LDPS- Technical Design
Assignment
Band-Pass Filter and PCB Design
Payam Barnaghi
Centre for Communication Systems Research
Electronic 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
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Image source: http://www.electronics-tutorials.ws/filter/filter_1.html
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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LPF Gain
Xc = 1/2πfC
Vout= Vin x R2/(R1+R2)
Rt = R1 + R2
Z=
(R2 + Xc2)
Vout = Vin x Xc/Z
Gain (db) = 20 log Vout / Vin
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LPF cut-off frequency
fc = 1/2πRC
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 = 1/2πfcR
C= 1/2 x π x 30000 x 20 x 1000
C= 26.5 nF
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The High-Pass Filter
− Conceptually similar to the LPF
Cut-off frequency:
Gain (db) = 20 log Vout / Vin
fc = 1/2πRC
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/usbnesadapter
Image 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
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