Week 3 activities

Because of the failure last week, this week we concentrated on the circuit of receiver.

We have redesigned the circuit more stable and significative, we reconstructed the circuit for receiver.   The circuit of the receiver has four parts in theory, a rank circuit, a radio-frequency (RF) amplifier, Detector and an audio amplifier. In order to simplify the circuit, we remove the detector and audio amplifier; however, the testing results was not as well as expected.

That means the detector and the audio amplifier is important and necessary. We then, improved the design of the receiver.

For the purpose of success, we also constructed the audio amplifier independently and tested it with oscilloscope, the output was tested correct.

Therefore, the final design for receiver is indicated below. In this circuit, LC is resulting in tank circuit, IC1 is used for demodulation; the circuit for diode is detector; while the IC2 and the components around consists of an audio amplifier. When Pin 1 and Pin 8 are disconnected, the gain for the chip is minimum for 20, therefore we may add a 10μF Capacitor between them to achieve a larger gain according to the real situation.

The diode and adjacent capacitor and resistors are formed as a detector to ensure the output of IC1 will be higher than 0.7V, which make the current and signal can go through the diode. The value of L1 should be the same as the receiver for better cooperation of the transmitter and receiver.

The main chip for transmitter was 555 timer before, but the circuit is not stable, so that we changed our design with two BC109C transistors.

The transmitter was also tested but was not able to work. Therefore, we checked wether all the components were connected properly. Also, the inductors we use is ready-made inductors; however, for better LC circuit of AM transmitter and receiver, it is more sensitive to make the inductors with a magnetic rod (D10mm × L190mm) and some enameled wires (D0.40mm). The wires are wrapped for 60-80 circles, but may be adjusted for several trials and best testing results.

The circuit design is shown below. As we can seen, the frequency of the input signal was set to 1MHz which is in the frequency range of an AM transmitter. Q1 is constructed as an RF amplifier, C5 decouples the Re and unleashes full gain of this stage.

Q2 and adjacent components are connected as an oscillator, which used for modulation. C3 couples signals from base to the top of L1. L1 and C6 are in rank circuit, the variable capacitor can control the frequency of the transmitter from 500kHz to 1.6MHz. C7 ensures the oscillation is transferred from collector to emitter and through the internal resistor Rbe of Q2 and back to base again. R7 ensures the oscillation will not be shunted to the ground through the very low value Re of Q2 and also increases the Ri; so that the modulation signal will not be shunted to the ground.

The antenna was replaced by the 8Ω resistor as an load. Additionally, the variable resistor R1 can adjust the power of the circuit.

The transistors used in the real circuit are two BC109C, but the component library does not contain BC109C. We use BC239BP to replace them.

Many improvements for better circuits are considered ,which may help us to better complete the project. The circuits for both transmitter and receiver still have some problems, we need to find out the errors with dividing the circuits into several parts.