RF Data Transceiver

Objective

Data transferring can be supplied by using wireline and wireless systems. In our project we need to use one of the wireless data transfer type that is RF data transfer system. Our project is based on the data transfer that has both transmitter and receiver part called transceiver. The main objective of this project is constructing the system that transfers the information (this is text in our project) from one computer to another computer by using circuit. The communication between these two computers could be supplied by using transmitter and receiver part with the distance at least 5 meters. Transmitter and receiver part include modulation, demodulation, power amplifier, antenna and filter.

 1. Introduction

While we was designing this project, circuit was divided into two parts in basically.

Transmitter Part

            In order to transmit a data from one computer to another computer, firstly data must be converted to electrical signals. Since these signals have low power, transmitting is difficult by itself. That’s why message signals are required to be modulated, after this process obtained new signal is converted to RF waves by using loop antenna.

Receiver Part

            Transmitted data are received by another loop antenna and then demodulation process must be applied in order to obtain message signal, since these message signals have not enough power to perceive, amplifier circuit must be used for this process. Finally message signals (electrical signals) are converted to binary data by using TTL 232.

                                                                       

Figure 1: Block Diagram of RF Data Transceiver

We have encountered some problems when we are designing this project. Some of them are listed below.

·      Setting Antenna

While electrical signal are converted into RF wave, we must use loop antenna. However our loop antenna did not work during transferring process. There were some reasons, main reason was that designed antenna circuit was wrong, second factor was that since common supply voltage was used, coupling effect occurred. In order to remove this effect, we tried to use a capacitor but this element did not work properly. Last reason was that we did not adjust correct number of turn of the cable.

·      Setting Modulation Circuit

           

 Since we used and gate for modulation process, frequency of signal decreased to 2 MHz. However according to modulation rule, this frequency could be around of 4 MHz .By the end of effort, we have overcome this frequency decreasing. Our problem was because of the oscilloscope’s time division part. When we put our message signal on the modulated signal there were perfect matching

·      Adjustment Carrier Voltage

In our first design of the local oscillator circuitry, our carrier voltage was found around 2.5 V. However this voltage was not sufficient for modulation process to get true result because this is near the high and low level transition points (low level=2.2-high level=2.5V). That’s why we tried to increase to nearly 5 V.

1.1 Serial Data Transmission

Data tranmission which is physical transfer of data from a point to a point in channel. There are two ways of this system.One of them is parallel data transmission ,the other is serial data tranmission.Bits are sent one by one for serial way.Advantages of serial transmission is that error rate is less than parallel and transfer rate may be faster.This way can be used for long distance by adding a check bit (parity bit).In this project, we used TTL 232 cable for serial transmission.Thanks to this cable ,we got message signal and message signal was transferred to modulation circuit.This cable has six pins and 9600 baud rate .

1.2 Crystal Oscillator

Oscillator which is an electronic device, produces square, triangle and sine waves. We had to use oscillator to design modulation circuit because carrier signal is necessary for modulation. Oscillator is a way to generate a carrier signal. Although there are many kinds of oscillator devices, we used crystal oscillator with Schmitt Trigger circuitry. We could use crystal oscillator which helps us to produce high level frequency oscillator circuitry. Operating frequency was 10 MHz for our project but when we need an amplifier this carrier causes some problems in bread board. So we should change it with lower frequency than we obtained 4 MHz carrier frequency.

1.3 Modulation and Demodulation

 Modulation and demodulation part was the  most important in our project since message signals have low frequencies.Modulation technique was developed in telecomunication in order to send message signal.In generally modulation process is multiplying message signal with carrier signal because message signal does not transfer itself without carrier signal. Frequency of message signal is increased to high frequencies with modulation.There are two types of modulation techniques.One of them is amplitude modulation and the other is frequency modulation. In this project, we used amplitude modulation namely we changed amplitude of message signal with carrier signal because amplitude modulation and demodulation is easier than frequency modulation and demodulation.At the end of this process, we obtained modulated signal with amplitude variations as carrier signal ( in terms of frequency). General amplitude  modulation equation is below .

c(t)=Ac*cos(2*pi*fm*t)   s(t)=Ac[1+ka*m(t)]cos(2*pi*fc*t)

-c(t) is carrier signal.   -s(t) is modulated signal.  -ka is modulation index.

