Analog and Telecommunication Electronics

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1 Politecnico di Torino - ICT School Analog and Telecommunication Electronics E1 - Filters type and design» Filter taxonomy and parameters» Design flow and tools» FilterCAD example» Basic II order cells with Op Amp 19/05/ ATLCE - E DDC 2014 DDC 1

2 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.: Filtri attivi 19/05/ ATLCE - E DDC 2014 DDC 2

3 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, 19/05/ ATLCE - E DDC 2014 DDC 3

Many types of filters RF filters (between antenna and RX/TX amplifiers): tuned circuits (LC) or mechanical (ceramic resonators) IF channel filters (narrow

4 Many types of filters RF filters (between antenna and RX/TX amplifiers): tuned circuits (LC) or mechanical (ceramic resonators) IF channel filters (narrow band-pass), isolate single radio channels). Same technologies as RF Baseband and audio filters (lowpass). Active filters 19/05/ ATLCE - E DDC 2014 DDC 4

5 Filter types and parameters Function of a filter: Get a defined frequency response Vi H(ω) Vo H(ω) = Vo/Vi Transfer function type High-pass Low-pass Band-pass/reject H(ω) H(ω) H(ω) ω Lin/log representation 19/05/ ATLCE - E DDC 2014 DDC 5

6 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 19/05/ ATLCE - E DDC 2014 DDC 6

7 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 19/05/ ATLCE - E DDC 2014 DDC 7

8 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 19/05/ ATLCE - E DDC 2014 DDC 8

9 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, 19/05/ ATLCE - E DDC 2014 DDC 9

10 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) 19/05/ ATLCE - E DDC 2014 DDC 10

11 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 19/05/ ATLCE - E DDC 2014 DDC 11

12 Bessel approximation Linear phase, constant group delay, no distortion No ripple in pass-band 19/05/ ATLCE - E DDC 2014 DDC 12

13 Butterworth approximation No ripple in pass-band 19/05/ ATLCE - E DDC 2014 DDC 13

14 Chebicheff approximation Ripple Very steep 19/05/ ATLCE - E DDC 2014 DDC 14

15 Cell parameters Each cell has a II order response ω 0 and ξ cannot be directly measured Design from ω 0 and ξ Test and tuning from peak position (ω α ) and amplitude Pole number cell Design Tuning real pole 19/05/ ATLCE - E DDC 2014 DDC 15

16 Filter design: example 1 - a Specs definition, or filter mask Passband gain Passband ripple (R) Stopband attenuation (A) Passband limit (Fc) Stopband limit (Fs) 19/05/ ATLCE - E DDC 2014 DDC 16

17 Filter design: example 1 - b Design of the filter Which approximation? How many poles/cells needed? Which parameters for each cell? 19/05/ ATLCE - E DDC 2014 DDC 17

18 Filter design: example 1 - c Frequency response time domain step response 19/05/ ATLCE - E DDC 2014 DDC 18

19 Filter design: example 1 - d Select technology Switched capacitor or R + C + A.O. (active RC)? Which basic cell circuit? 19/05/ ATLCE - E DDC 2014 DDC 19

20 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, 19/05/ ATLCE - E DDC 2014 DDC 20

21 II order cell with Op Amp: example 1 A v A Can be low/high/band pass, depending on choices of Yi 19/05/ ATLCE - E DDC 2014 DDC 21

22 Example circuit: low-pass cell analysis R1 = R4 R3 = R4 = R1 A R3 - C5 C2 = C5 = V I C2 + AO V U Evaluate n =? =? H(0) =? 19/05/ ATLCE - E DDC 2014 DDC 22

23 Example circuit: frequency response R4 Bode plot (on the web: Simulators, II order functions, or SPICE analysis) R1 R3 - A V I C2 + C5 AO V U 19/05/ ATLCE - E DDC 2014 DDC 23

24 Example circuit: time-domain response R4 Step response (on the web: Simulators, II order functions, or SPICE analysis) R1 R3 - A V I C2 + C5 AO V U 19/05/ ATLCE - E DDC 2014 DDC 24

25 II order cell with Op Amp: example 2 Finite gain (K) circuit 19/05/ ATLCE - E DDC 2014 DDC 25

26 II order cell with Op Amp: example 3 2-integrator cell Same circuit provides low/band/high-pass 19/05/ ATLCE - E DDC 2014 DDC 26

27 Comparison with LTC1562 cell A I2 I1 I2 I1 A 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: 19/05/ ATLCE - E DDC 2014 DDC 27

28 Basic cell of LTC1562 filter IC 4 double integrator cells Parameters defined by external components Data sheet: 19/05/ ATLCE - E DDC 2014 DDC 28

29 Filter design: example 1 - e Final complete circuit diagram (from FilterCAD) 19/05/ ATLCE - E DDC 2014 DDC 29

30 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. 19/05/ ATLCE - E DDC 2014 DDC 30

Analog and Telecommunication Electronics

Politecnico di Torino - ICT School Analog and Telecommunication Electronics E1 - Filters type and design» Filter taxonomy and parameters» Design flow and tools» FilterCAD example» Basic II order cells