Download voltage-controlled oscillator for fm broadcast radio receiver

RADIO
PROJECT
RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
2011
VOLTAGE-CONTROLLED OSCILLATOR
FOR FM BROADCAST RADIO RECEIVER
Aijun Lu
Yang Gao
Supervisor: Göran Jönsson
Department of Electrical and Information
Technology, Lund University
This report demonstrates the full design process from schematic
design to after layout measurement that focuses on the local
oscillator using for the broadcast radio receiver. This design includes
three main stages which are common collector oscillator, common
emitter amplifier and fifth-order low-pass filter.
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Abstract
We designed a local oscillator for the superheterodyne receiver during the
project. Frequency tuning is voltage-controlled by using variable capacitance
diode BBY40 and the oscillator frequency can be variable for reception of
the FM broadcast band 88-108 MHz. The design is implemented using Clapp
oscillator structure. Besides, a fifth-order low pass filter was designed to
reject the harmonic.
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Table of Content
1 Preface .................................................................................................4
1.1 Introduction of RF receiver ............................................................4
1.2 Introduction of oscillator ................................................................4
1.3 Specification ..................................................................................6
2 Circuit design ....................................................................................6
3 Parameter calculations ..................................................................8
3.1 Oscillator circuit ...............................................................................8
3.2 Filter design .......................................................................................8
4 PCB layout design .........................................................................10
5 Results ...............................................................................................11
6 Conclusions ........................................................................ 13
7 Acknowledgments .........................................................................14
8 References .................................................................................... 14
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
1 Preface
1.1 Introduction of RF receiver
A superheterodyne receiver uses frequency mixing or heterodyning to
convert a received signal to a fixed intermediate frequency, which can be
more conveniently processed than the original radio carrier frequency.
Virtually all modern radio and television receivers use the superheterodyne
principle. Oscillator is one of the essential components in superheterodyne
receiver.
Figure1 The superheterodyne receiver [1]
As we know the FM broadcast band is 88-108 MHz, and usually a radio
receiver operates with an intermediate frequency, or IF of 10.7MHz. In order
to mix down the received frequency to the IF, the local oscillator should
work between 98.7MHz and 118.7MHz.
1.2 Introduction of oscillator
To be able to analyze the oscillating conditions Black’s feedback model is
used. Here the oscillator is split into two blocks: An amplifier which is
considered to be wideband and a feedback network that is usually frequency
selective as shown in figure2.
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Figure 2 black feedback model for oscillator
The transfer function for the amplifier with feedback is
So
when A* =1 and ( A* )=
=
。 the
feedback gain
will becomes infinite.
These equations are called the Barkhausen oscillation criteria. It’s the
condition that the system will be self-generating and we will perform
continuous oscillation.
There are some standard types of oscillators depending on how the feedback
is arranged. As shown in figure 3. One of the most common oscillator
arrangement found is the Colpitt oscillator with its feedback path through a
capacitive voltage divider, see figure 3.a. The feedback can also be through
an inductive tap as in figure 3.b. This configuration is called a Hartley
oscillator. The third circuit is a Clapp oscillator; it has a capacitor in series
with the inductor.
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Figure 3 Oscillator configuration: (a) Colpitt
(b)Hartley (c)Clapp [2]
1.3 Specification
For a superheterodyne receiver, the tuning is mechanical or voltage
controlled. The oscillator frequency should be variable for reception of a
specified frequency band 88 to 108 MHz.
Supply voltage is 12 V
Minimum output Power should be 8 dBm
Any harmonics should be at least -16 dBc
Other spurious should be at least -70 dBc
2 Circuit design
The design of our oscillator system can be split into three blocks: the
oscillator circuit, the buffer and the filter. The oscillator circuit is used to
generate a sinusoidal signal at a fixed frequency, the buffer is a common
emitter amplifier used to amplify the signal and the low-pass filter is used to
reject the harmonics and spurious. The schematic is shown in figure 4.
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Figure 4 Circuit schematic
Here we design oscillator structure with Clapp configuration by using the
bipolar amplifier BFR92A in common collector configuration. Also we
design a common emitter amplifier to amplify the signal to satisfy the 8dBm
output power requirement.
