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LDPS- Technical Design Assignment

Band-Pass Filter and PCB Design

Payam BarnaghiCentre for Communication Systems ResearchElectronic Engineering Department University of Surrey

Spring Semester 2014

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Assignment Aim

−The aim of this work is to create a professional report on how to design a Band-Pass Filter (BPF):

− Discuss the theory behind a BPF.

− Compute the necessary component values and make sure that the filter response is correct

− Design the Printed Circuit Board (PCB) using the Eagle software.

− Provide a report reader with all the information that a reader will need to build her/his own version of a BPF.

− You will NOT be fabricating a PCB or building anything.

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Band-Pass Filter Introduction

-The BPF will be a filter used in telephony communications: Pass-band between 200Hz and 4.8kHz.- A BPF can be constructed by a Low-Pass Filter (LPF) and a High-Pass Filter (HPF) connected together- Decide the lower and higher cut-off frequencies of the BPF.- Use a HPF cut-off to generate the lower cut-off frequency of the BPF.- Use a LPF cut-off to generate the higher cut-off frequency of the BPF.

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Commonly used filter designs

− Low Pass Filter – only allows low frequency signals from 0Hz to its cut-off frequency 

− High Pass Filter – allows high frequency signals from its cut-off frequency 

− Band Pass Filter – allows signals within a frequency range between two points to pass through and blocks both the lower and higher frequencies on either side of this frequency range.

Capacitive Reactance Capacitive Reactance

Image source: http://www.electronics-tutorials.ws/filter/filter_1.htmlImage source: http://www.electronics-tutorials.ws/filter/filter_1.html

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The Low-Pass Filter

− A 1st order, low-pass RC filter is simply an RC series circuit across the input, with the output taken across the capacitor.

− We assume that the output of the circuit is not connected, or connected only to high impedance, so that the current is the same in both R and C.

− The voltage across the capacitor is IXC = I/ωC.

− The voltage across the series combination is IZRC = I(R2 + (1/ωC)2)1/2, so the gain is:

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Rt = R1 + R2

Z= (R2 + Xc2)

Vout = Vin x Xc/Z

Rt = R1 + R2

Z= (R2 + Xc2)

Vout = Vin x Xc/Z

LPF Gain

Gain (db) = 20 log Vout / VinGain (db) = 20 log Vout / Vin

Xc = 1/2πfC Xc = 1/2πfC

Vout= Vin x R2/(R1+R2)Vout= Vin x R2/(R1+R2)

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LPF cut-off frequency

fc = 1/2πRC fc = 1/2πRC

Phase Shift ф = - arctan (2πfRC)Phase Shift ф = - arctan (2πfRC)

For example: An LPF will only allow frequencies to pass below 30kHz (30,000Hz). Assuming that the resistor used in the design has a values of 20kΩ´s, calculate the values of the capacitor required.

C = 1/2πfcRC= 1/2 x π x 30000 x 20 x 1000C= 26.5 nF

For example: An LPF will only allow frequencies to pass below 30kHz (30,000Hz). Assuming that the resistor used in the design has a values of 20kΩ´s, calculate the values of the capacitor required.

C = 1/2πfcRC= 1/2 x π x 30000 x 20 x 1000C= 26.5 nF

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The High-Pass Filter

− Conceptually similar to the LPF

Cut-off frequency:Cut-off frequency: Gain (db) = 20 log Vout / VinGain (db) = 20 log Vout / Vin

fc = 1/2πRC fc = 1/2πRC

Phase shift ф= Tan-1 (1/2πfRC )Phase shift ф= Tan-1 (1/2πfRC )

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Qualitative Example

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Band-pass filter

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Band-pass filter

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PCB Design

Image source: http://www.bidouille.org/elec/usbnesadapterImage source: http://www.bidouille.org/elec/usbnesadapter

Image source: http://www.wb5rvz.com/sdr/RXTX_V6_3/06_mixer_rx.htmImage source: http://www.wb5rvz.com/sdr/RXTX_V6_3/06_mixer_rx.htm

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PCB Design

− The Schematic: before beginning to lay out your PCB, you need to have a complete and accurate schematic diagram.

− Measurement units- − what measurement units are used and their common

terminologies.− Tracks; there is no recommendation for track sizes.

− Track size will depend on the electrical requirements of the design;

− the routing space and clearance that is available− The lower limit of your track width will depend upon the

“track/space” resolution of your PCB manufacturer. − In practice, the track size will be defined by the flowing current

and maximum temperature rise of the track that can be tolerated;

− Remember that tracks will have a certain amount of resistance. The wider the track the lower the resistance.

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PCB Design - continued

− Routing is the process of laying down tracks to connect components on your board.

− Silk Screen: this layer is also known as component overlay is the layer on top of your board (and bottom if needed) that includes outline of the components, designators (e.g. R1, C1) and text labels.

− Mechanical layer: is used to provide an outline for your board and other manufacturing instructions. It is not a part of the actual design.

− Keepout: this layer usually defines areas that you do not want to be automatically or manually routed.

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Assignment Preparation

−Before coming to the lab sessions you should have:− Analysed the circuit you will be using and developed

the equations that determine its response− Demonstrated that it is suitable for the task in hand

by plotting its frequency response (amplitude and phase)

− Determined the component values you will be using

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Recommended Report Structure

− Your report should include the following elements:− An Introduction (state the purpose of the document)− Theory and Design

− A higher-level block diagram of the system and a summary of its operation and specifications

− Lower-level circuit diagrams for each of the low-pass and high-pass filters above

− A description of the operation of each of the above filters using standard formulas from circuit analysis, an interpretation of these formulas, the determination of the filter cut-off points and plots of amplitude and phase responses.

− A PCB layout of your designed filter. Don’t include any labelling on the “top-silk”. You should only show the PCB board so that a technician can use your layout as a mask.

− Intermediate design steps can be shown in an Appendix to illustrate how the component names would be shown on the board.

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Report Structure (Continued)

−Assembly Drawings:− Box with co-axial input and output sockets− PCB board position− Any wiring− Any mounting hardware− Dimensions!− Anything you think a technician would need to know to build your

design.

−Parts/Components List− Should contain ALL materials that are required to build the filter

(prices, part numbers, component designation relative to the PCB, etc.)

− You will be working in pairs but an independent report is expected from each of you. The PCB boards, assembly drawings and components used can be the same but everything should be your own work!

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Submission due date

−All reports are due at 4:00pm, Tuesday of Week 3.

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Assessment

−Please note:− The emphasis is on presentation quality and the

expectation is to produce a professional document that will allow a technician or someone skilled in the electronics to be able to build what you have described.

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Questions?

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Acknowledgments

− Some of the slides are adapted from Dr. William Headley’s (CCSR, University of Surrey) EDPS assignment, 2012.

− Images in slide 4 are from: http://www.electronics-tutorials.ws/− David L. Jones, PCB Design Tutorial,

http://alternatezone.com/electronics/pcbdesign.htm