-Ac is amplitude of carrier signal.  -m(t) is message signal.

In this project,we used and gate (74LS08) for modulation actually there are more than one tecnquies.However we prefered this method since operating frequency was 4 MHz and the most optimal device was and gate for this frequency.At the same time,that was the easiest way.Demodulation which is a way of getting message signal from modulated signal.We used envelope dedector for this process. This device includes a capacitor ,a resistor and a 1N4148 diode. Again there are more than one ways but the technique was the easiest .

1.4  Antenna

Antenna that is a electronic device , takes electrical signal and converts to RF waves or vice versa.There are many kinds of it.That’s why we have chosen the simplest way for antenna since the complexity increases with the distance proportionally. For our project , straight copper wire was necessary because if  the current flows in a wire, magnetic field is generated and so that radio waves occur in magnetic area.However our antenna circuit did not work properly so we obtained very weak signal and noise always interferenced into signal.

2. Technical Description

RF data transceiver project depends on two main parts, one of them is transmitter part, the other is receiver part. These two parts connected each other with two loop antennas each one of them in one part.

In the transmitter part, the first process is transferring data from computer to the serial data transfer system that could be supplied by RS 232 .The data coming from RS 232 with the TTL level which means analog data comes from the cable[1]. To send and receive signal with computer we used Serial Port Terminal V1.1 which is easy to use for this project. After serial data comes as message we need modulation process. Modulation processes supplies us to transfer data efficiently during the process, but we need the high frequency oscillator to send message throughout with channel. Firstly we obtained the frequency 4 MHz frequency and to supply 4 MHz carrier frequency we used Schmitt trigger and 4 MHZ crystal. Output of the local oscillator circuit, we had 4 MHz carrier signal that is ready for modulation process.  After obtaining the carrier signal we modulate signal by applying and gate 74LS08 which supplies us 4 MHz modulated signal and sending modulated signal via our antenna which will be explained in the coming part. Block diagram (under) basically shows the parts that needs to be completed in transmitter part.

  

Figure 2: Block Diagram of Transmitter

After signal sent throughout transmitter antenna, receiver antenna takes the signal. Modulated and transmitted signal has weakened also some noise added to the original signal. Firstly we applied power amplifier because this signal loss some power because of wireless transferring and our amplifier increases voltage from 40 mV to 4,6V without any problems. Then we applied to envelope detector to the coming signal. Envelope detector works to detect message signal with specified capacitance and resistance values. Besides envelope detector also helps us to decrease effects of the noises comes from the other devices and noise sources. After envelope  we send data to RS 232 TTL and read message by computer.

Figure 3: Block Diagram of Receiver

2.1 Local Oscillator

Modulation is the most important part during transmission time, there are two signals that we need. One of them is message signal and the other is carrier signal in order to modulation. There are a few ways to generate carrier signal. Our method is crystal oscillator with schmitt trigger circuitry. We could use crystal oscillator which helps us to produce high level frequency oscillator circuitry. Oscillator circuit’s output that we created, is an input of modulator. For this project, carrier signal frequency is 4 MHz. Because after 20 MHz cosmic noises effects our signal also 4 MHz carrier frequency was not so hard. So we obtained our carrier frequency 4 MHz. Also we obtained message signal’s frequency around 9.6 kHz. So this is an amplitude modulation and bandwidth is 2*Fm=19.2 kHz. In our local oscillator circuitry, at the output we generated 4 MHz sinusoidal carrier signal. [2]

Figure 4: Local Oscillator

2.2 AM Modulator

                  