In order to satisfy that the tuning is mechanical or voltage controlled, we add
a variable capacitance diode named BBY40 in series with the inductor to
achieve the frequency tuning between 98.7 MHz and 118.7 MHz .The
component BBY40 has a capacitance which is controlled by the voltage
across it, and its characteristics is shown in figure 5.
Figure 5 Diode capacitance as a function of reverse voltage; typical values
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RADIO
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VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
3 Parameter calculations
3.1 Oscillator circuit
The oscillation frequency can be calculated by
Biasing
The emitter voltage is 5.9V-0.7V=5.2V, which can be used as the biasing
voltage of the next buffer stage. So we connect the emitter of the oscillator
bipolar to the base of the buffer amplifier bipolar directly.
3.2 Filter design
A low pass filter is designed to reject harmonics and spurious. As the local
oscillator works between 98.7MHz and 118.7MHz, so we that for the filter at
119MHz the gain should be larger than -2dB and at 200MHz the gain should
be less than -20dB.
We use Butterworth structure and the amplifier function is given by
For 119MHz
We can get
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
For 200MHz
We can get
=>n=4.94 thus we select the nearest higher order
n=5.
By looking up table, we get the result:
L1’ = 0.6180
C2’ = 1.6180
L3’ = 2.0000 C4’ = 1.6180
And demoralization gives
Figure 6 The fifth‐order low pass filter
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L5’ = 0.6180
RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
4 PCB layout design
After we complete the schematic, we use the EAGLE PCB tool to do our
printed board layout. Figure 7 and Figure 8 shows the PCB layout and the
final board with components
Figure 7 PCB layout
Figure 8 The final board with components
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RADIO
PROJECT
VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
5 Results
We utilize spectrum analyzer in Radio lab, LTH to test our circuit board.
Firstly we test the result around the lowest frequency 98.7MHz and the top
frequency 118.7 MHz. The results are shown in figure 9 and figure 10.
Figure 9 The output signal around 98.7 MHz
Figure 10 The output signal around 118.7 MHz
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RADIO
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VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
We can see from the figures that at 98.7MHz the output power is about
10dBm which is a little larger than 8dBm and can satisfy the requirement.
But at 118.7MHz the output power is only 2dBm, cannot satisfy the
requirement. The tuning frequency achieved by controlling the power supply
of BBY40.
We also test the harmonics. Figure 11 and 12 show the harmonics test
results.
Figure 11 The harmonics test result around 98.7 MHz
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RADIO
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VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
Figure 12 The harmonics test result around 118.7 MHz
From figure 11 we can see when the output power is about 10dBm the
neighboring harmonic is only about -25dBm, which satisfy the -16dBc
requirement. And at 118.7MHz the output power is around 2 dBm , the
neighboring harmonic is about -40dBm which also meet the -16dBc
requirement.
6 Conclusions
A local oscillator for a superheterodyne receiver with the tuning is voltage
controlled is designed and verified in this project. We design the
schematic and layout the PCB; finally we test our board at the radio
lab. The results show that we can adjust the control voltage to tune the
output frequency from 98.7MHz to 118.7MHz successfully. Although
we can achieve 10dBm output power at 98.7MHz, the output power
decreases when the frequency increases, and the output power is about
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RADIO
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VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER
2dBm at 118.7MHz, which cannot meet the requirement. We also test
the harmonics and they all are at least -16 dBc, which meet the
requirement.
To increase the output power, we can introduce a voltage divider circuit for
the second transistor to offer a larger basing voltage, which may increase the
power gain of the amplifier.
7 Acknowledgments
We would like to thank our supervisor, Göran Jönsson, for his suggestion in
circuit design and layout assistance as well as giving us knowledge during
lectures and guiding us during the labs. We would like to thank Lars
Hedenstjerna, for producing our circuit board.
8 References
[1] G.Jönsson, Department of Electroscience, Lund University, Slides from
the Radio course, 2010
[2] L.Sundström, G.Jönsson, H.Börjesson, Department of Electroscience,
Lund University, Radio Electronics, 2004
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