24-01-2013, 11:02 AM
Integration and Analysis of a 24.3MHz FM Transmitter/Receiver System
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ABSTRACT
We designed and built an FM transmitter / receiver
system to operate at 24.3MHz and 9V battery
power supplies. The transmitter consumes
360mW of DC power and transmits a 2.75dBm
signal.
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.
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.
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).
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.
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 Ω) 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.)
The Receiver
The receiver end of the system captures the transmitted
signal through an antenna and amplifies
that signal so that it may be downconverted to IF,
filtered, and demodulated to the original signal.
The following components together perform this
functionality: an input filter, low-noise amplifier,
mixer, IF amplifier and filter, phase-locked loop,
and an audio speaker.
Transmitter
The following is a discussion of the performance
of each of the blocks within the transmitter circuit.
Audio Input and Amplification
The audio amplifier circuit provides sufficient signal
gain while allowing for adjustment to large
input signals. We were able to adjust the potentiometer
at the input to the op-amp to accommodate
a comfortable speaking distance from the microphone.
The circuit consumes 36mW of DC power
and has a gain of 20.7 dB at 1kHz. We found the
frequency response to be less than ideal, however,
as the output peaks at 4.63kHz, and from measurements
made on the spectrum analyzer the –3dB
bandwidth is 2.75kHz to 8.5kHz. Since the audio
range is defined from 20Hz to 20kHz and most
vocal signals fall under 1kHz, this is not the optimal
bandwidth for the desired input signals. Because
the frequency response of this circuit is
largely determined by the response of the op-amp,
which conceivably has a more than adequate slew
rate, it is uncertain as to why the circuit would
yield such an undesirable response. It is possible
that measurements were made without properly
tuning the input potentiometer. This could cause
distortion or attenuation of the signal at the measured
frequencies. The output potentiometer sets
the output to a DC voltage of 7.5V. See Appendix
A-4 for further discussion of this circuit.
Transmitter/Receiver System
The transmitter and receiver system collaborated
well, with a maximum transmittable distance of
about 1/2 mile. In general, we found that matching
of impedances is a key factor in maximizing system
performance. Matching impedances allows for
maximum power transfer as well as minimization
of travelling wave reflections in the transmission
lines. The first improvement in this respect can be
made in matching the output of the transmitter and
the input of the receiver to their respective antennae
will help deliver maximum power to the respective
loads. In addition, matching between
stages within the two component blocks can be
improved, most notably between the transmitter
mixer and the power amplifier. We did attempt to
build matching networks in each of these locations,
but they tended not to yield the desired frequency
response, due to variations in component
values, stray parasitic capacitances and transistor
input capacitances. We need to conduct a more
thorough characterization of input / output impedances
before we can attempt to match to them.
CONCLUSION
We designed and built a 24.3 MHz FM transmitter
/ receiver system with both functional chips and
discrete components. We successfully integrated a
number of circuit blocks to perform the required
FM modulation and demodulation. Maximization
of performance proved to come through enough
power amplification in transmission and low-noise
amplification in reception. We found that there is
such a thing as too much amplification, however,
which can cause distortion as with the IF amplifier.
In addition, impedance matching is an important
consideration, although reliable characterization
of the impedances to be matched is necessary
for this to be an effective improvement to the
system. In addition, tuning the two system components
to work together at the correct frequencies
helps the system function cohesively.