Integration and Analysis of a 24.3MHz FM Transmitter/Receiver SystemAlex TungLab Partner: Michael WiemerEE133: Prof. Bob DuttonFinal Project Write-up2TABLE OF CONTENTSABSTRACT 3INTRODUCTION 3CIRCUIT DESIGN THEORY 3DISCUSSION/RESULTS 5CONCLUSION 9APPENDIX A-1: POWER AMPLIFIER 11APPENDIX A-2: LOW-NOISE AMPLIFIER 13APPENDIX A-3: 4.5V REFERENCE 15APPENDIX A-4: AUDIO INPUT 16APPENDIX B: FORMAL LAB WRITE-UP REFERENCES 173ABSTRACTWe designed and built an FM transmitter / re-ceiver system to operate at 24.3MHz and 9V bat-tery power supplies. The transmitter consumes360mW of DC power and transmits a 2.75dBmsignal. at an output power of 2.7dBm. The receiver has aclear minimum detectable signal of –90dBm and amaximum receivable distance of 5/8 miles whenused in conjunction with a 20dBm transmitter.INTRODUCTIONOptimal performance of an FM transmitter / re-ceiver system depends on a number of factors, in-cluding solid design and a precise implementationof each component block of the system. Designingand tuning the transmitter and receiver to functionwell with one another also presents a key chal-lenge. In addition, implementation of filters andlow-noise amplifiers helps to reduce the degrada-tion of system performance due to outside noisesources while also improving maximum receiv-able distance. This paper will discuss the designand implementation of a 24.3 MHz transmit / re-ceive system in terms of expected performance,measured performance, and improvements made.CIRCUIT DESIGN THEORYWe designed the transmitter and receiver to oper-ate at a frequency of 24.3 MHz with an intermedi-ate frequency (IF) of 300 kHz. Each of the twosystem components consists of a number of circuitblocks, which perform various functions withinthe system. We discuss each of these blocks sub-sequently.I. The TransmitterThe transmitter uses an input audio signal tomodulate an intermediate frequency, mixes thesignal to a higher transmit frequency, and outputsthe modulated signal from an antenna. The fol-lowing blocks combine to achieve these functions:an audio input and amplifier, a voltage-controlledoscillator, a mixer, a local oscillator, and a poweramplifier (See Figure 1).1. Audio Input and AmplificationThe audio input and amplification circuitry con-sists of a microphone and non-inverting amplifier,which produces a nominal 100 V/V voltage gain atits output. The input level of the microphone canbe adjusted using a potentiometer so as to mini-mize distortion of loud signals. The output DClevel of the circuit is also adjustable, so that theVCO maybe set to the correct free-running fre-quency (see Appendix A-4).2. Voltage-Controlled OscillatorThe voltage-controlled oscillator performs the fre-quency modulation of an intermediate frequencywith the input audio signal by converting the volt-age input to a frequency output. The output DClevel from the audio amplifier sets the free-running frequency of the VCO, and an applied in-put signal produces a frequency-varied output thatcorresponds to the input voltage fluctuations. Weused the LM566 VCO to implement this oscillatorin our transmitter. With a DC input level of 7.5V,the free-running frequency of the oscillator can beadjusted to the needed 300kHz with a variable ca-pacitor in its timing regulation circuitry. For fur-ther discussion of the VCO, see Appendix B-2.3. MixerThe SA602 Analog Multiplier serves as the mixerfor this system. The mixer takes as input the300kHz IF signal and upconverts it by multiplyingit with a 24MHz carrier signal from a local oscil-lator. This multiplication produces an output sig-nal at the carrier frequency of 24MHz and twosideband signals at 24.3MHz and 23.7MHz. InFigure 1: Transmitter Circuit Blocks4Figure 2: Receiver System Blockstheory, one would like to suppress the excess24MHz carrier and 23.7MHz negative sidebandsignals, as they do not transmit the desired infor-mation. Performing this single-sideband transmis-sion requires more complicated techniques thanare within the scope of this project.4. Local OscillatorThe SA602 contains the added functionality of anon-chip local oscillator, the frequency of whichcan be set using a crystal and capacitive divider.We used this oscillator output as the carrier signalfor our mixer, setting the oscillation frequencywith a 24MHz crystal. The ease of this imple-mentation makes construction of a separate, dis-crete oscillator (e.g. Colpitts or Weinbridge) un-necessary.5. Power AmplifierOnce the input signal is upconverted to the desiredtransmission frequency, it must be amplified toachieve maximum transmittable distance. We de-signed the power amplifier to deliver 100mW ofRF power to the load, which is the antenna. Weused a two-transistor cascode configuration toconstruct a class A amplifier, placing an LC matchon the output to allow resonant transformation to a50ohm load impedance (See Appendix A-1). Theinput to the amplifier comes from the SA602mixer through a coupling capacitor. Although theclass A design of the amplifier causes it to con-sume a great deal of DC power, the cascode im-plementation allows appreciable RF power gainwhile minimizing the effects of the Miller capaci-tance of the input transistor on the amplifier fre-quency response.II. The ReceiverThe receiver end of the system captures the trans-mitted signal through an antenna and amplifiesthat signal so that it may be downconverted to IF,filtered, and demodulated to the original signal.The following components together perform thisfunctionality: an input filter, low-noise amplifier,mixer, IF amplifier and filter, phase-locked loop,and an audio speaker.1. Input FilterIn order to reduce the amount of distortion causedby harmonic signals, we designed the LNA with aseries LC input filter centered at 24.3MHz with a10MHz bandwidth.2. Low-Noise AmplifierIn order to maximize receivable distance, the re-ceiver end of the system includes an input amplifi-cation stage in the form of a single-transistor low-noise amplifier. This stage amplifies the power ofthe incoming signal while minimizing distortion atits output. The LNA consists of a bipolar transistorwith resistive feedback and an inductive load. Theoutput of the amplifier consists of an LC matchwhich transforms the actual load impedance(1.5kΩ) of the mixer to the load desired for thespecified amount of power gain (See AppendixA-2). The input impedance should ideally matchthe impedance of the input source through someLC transformation network (i.e. it should bematched with the