Week 4 Activities

It is week 4 now, which is very close to the end of the project.

Last week we reconstructed and improve the circuits for the receiver and transmitter. So this week, we tested the receiver and transmitter at the same time. The receiver still has problems. However, the transmitter seemed to be working.

As a consequence, in this week, our group concentrated on the transmitter for the simulated design. MP3 was connected into the circuit as the input signal. Fortunately, the transmitter circuit was working, we can hear the input sound from the AM radio; however, there was some noise when listening to the AM radio.

Therefore, we considered to change some resistors (increase R4 and R6 and reduce R2 )to reduce the gain of the transistor, which might reduce the noise. However, the values for resistors were not correct, so that it was not effective.

After that, the resistors were changed back to original ones. What was unfortunate was that the circuit did not work anymore. We wanted to check out whether the output is available measured by using oscilloscope and the result came out with zero output.

Then, we thought that the components of the circuit must be damaged, especially capacitors.

In order to simplify the circuit, the changes of each LC tank circuit are that the inductor L1 was changed to approximately 540μH for 60 turns and the variable capacitor in transmitter was changed to a fixed-value capacitor 100pF.

According to the frequency equation for LC tank circuit, f=1/(2π×√LC)≈680kHz, which is in the range of AM radio.

A can is used for antenna. The real transmitter circuit was built and shown below:

However, at the end of the week 4, we still failure to make progress of the transmitter for the circuit not working. Nevertheless, the circuit was working before and the simulation demonstrates the design   should have no errors. Therefore, there might be something ignored that affects the circuit.

As a consequence, the next week is the last time for us to conduct the project, the transmitter should be done well. I think there is few time for us to complete the receiver; however, we will try efforts to do as much as possible for the receiver.

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.

Week 2 Activities

This is the second week of this project and the components we ordered were all arrived. 

The circuits were done for the first time for both receivers and transmitters which spent the half day. However, the circuit of the receiver was tested at first. 

LM376 is an op amp chip, which totally has eight pins. Pin 1 and Pin 8 are GAINs, which control the gain of this amplifier. When Pin 1 and Pin8 are disconnected, the gain of the receiver circuit is 20. However, we want to achieve the gain as large as possible, so a capacitor was used to achieve the maximum gain 200. 

Pin 2 and Pin 3 are -INPUT and +INPUT. The Pin 2 is connected with the ground and Pin 3 is an input terminal. An output capacitor is also connected between the antenna and earphone, which is considered as coupling capacitor to reduce the noise. Pin 4 is GND, which is connected to the ground. Pin 5 is OUTPUT. Moreover, Pin 6 is linked with positive terminal of the voltage. 

Additionally, Pin 7 is BYPASS, between the ground and Pin 7, there is a shunt capacitor to filter the high frequency. 

The original design of the receiver is shown below. 

After the test, we could hear some noise in the earphone, but no sound or program. Thus, we found that the adjustment of the frequency is pretty necessary; in order to find an appropriate frequency that the receiver can receiver the signal. 

There are three main changes of the design. A variable capacitor and a inductor in parallel  are connected between the input signal (i.e. antenna)and the ground to achieve the adjustment of the frequency. Besides, we add a variable resistor to adjust the gain of the signal in order to adjust the volume(i.e. change the gain of the circuit). 

Moreover, some values of the components were also changed to achieve better resulting test. In addition, a 10Ω resistor is series with a 0.1μF capacitor is connected with Pin 5 to make the circuit more stable.

The updated design is indicated in the figure below.

Because the time was limited during the Friday, so that there is no enough time for us to test the new circuit. 

After that, the circuit for transmitter is also changed for better design, but some problems are found and needed to be solved.

It is unfortunate for us to make no progress of the real circuit this week. Therefore, what we can do is to check the updated designs and use PSpice to simulate the circuit to ensure the circuit can work effectively and normally.

Week 1 Activities

Last Friday is the beginning of the project, and a lot of activities has been conducted.  Firstly, we decided to design a AM transmitter and a AM receiver, because AM circuits are easier to be design when compared with FM circuits.

For better research, we surfed on the internet and find a large quantity of information of the AM transmitters and receivers.

The short range wireless AM transmitter and receiver can work at the frequency range from 540kHz to 1.6 MHz.

Transmitter modifies the input signals in order to cope with the limitations imposed by the channel. For lower-power transmitter, it is often used in communication systems and radio systems. A simple AM transmitter achieves amplitude modulation.

Receiver processes the received signal by reversing the signal modifications made at the transmitter. When the AM receiver receives the signal, after modulation passing through the audio amplifier, then spread to the earphone or speaker. Therefore, we can hear the sound or program.

Therefore, the transmitter and receiver as a pair are specifically designed to combat the deleterious effects of the channel to achieve more efficient information transmission.

The link below contains a number of designs of the AM circuits, which can be referred and is helpful for us to design the circuit.

Instructables: http://www.instructables.com

After working out the circuit design, we ordered the components.

For transmitter circuit,
555 timer chip
An NPN transistor
10nF capacitors ×2
1nF capacitor
1 kΩ resistors ×2
10 kΩ resistor
5 kΩ potentiometer
3.5mm female audio jack
male audio jack to jack connector
antenna
breadboard

For receiver circuit,
LM 386 op amp IC chip
1000μF capacitors ×2
100μF capacitor
9V battery
3.5mm female audio jack
earphones

The software Multisim and PSpice are also be used to build the circuits and analysis the characteristics  of the circuits.

All the goals were achieved in Week 1. Next week, The real circuit should be made, especially, the receiver. Hope we can success next week :-D