Integration and Analysis of a 24 3MHz FM Transmitter Receiver System Alex Tung Lab Partner Michael Wiemer EE133 Prof Bob Dutton Final Project Write up TABLE OF CONTENTS ABSTRACT 3 INTRODUCTION 3 CIRCUIT DESIGN THEORY 3 DISCUSSION RESULTS 5 CONCLUSION 9 APPENDIX A 1 POWER AMPLIFIER 11 APPENDIX A 2 LOW NOISE AMPLIFIER 13 APPENDIX A 3 4 5V REFERENCE 15 APPENDIX A 4 AUDIO INPUT 16 APPENDIX B FORMAL LAB WRITE UP REFERENCES 17 2 tery power supplies The transmitter consumes 360mW of DC power and transmits a 2 75dBm signal ABSTRACT We designed and built an FM transmitter receiver system to operate at 24 3MHz and 9V batat an output power of 2 7dBm The receiver has a clear minimum detectable signal of 90dBm and a maximum receivable distance of 5 8 miles when used in conjunction with a 20dBm transmitter 1 Audio Input and Amplification The audio input and amplification circuitry consists of a microphone and non inverting amplifier which produces a nominal 100 V V voltage gain at its output The input level of the microphone can be adjusted using a potentiometer so as to minimize distortion of loud signals The output DC level of the circuit is also adjustable so that the VCO maybe set to the correct free running frequency see Appendix A 4 INTRODUCTION Optimal performance of an FM transmitter receiver system depends on a number of factors including solid design and a precise implementation of each component block of the system Designing and tuning the transmitter and receiver to function well with one another also presents a key challenge In addition implementation of filters and low noise amplifiers helps to reduce the degradation of system performance due to outside noise sources while also improving maximum receivable distance This paper will discuss the design and implementation of a 24 3 MHz transmit receive system in terms of expected performance measured performance and improvements made 2 Voltage Controlled Oscillator The voltage controlled oscillator performs the frequency modulation of an intermediate frequency with the input audio signal by converting the voltage input to a frequency output The output DC level from the audio amplifier sets the freerunning frequency of the VCO and an applied input signal produces a frequency varied output that corresponds to the input voltage fluctuations We used the LM566 VCO to implement this oscillator in our transmitter With a DC input level of 7 5V the free running frequency of the oscillator can be adjusted to the needed 300kHz with a variable capacitor in its timing regulation circuitry For further discussion of the VCO see Appendix B 2 CIRCUIT DESIGN THEORY We designed the transmitter and receiver to operate at a frequency of 24 3 MHz with an intermediate frequency IF of 300 kHz Each of the two system components consists of a number of circuit blocks which perform various functions within the system We discuss each of these blocks subsequently Figure 1 Transmitter Circuit Blocks I The Transmitter The transmitter uses an input audio signal to modulate an intermediate frequency mixes the signal to a higher transmit frequency and outputs the modulated signal from an antenna The following blocks combine to achieve these functions an audio input and amplifier a voltage controlled oscillator a mixer a local oscillator and a power amplifier See Figure 1 3 Mixer The SA602 Analog Multiplier serves as the mixer for this system The mixer takes as input the 300kHz IF signal and upconverts it by multiplying it with a 24MHz carrier signal from a local oscillator This multiplication produces an output signal at the carrier frequency of 24MHz and two sideband signals at 24 3MHz and 23 7MHz In 3 theory one would like to suppress the excess 24MHz carrier and 23 7MHz negative sideband signals as they do not transmit the desired information Performing this single sideband transmission requires more complicated techniques than are within the scope of this project In order to reduce the amount of distortion caused by harmonic signals we designed the LNA with a series LC input filter centered at 24 3MHz with a 10MHz bandwidth 2 Low Noise Amplifier In order to maximize receivable distance the receiver end of the system includes an input amplification stage in the form of a single transistor lownoise amplifier This stage amplifies the power of the incoming signal while minimizing distortion at its output The LNA consists of a bipolar transistor with resistive feedback and an inductive load The output of the amplifier consists of an LC match which transforms the actual load impedance 1 5k of the mixer to the load desired for the specified amount of power gain See Appendix A 2 The input impedance should ideally match the impedance of the input source through some LC transformation network i e it should be matched with the impedance of the antenna if that impedance is known 4 Local Oscillator The SA602 contains the added functionality of an on chip local oscillator the frequency of which can be set using a crystal and capacitive divider We used this oscillator output as the carrier signal for our mixer setting the oscillation frequency with a 24MHz crystal The ease of this implementation makes construction of a separate discrete oscillator e g Colpitts or Weinbridge unnecessary 5 Power Amplifier Once the input signal is upconverted to the desired transmission frequency it must be amplified to achieve maximum transmittable distance We designed the power amplifier to deliver 100mW of RF power to the load which is the antenna We used a two transistor cascode configuration to construct a class A amplifier placing an LC match on the output to allow resonant transformation to a 50ohm load impedance See Appendix A 1 The input to the amplifier comes from the SA602 mixer through a coupling capacitor Although the class A design of the amplifier causes it to consume a great deal of DC power the cascode implementation allows appreciable RF power gain while minimizing the effects of the Miller capacitance of the input transistor on the amplifier frequency response Figure 2 Receiver System Blocks 3 Mixer We implemented the mixer on the receiver board with the same SA602 analog multiplier chip as we used on the transmitter board This mixer multiplies the 24 3MHz input signal from the LNA with a 24MHz local oscillator signal and outputs the original modulated 300kHz IF signal II The Receiver The receiver end of the system captures the transmitted signal through an antenna and