for use Supervisor: on chip

Transcription

1 Local Oscillator for use in FM Broadcast Radio Receiver ETI 041: Radio Project Supervisor: Göran Jönsson Student: Yelin Wang and Hao Cai Master Program: System on chip Lund University

2 Abstract Oscillator is one of the essential components in superheterodyne receiver. These project main aims to build a local oscillator for use in an FM broadcast radio receiver. The design is implemented by using Clapp structure and bipolar transistor NXP BFG520X in emitter follower structure. Frequency can be tuned by voltage control of capacitance diode BBY40. Harmonic and spurious signal rejection is involved in the project too. i

3 Table of Contents CHAPTER 1 Introduction Introduction of RF Receiver System Requirement 2 CHAPTER 2 Circuit Design Oscillator Buffering Circuit Filter Design 4 CHAPTER 3 Parameter Calculation Oscillator Circuit Amplifier Biasing Filter Design Matching Network 6 CHAPTER 4 PCB Layout Design 7 CHAPTER 5 Result 8 Reference 10 Acknowledgments 11 ii

4 Introduction 1. Introduction 1.1. Introduction of RF receiver Oscillator is one of the essential components in superheterodyne receiver. Virtually all modern radio and TV receivers use the superheterodyne principle. Figure 1.1 shows the system of the superheterodyne receiver. Figure 1.1: The superheterodyne receiver [3] Oscillators can generally be categorized as either amplifiers with positive feedback satisfying the well known Barkhausen Criteria (Ref. 1), or as negative resistance circuits (Ref. 2). Both concepts are illustrated in Figure 1.2. At RF and Microwave frequencies the negative resistance design technique is generally favored. Figure 1.2: Negative resistance circuits and amplifiers circuit It is customary to assume that for startup of oscillations the loop gain must be greater than one, so that as the transconductance g m of the transistor starts to reduce as the signal levels grow, and the limiter starts to limit, the Barkhausen criterion is satisfied at steady state (i.e., loop gain is exactly equal to one) at a reasonable amplitude of oscillation. Over the years several RF oscillator configurations have become standard. These are illustrated in Figure 1.3. The Colpitts, Hartly and Clapp circuits are examples of negative resistance oscillators shown here using bipolar as the active devices. 1

5 Figure 1.3: Colpitts and Hartly configuration An oscillator circuit known as the Clapp circuit or Clapp Gouriet circuit is shown in Figure 1.4. The Clapp oscillator is a modified Colpitts oscillator, with a series LC replacing the lone inductor. This oscillator circuit has the practical advantage of being able to provide another degree of design freedom by making C 0 much smaller than C 1 and C 2, like the Colpitts, the Clapp obtains its feedback via a capacitive voltage divider. Figure 1.4: Clapp configuration 1.2. System Requirement 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 = 12 V regulated Output Power = 8 12V (minimum) Any harmonics = 16 dbc at least Other spurious = 70 dbc at least This report will now concentrate on a worked example of a Clapp oscillator, using a varactor tuned capacitor for voltage control of the output frequency. The frequency under consideration will be between 98.7 MHz and MHz, which is purposely set in order to mix down to the IF frequency (10.7 MHz). 2

6 Circuit Design 2. Circuit design The simulation tools, Multisim 10.0 and Advanced Design System (ADS) have been used in oscillator circuit simulation. The whole circuit can be mainly divided into three parts: oscillator circuit, buffering circuit and low pass filter. The schematic is shown in Figure 2.1. Figure 2.1: Schematic (oscillator, buffer and filter) Important: We need a coupling capacitance at the output. Otherwise, the connected instrument may suffer dangerous from DC signal Oscillator We use the oscillator circuit in Clapp configuration with a bipolar amplifier (using NXP BFG520X) in common collector configuration (also known as an emitter follower or voltage follower) amplifier. The emitter follower will offer a voltage gain just under one. The circuit will generate a fixed frequency sinusoidal wave. Here we introduce a VHF variable capacitance diode named BBY40 to achieve the frequency tuning between 98.7 MHz and MHz. The characteristic of BBY40 is shown in Figure

