Electronic – Radio Transmitter using Crystal Oscilliator


I am on the mission to control my RC car using arduino. The simplest way I thought of doing this is to add arduino outputs into the transmitter. Long story short I wanted to learn more about how these transmitters worked.

I have done some research and managed to find a schematic that is similar to the transmitter circuit that I currently have posted below. My transmitter is using a 27Mhz crystal oscillator to create a carrier wave.

RC transmitter

Bigger picture click here

But I do not understand how the crystal oscillator is biased and how also the carrier wave is mixed with the transmission signal (message Left, Right , Forward, Reverse)from the IC.

Sorry if I have not used the terms correctly I am new to RF and want to learn 🙂

Best Answer

The voltage from pin 8 of SCTX2BC changes the DC component of the voltage on the base of Q2 transistor, modifying its quiescent IC current and its gain. So this is how the RF signal amplitude is modulated.

The RF signal from the Q1 based oscillator is connected via the capacitive voltage divider consisiting of C5 and C6.

The bias of Q1 may be found by the following equation:

$$V_{bias} = R7\,\frac{I_E}{\beta+1}+0.65V+I_E\times R8 = 9V$$

$$V_{CE} = 9V - I_E \times R8$$

$$I_C = \frac{\beta}{\beta + 1}I_E$$

C4 assures the positive feedback.