voltage-controlled oscillator for fm broadcast radio receiver

RADIO PROJECT

VOLTAGE-CONTROLLED OSCILLATOR FOR FM BROADCAST RADIO RECEIVER

1

Aijun Lu

Yang Gao

Supervisor: Göran Jönsson

Department of Electrical and Information

Technology, Lund University

RADIO

PROJECT

VOLTAGE-CONTROLLED OSCILLATOR

FOR FM BROADCAST RADIO RECEIVER

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.

2011

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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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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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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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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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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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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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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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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 L5’ = 0.6180

And demoralization gives

Figure 6 The fifth‐order low pass filter

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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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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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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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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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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

voltage-controlled oscillator for fm broadcast radio receiver

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