Transcript Guitar Signal Transmitter (TA: Saurav K. Bandyopadhyay)

Guitar Signal Transmitter
Project by
Santiago Yeomans, Chad Cummins,
Gboyega Adeola
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
FM
PM
Fair S/N ratio
Best S/N ratio
Variable
modulation index
Easier
implementation
Constant
modulation index
Phase lock loop to
demodulate
signal
Multipliers
needed to
produce high
index PM
VCO can produce
high index freq.
modulation
Flow Diagram
Buffer Amplifier
Carrier Oscillator
Reactance
Modulator
Frequency
Multipliers
Power Output
Reactance
Buffer
Frequency
Amplifier:
Modulator:
Multipliers:
SectionThe buffer
The
Develops
To avoid
nature
amplifier
frequency
the
of FM
final
actsiscarrier
as
that
drifts
a high
when
of
power
the
input
theLC
to tank
baseband
impedance
while
be
transmitted.
modulating
signal
withAlso
aisthe
low
zero,
included
carrier
gain
the and
carrier
by
here
low
theis
isbaseband
output
an
at its
impedance
“carrier”
impedance
with amatching
frequency,
high
associated
whenit.it The
with
modulation
network,
peaks
in index,
which
high
theinput
carrier
the
modulation
output
impedance
deviation
impedance
can prevents
take
is at place
aismaximum
the
loading
atsame
lower
effects
and
asfrequencies
that
when
fromit
troughs
the
with
on
the
oscillator
a load
higher
the (antenna)
deviation
section.
Q factorisofatthe
its oscillator.
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.
Driver
Amplifier
Power output
amplifier
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
6-Band Equalization Stage
Chebyshev Filters
LPF 682 Hz
1 kHz
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