waveform i got is a capacitor charging and discharging wave at a the same frequency of output of the 555 timer.

besides that, can i just change the crystal value? i cant find exactly 27MHz in Malaysia, just found 27.125MHz... if i want to change it to other values, can i just change the crystal at the transmitter and receiver?

thanks
 
What are you using to measure the frequency with?

A multimeter might not work above 20kHz, so it will only see the waveform from the 555.

I don't think you can buy 27MHz crystals, it just means a crystal in the CB band which covers 27MHz.
 
There are a number of specific radio control channels in the 27MHz band, these are colour coded (to make it simple to avoid clashes), I've no idea what the exact frequencies of them are?.
 
There are also different crystals for the recievers to give an appropriate IF for the oscillator.
 
i used the oscilloscope to view the waveform..

actually i havent learnt about crystal.. so i did not know the waveform of a crystal until i do this project...
 
Well what's the bandwidth of your scope?

You might be able to get away with 25MHz but you really need 50MHz minimum.
 
Can I know what is the application of this circuit? The output from the 555 timer is the signal to be transmitted? What about the receiver?

Thanks
 
well, the circuit is actually a remote control circuit which uses radio....

now i can see differences in the receiver output when i turn on or off the switch at the transmitter. so i assume its working, but i havent actually test it out. i will complete the circuit tomorrow.

but first of all i need to find out how the circuit operates. i really need to know as later i will have to modify it ~(part of assignment given)

thanks
 
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