Project by Santiago Yeomans, Chad Cummins, Gboyega Adeola Guitar Signal Transmitter

Introduction Electric guitar output will connect to FM transmitter Transmitted audio will be received at receiver and connected to guitar amplifier ¼ “ audio plugs used for connections –output of guitar to transmitter –output of receiver to amp

Frequency Modulation vs. Phase Modulation FMPM Fair S/N ratioBest S/N ratio Variable modulation index Constant modulation index Easier implementation Phase lock loop to demodulate signal VCO can produce high index freq. modulation Multipliers needed to produce high index PM Cost efficientExpensive

Flow Diagram Reactance Modulator Carrier Oscillator Buffer Amplifier Frequency Multipliers Driver Amplifier Power output amplifier Reactance Modulator: The nature of FM is that when the baseband signal is zero, the carrier is at its “ carrier ” frequency, when it peaks the carrier deviation is at a maximum and when it troughs the deviation is at its minimum. This deviation is simply a quickening or slowing down of frequency around the carrier frequency by an amount proportional to the baseband signal. In order to convey this characteristic of FM on the carrier wave, the capacitance must be varied. Buffer Amplifier: The buffer amplifier acts as a high input impedance with a low gain and low output impedance associated with it. The high input impedance prevents loading effects from the oscillator section. Frequency Multipliers: To avoid frequency drifts of the LC tank while modulating the carrier by the baseband with a high modulation index, modulation can take place at lower frequencies with a higher Q factor of the oscillator. Power Output Section- Develops the final carrier power to be transmitted. Also included here is an impedance matching network, in which the output impedance is the same as that on the load (antenna)

Block Diagram In Out

Three Stages Signal Source (Guitar, CD player) –CD Audio Output: >1.5 V –Guitar Signal Output: 150mv –Output Impedance: Transmitter –Colpitts Oscillator ---  –Circuit Self-Amplification Receiver –Hartley Oscillator ---  –Audio amplifier

Oscillator Resonant Circuits. Resonant frequency is that at which the impedance of capacitor & inductor is the same; it represents the oscillator carrier frequency in Hertz. The parallel resonant circuit we used, known as an LC tank, takes the advantage of the resonant frequency and allows the impedance to be at a maximum & the current at a minimum at Fc. Q : ratio of maximum energy stored to the amount lost per ac cycle. It determines the 3dB bandwidth of resonant circuits. Since we didn ’ t have a resistor in the LC tank, the inherent properties of inductor & capacitor at high frequencies had to be taken into account.

Essential Circuit Elements Transistors: –Transmitter: 2 (2N3904) –Receiver: BF256, 2N3904 Inductors –Copper Coils 5 turns 16 turns

Transmitter

Receiver

LPF 682 Hz 1 kHz Chebyshev Filters 6-Band Equalization Stage

2 kHz 4 kHz

8 kHz 10.9 kHz

Pspice Simulation

Output of receiver connected to guitar amplifier

Project Achievements Achieved both FM RF Transmission and reception –Carrier frequency of 100 MHz Audio received and sent to guitar amplifier Audio from cd player worked well Temporarily had guitar audio transmitting and receiving Circuits were low cost to build

Performance Quality of Audio –Clear at times, some noise occasionally Transmission –Better transmission was achieved with a source device having a larger input impedance.

Project Challenges Setup both transmitter and receiver for same carrier frequency (100MHz) –Variable Capacitor range was unknown, not sure about the pins (Variable cap taken from $5 handheld radio bought from Wal-Mart)

Challenges continued Working with the LM741 Op Amp Working with a breadboard Parasitic Capacitances Unable to Effectively Simulate Inductors Parasitic Capacitances inherent in high frequency engineering

Oversights Impedance Mismatching –Between Amplifier and Receiver Under-estimated difficulty of amplifying guitar audio before transmission

Timeline

Factors for obtaining better S/N Resistive properties in LC tank A) Skin effect - at high frequencies, there is less cross sectional area for carriers to move, so the resistance increase; when the magnetic field at the centre of the wire increases and local inductive reactance takes over, that is, stray capacitances begin to build up between adjacent turns. B) Dielectric permittivity Temperature Stability of the oscillator Components in oscillator have non-zero temperature coefficients. To find the change in frequency for a given temperature change, simply multiply the coefficient by the temperature change & the centre frequency. Major source of frequency instability: Capacitor & Transistor (junction capacitance) More compact circuitry Wrapping the circuit with aluminum foil to electromagnetically shield the RF stage. Unwanted electromagentic radiation had to be stopped from destructively interfering With the carrier modulation.

Future Improvements Replace the common 2N3904 transistor with a BC549 which would perform better with high frequency Use a ground plane for better performance of sensitive circuits Solder all connections