Analog and Telecommunication Electronics

Transcription

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 29/06/ ATLCE - E DDC

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 Reference: Text 1 (Sedra): 2, 12, Text 2 (Del Corso): 2.1.3, /06/ ATLCE - E DDC

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, 29/06/ ATLCE - E DDC

4 Many types of filters RF filters (between antenna and RX/TX amplifiers) IF channel filters (narrow band-pass), isolate single radio channels) Baseband and audio filters (low-pass) 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC

9 Filter implementation techniques 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: Need A/D and D/A conversion» Sampling aliasing, quantization errors» Need processing power, memory, Mostly automated design Can use microp, DSP, FPGA (easy to modify) 29/06/ ATLCE - E DDC

10 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, 29/06/ ATLCE - E DDC

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» No ripple in passband» Most steep around cutoff 29/06/ ATLCE - E DDC

12 Bessel approximation Linear phase, constant group delay, no distortion No ripple in pass-band 29/06/ ATLCE - E DDC

13 Butterworth approximation No ripple in pass-band 29/06/ ATLCE - E DDC

14 Chebicheff approximation Ripple Very steep 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC

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) 29/06/ ATLCE - E DDC

17 Filter design: example 1 - b Design of the filter Which approximation? How many poles/cells needed? Which parameters for each cell? 29/06/ ATLCE - E DDC

18 Filter design: example 1 - c Frequency response time domain step response 29/06/ ATLCE - E DDC

19 Filter design: example 1 - d Select technology Switched capacitor or R + C + A.O. (active RC)? Which basic cell circuit? 29/06/ ATLCE - E DDC

20 II order cells The basic II order cell can use: L, C, (R) actually used only for RF Specific IC (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, 29/06/ ATLCE - E DDC

21 II order cell with Op Amp: example 1 A v A Can be low/high/band pass, depending on choices of Yi 29/06/ ATLCE - E DDC

22 Example circuit: low-pass cella anlysis R1 = R2 R2 = R3 = R1 A R5 - C6 C4 = V I C4 + AO V U R5 = C6 = Evaluate n =? =? H(0) =? 29/06/ ATLCE - E DDC

23 Example circuit: frequency response R2 Bode plot (on the web: Simulators, II order functions, or SPICE analysis) R1 R5 - A V I C4 + C6 AO V U 29/06/ ATLCE - E DDC

24 Example circuit: time-domain response R2 Step response (on the web: Simulators, II order functions, or SPICE analysis) R1 R5 - A V I C4 + C6 AO V U 29/06/ ATLCE - E DDC

25 II order cell with Op Amp: example 2 Finite gain (K) circuit 29/06/ ATLCE - E DDC

26 II order cell with Op Amp: example 3 2-integrator cell Same circuit provides low/band/high-pass 29/06/ ATLCE - E DDC

27 Comparison with LTC1562 cell I2 I1 I2 I1 The LTC 1562 cell is actually a two-integrator loop. The adder uses the inverting input of the Op Amp (integrator 1) Complete data sheet: 29/06/ ATLCE - E DDC

28 Basic cell of LTC1562 filter IC 4 double integrator cells Parameters defined by external components Data sheet: 29/06/ ATLCE - E DDC

29 Filter design: example 1 - e Final complete circuit diagram (from FilterCAD) 29/06/ ATLCE - E DDC

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 29/06/ ATLCE - E DDC