7 Figure 2.2: Diode capacitance as a function of reverse voltage 2.2. Buffering circuit The second BFG520X is a simple emitter follower that is used only as a buffer between the oscillator and the filter. A buffer amplifier is one that provides electrical impedance transformation from one circuit to another. Large signals can often then be reduced by a 3 or 6 db attenuator which also has the benefit of presenting well defined load impedance to the amplifier. If the stage is feeding a mixer, as is most often the case, then another benefit is the mixer, also see a source impedance of 50 ohms Filter design A low pass filter is used in the circuit for harmonic rejection. Figure 2.3: Low pass filter for harmonic rejection 4

8 Parameter Calculation 3. Parameter calculation 3.1. Oscillator circuit The oscillation frequency can be calculated by follow equations: The VHF variable capacitance makes the tuning of frequency, f 0 = π 3.2. Amplifier biasing The first stage: V B = V Ω Ω. Ω. Ω. Ω Ω Ω = 5.56 V I E =. V. V = 10.8 ma (V BE = 0.7 V) Ω The second stage: V B = V Ω. Ω. Ω. Ω Ω = V I E =. V. V = ma (V BE = 0.7 V). Ω 3.3. Filter design A low pass filter designed to be use in harmonic rejection. Figure 3.1 shows the passband and stop band of this filter. 5

9 Ω 2 = 2π * M (rad/s) Ω 3 = 2π * 200 MHz (rad/s) A 2 = 2 db A 3 = 20 db Figure 3.1: Low pass filter design [4] Filter approximation use Butterworth ladder filter structure. A(f) = n = 4.92, we choose n = 5. By using table [5], we get the result: L 1 = C 2 = L 3 = C 4 = L 5 = The actual component value is demonstrated in Figure 3.2. Figure 3.2: The fifth order low pass filter 3.4. Matching network In order of maximum transfer of power and output power, we need to design a 50 ohm matching in the emitter resistance in the second stage. R E = 10.7 kohm 6

10 PCB Layout Design 4. PCB layout design Easily Applicable Graphical Layout Editor (EAGLE 5.0) plays the role in the printed board layout. Figure 4.1 and Figure 4.2 shows the PCB layout and a close look to the board separately. Figure 4.1: PCB layout Figure 4.2: A close look to PCB 7

11 Result 5. Result Figure 5.1: The output signal and the harmonics around 98 MHz Figure 5.2: The output signal and the harmonics around 118 MHz 8

12 Test instrument is using spectrum analyzer in Radio lab, LTH. Figure 5.1 and Figure 5.2 shows the test result around the lowest frequency 98.7MHz and the top frequency MHz. According to the requirement, the power of the output is too week, only 16 dbm. The tuning frequency achieved by controlling the power supply of BBY40. The neighboring harmonic is 20 db below the desired frequency which satisfied the requirement. Figure 5.3 shows the power supply variation effects on output frequency. Figure 5.3: The power supply variation effects on output frequency The power supply voltage is adjusted from 10V 14V, the variation of the resonant frequency is 1.46MHz, which indicates that the changing speed of the resonant frequency according to the variation of power supply is 0.365MHz/V. 9

13 Reference Reference [1] L.Sundström, G.Jönsson, H.Börjesson, Department of Electroscience, Lund University, Radio Electronics, 2004 [2] Paul H.Young, Electonic Communication Techniques [3] G.Jönsson, Department of Electroscience, Lund University, Slides from the Radio course, 2008 [4] G.Jönsson, Department of Electroscience, Lund University, Slides from the Radio Electronics course, 2009 [5] L.Sundström, G.Jönsson, Department of Electroscience, Lund University, Radio Electronics Formulas and Tables,

14 Acknowledgments We wish to thank our supervisor, Goran Jönsson, for his suggestion in circuit design and layout assistance, and reviewing of our design. Additionally, we would like to thank Ping Lu, for her supplement design document; Lars Hedenstjerna, for producing our circuit board. 11