analog and telecommunication electronics...• understanding of filter types and parameters –...

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Analog and Telecommunication Electronics

E1 - Filters type and design» Taxonomy and parameters» Design flow and tools » FilterCAD example» Basic II order cells

AY 2015-16

Politecnico di TorinoElectronic Eng. Master Degree

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Lesson E1: Filters type and design

• Filter taxonomy and parameters• Design flow• Design tools

– FilterCAD example

• Basic II order cells with Op Amp– Multiple feedback – Finite gain– Two-integrator loop

• References:– Design with Op Amp …: 3, 4.1 Active Filters– Elettronica per Telecom.: 2.1.3 Filtri attivi

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Goals of this lesson

• Understanding of filter types and parameters– Low-pass, high-pass, band-pass/reject

• Knowledge of technologies to build filters (not RF)– Passive, active Op Amp, Switched Capacitor

• Use of CAD tools for filter design– Filter design process

• Knowledge of Op Amp circuits for basic cells

• Ability to design II order cells with Op Amps– Finite gain, multiple, feedback, …

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Many types of filters

RF filters(betweenantenna and RX/TX amplifiers): tuned circuits(LC) or mechanical(ceramicresonators)

IF channel filters (narrowband-pass), isolate single radio channels). Same technologies as RF

Baseband and audio filters (low-pass). Active filters

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Filter types and parameters

• Function of a filter:– Get a defined frequency response

H(ω) = Vo/Vi

• Transfer function type

– High-pass

– Low-pass

– Band-pass/reject

• Lin/log representation

ω

H(ω)Vi Vo

H(ω)

H(ω)

H(ω)

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Filter examples - a

• Pass-band– Radio Frequency (GHz)

» Remove outband signals for RX» Reduce harmonics and distortion for TX» Based on tuned circuits

– Intermediate frequency channel» Isolate single channels» Based on LC or mechanical resonators, or digital processing

• Low-pass – Anti-aliasing before ADC, reconstruction after DAC

» Active filters with R, C, Op Amp» Switched Capacitor circuits

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Filter examples - b

• High pass – Remove DC (or LF) from the signal

» Cancel offset and other DC errors» R, C, Op Amp, SC

• Band reject– Remove specific interferences

» Low frequency (50/60 Hz)R, C, Op Amp, SC

» EMC/Radio interferers

• Notch filters– Remove a single F

» Tuned circuits» Mechanical filters

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Structure of filters (low-pass)

• We can get only approximation of ideal H(ω)

– Causality principle» Hard F limit infinite in time

– Tolerances» Real value of devices

• Any p(s) with real coefficients can be decomposed in I and II order terms, with real coefficients.

– Any p(s) can be built with a cascade of I or II order cells

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Filters: design sequence

• Define specifications (filter mask)» Band-pass gain and ripple» Cutoff frequency and slope» Band-reject attenuation

• Filter design» Which approximation?» How many cells?

• Selection of technology» Analog/digital?» Which circuit for basic cells?

• Circuit design » schematic diagram, values of components, tolerances, …

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Filter technologies

• Analog filters:– Passive LC (R): inductors, capacitor, (resistors)

» Size, weight, parasitic– Active filters (Op Amps + RC)

» Active device constraints (need power, limited range, …)– SC (Switched Capacitors)

» Most common technique in current ICs

• Digital filters (not addressed in this course): – Need A/D and D/A conversion

» Intrinsic aliasing & quantization errors» Need processing power, memory, …

– Mostly automated design– Digital processing with microP, DSP, FPGA (easy to modify)

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Filters: approximation types

• Ideal transfer funct. approximated as polynomials ratio

• Several choices for approximation, such as:

– Bessel» Linear phase, no ripple in passband» Least steep

– Butterworth» No ripple in passband

– Chebicheff» Ripple in passband» Most steep around cutoff

– …. Many others, with different optimizations

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Bessel approximation

– Linear phase, constant group delay, no distortion– No ripple in pass-band

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Butterworth approximation

– No ripple in pass-band

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Chebicheff approximation

– Ripple – Very steep

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Cell parameters

• Each cell has a II order response

• ω0 and ξ cannot be directly measured– Design from ω0 and ξ– Test and tuning from peak position (ωα) and amplitude

Design TuningPole number cell

real pole

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Filter design tools

• Several deign tools availble on the web

• Linear Technology:http://ltspice.linear.com/software/FilterCAD.zip(simple)

• Texas Instruments:http://www.ti.com/lsds/ti/analog/webench/webench-filters.page

(complete)

• Others ….

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Filter design: example 1 - a

• Specs definition, or filter mask– Passband gain– Passband ripple (R)– Stopband attenuation (A)– Passband limit (Fc)– Stopband limit (Fs)

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Filter design: example 1 - b

• Design of the filter– Which approximation?– How many poles/cells needed?– Which parameters for each cell?

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Filter design: example 1 - c

• Frequency response

time domainstep response

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Filter design: example 1 - d

• Select technology– Switched capacitor or R + C + A.O. (active RC) ? – Which basic cell circuit?

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II order cells

• The basic II order cell can use:– L, C, (R) – actually used only for RF– Specific IC, with internal Op Amps (e.g. the LTC1562)

• Op Amp with feedback (R, C)– Multiple feedback, Constant gain, Double integrator, …– Critical issue: tolerances

» Need high precision passive components (R, C)» OK for “discrete”, difficult to get inside ICs

• Switched Capacitor circuits far better for integration– High precision ratio of the same component (C)– General trend to use SC to replace R

» Filters, amplifiers, ADC/DAC, …

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II order cell with Op Amp: example 1

A

vA

Can be low/high/band pass, depending on choices of Yi

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Example circuit: low-pass cell analysis

R1 =

R3 =

R4 =

C2 =

C5 =

Evaluate

n = ?

= ?

H(0) = ?

R3

AOVI

-

+ VU

C5R1

R4

C2

A

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Example circuit: frequency response

Bode plot (on the web: Simulators, II order functions, or SPICE analysis)

R3

AO

VI

-

+ VU

C5R1

R4

C2

A

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Example circuit: time-domain response

Step response (on the web: Simulators, II order functions, or SPICE analysis)

R3

AO

VI

-

+ VU

C5R1

R4

C2

A

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2-integrator cell

Same circuitprovideslow/band/high-pass

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Comparison with LTC1562 cell

The LTC 1562 cell is actually a two-integrator loop. The adder A uses the inverting input of the Op Amp (integrator 1)

Complete data sheet: http://www.linear.com/pdf/1562fa.pdf

I1

I2

I1I2

A

A

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Basic cell of LTC1562 filter IC

4 double integrator cells

Parameters defined by external components

Data sheet: http://www.linear.com/pdf/1562fa.pdf

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Filter design: example 1 - e

• Final complete circuit diagram (from FilterCAD)

Not the best type ofschematic(topographic/ functional)

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Lesson E1: Final test

• Describe filter taxonomy, based on frequency response.

• Which are the parameters that define a filter?

• For one of tf the filter types, describe the effect of changing the frequency response parameters on the time-domain step response.

• Describe the design flow for a filter.

• Which are the benefits and drawbacks of active filters built with Op Amps?

• Describe at least two circuits to get II order response from RC circuits.

• Draw he diagram of a Multiple Feedback low-pass cell.

• Draw the diagram of a Finite Gain low-pass cell.

• Turn the cell into high-pass.

analog and telecommunication electronics...• understanding of filter types and parameters –...

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