Figure 5: Modulator

Modulation is indispensable for transmission line, since message signals have low frequencies in order to send these low frequencies some ways have been developed in telecommunications. In this project we used amplitude modulation technique. According to this method, in order to transmit message signal, a high frequency signal is needed thaht is supplied with local oscillator circuitry. During modulation process, carrier signal and message signal must be multiplied since message signal has low frequency around 9600 Hz. There are a lot of ways to modulation we choose and gate 74LS08 which is good for basic multiplication. [3]

    

2.3 Loop Antenna

Antenna was the most important part of our project, because we spend almost half of our time to design a suitable antenna to transmit our data to the receiver part. In first try, we tried to send data with 10 cm cable but we couldn’t send any data. In the second try, we established R-L-C circuit also we couldn’t be successful although we calculated the value of the components as in the second report. At our third trial we connected our 5V supply in both transmitter and the receiver part, in this trial we observed the message signal at the end of the receiver output. But this was not a real transmission, this was just coupling effect so this was not a successful method. In last try, we turned to coil as inductor and paralleled 100 nF capacitor we observed some signal at the receiver but when we amplify this signal we couldn’t get a square wave .So we couldn’t be perfectly successful but we could send some data which is not noise or something like that because the shape of output were similar with expected.[4]

2.4 Envelope Detector

Envelope detector is an important part of the RF data transceiver project. Because modulated signal transferred to the unmodulated signal which is original message signal given from the transmitter computer. Envelope detectors designed as a half wave rectifier. Because when voltage increases capacitor charges, when voltage decreases capacitor discharges through to the resistor. Advantages of the envelope detector are cheap and easy to apply and set up. Disadvantage of envelope detector is decreases the voltage level. To eliminate this disadvantage we applied amplifier before this process. Also envelope detector are used as low-pass filter so this helps us to eliminate noises that was added to modulated signal in the transmission. We can calculate as;

Fc=1/(2*π*(R*C)) Fc is the carrier frequency

Fc=4 MHz , R=40k , C=10pF

Figure 6: Envelope Detector

2.5 Amplifier

          

           During transmission, because of the channel and the other effects signal’s power decreases. This weakness causes to unidentified signals coming from the transmitter. To solve this problem and not to change frequency of the demodulated signal we need a good amplifier method. For this reason we used amplifier circuit with the transistors. Because transistors are suitable for working in the sensitive frequency value. With 2 serial amplifier circuitry we can increase the voltage from 40 mV to 4.6V. Which is enough to send it to the serial line transmission part.

Figure 7: Amplifier[5]

3.  Test Results

3.1 RS232 TTL Cable Test Results

RS 232 serial port terminal cable’s output values are masured as;

Logic	Voltage	Frequency
Logic 0	0.1V	0
Logic 1	5.1V	9600 Baud rate

                               

3.2 Local Oscillator Test Results

To produce a carrier signal we used 74HC14 Schmitt trigger circuit and get these results.

Vmax	Vmin	Vp-p	Frequency
4.5V	-0.4V	4.9V	10 MHz

Figure 8: Carrier Signal

3.3 Modulator Test Results

And gate is the device that runs modulation process, has some important values for our circuitry

Carrier Frequency	Message Frequency	Carrier Voltage	Message Voltage	Modulated Signal Voltage	Modulated Signal Frequency
10 MHz	4.8 KHz	4.9V	5V	2.72	10MHZ

Figure 9: Modulated Signal

3.4 Receiver Antenna Test Results

At the output of receiver antenna we get;

Frequency	Voltage
3,98 MHz	1,2V

3.5 Receiver Amplifier Test Results

When the signal receives the antenna it need to be amplified. To be able to apply amplifier to the received signal we need a good designed amplifier circuitry specified to this frequency value.

Input Voltage	Output Voltage
1,2 V	6V

3.6 Demodulator Test Results

At the envelope detector’s output we had;

Voltage	Frequency
3.56 V	4.8 KHz

Figure 10: Demodulated Signal