EE247 Lecture 7 Summary last lecture Continuous time filters Bandpass filters Example Gm C BP filter using simple diff pair Linearity Noise Various Gm C Filter implementations Comparison of continuous time filter topologies EECS 247 Lecture 7 Filters 2004 H K Page 1 Summary last lecture Automatic on chip filter tuning Continuous tuning Master slave tuning Periodic off line tuning Systems where filter is followed by ADC DSP existing hardware can be used to periodically update filter freq response EECS 247 Lecture 7 Filters 2004 H K Page 2 Master Slave Frequency Tuning Reference Voltage Controlled Oscillator VCO Instead of VCF a voltage controlledoscillator VCO is used VCO made or replica integrators Tuning circuit operates exactly as a conventional phase locked loop PLL Tuning signal used to tune main filter Ref K S Tan and P R Gray Fully integrated analog filters using bipolar FET technology IEEE J Solid State Circuits vol SC 13 no 6 pp 814 821 December 1978 EECS 247 Lecture 7 Filters 2004 H K Page 3 Master Slave Frequency Tuning Reference Filter VCF Replica Filter Master Main Filter Slave 1 Vo Vin R Rs 1 1 1 Phase Comparator Amp Filter s 1 s 0 VLP 1 1 s 2 1 s 10 1 Q Vtune s 3 s 4 R R 1 s 5 Vr e f Ref H Khorramabadi and P R Gray High Frequency CMOS continuous time filters IEEE Journal of Solid State Circuits Vol SC 19 No 6 pp 939 948 Dec 1984 EECS 247 Lecture 7 Filters 2004 H K Page 4 L Reference C Gm Locked to Ref Frequency P2 high S2 closed S2 Vref S3 A Gm I Gm Vref C1 C2 Charge C1 with I Gm Vref P2 VC1 G m Vref T 2 C1 VC1 T1 T2 EECS 247 Lecture 7 Filters 2004 H K Page 5 Reference C Gm Locked to Ref Frequency P3 high S3 closed S2 Vref S3 C1 T1 T2 A Gm C2 Charge on C1 shared with C2 Feedback forces Gm to assume a value VC1 VC2 V r e f s i n c e VC1 Gm Vref T 2 C1 t h e n Vref Gm Vr e f T 2 or EECS 247 Lecture 7 Filters C1 Gm C1 T 2 N fclk 2004 H K Page 6 Reference C Gm Locked to Ref Frequency Incorporating Offset Cancellation P2 Vref 2 P2B P3 C3a Vcm Vref 2 P2B C3b P2 P3 P2 C1 P1 C2 P1 P3 P2 P3 Vtune Gm cell two sets of input pairs Aux input pair C3a b Offset cancellation Same clock timing EECS 247 Lecture 7 Filters 2004 H K Page 7 DC Tuning of Resistive Timing Element Vtune Rext used to lock Gm or onchip R I Gm Feedback forces Gm 1 Rext I Account for Cap variations in the gm C implementation by trimming Rext Ref C Laber and Gray A 20MHz 6th Order BiCOM Parasitic Insensitive Continuous time Filter and Second Order Equalizer Optimized for Disk Drive Read Channels IEEE Journal of Solid State Circuits Vol 28 pp 462 470 April 1993 EECS 247 Lecture 7 Filters 2004 H K Page 8 Off line Frequency Tuning Example Wireless Receiver Baseband Filters A D Digital Signal Processor DSP Osc IF Stage 0 to 2 RF Amp 2 A D Systems where filter is followed by ADC DSP Take advantage of existing digital signal processor to periodically update the filter critical frequency Filter tuned only at the outset of each data transmission session off line tuning EECS 247 Lecture 7 Filters 2004 H K Page 9 Offline Filter Tuning Concept EECS 247 Lecture 7 Filters 2004 H K Page 10 Summary Filter Frequency Tuning Trimming Expensive Does not account for variations associated with temperature and supply etc Automatic frequency tuning Continuous tuning Master VCF used in tuning loop Tuning quite accurate Issue reference signal feedthrough to the filter output Master VCO used in tuning loop Design of reliable stable VCO challenging Issue reference signal feedthrough Single integrator in negative feedback loop forces time constant to be a function of accurate clock frequency More flexibility in choice of reference frequency less feedthrough issues Locking a replica of the Gm cell to an external resistor DC offset issues Does not account for integrating capacitor variations Periodic tuning Requires digital capability minimal additional hardware Advantage of no reference signal feedthrough since tuning performed off line EECS 247 Lecture 7 Filters 2004 H K Page 11 Bandpass Filters Bandpass Filters Q 5 Combination of lowpass highpass Lowpass Highpass H j H j H j Q 5 H j Q 5 Direct implementation Q 5 EECS 247 Lecture 21 Filters 2004 H K Page 12 Direct Implementation Narrow Band Bandpass Filters Lowpass Freq Mask s s Q c s c Bandpass Freq Mask s s2 s1 c B2 B1 Design based on lowpass prototype for narrow band filters Same lowpass tables used EECS 247 Lecture 7 Filters 2004 H K Page 13 Lowpass to Bandpass Transformation Lowpass pole zero s plane Bandpass pole zero s plane Pole Zero From Zverev Handbook of filter synthesis Wiley 1967 p 156 EECS 247 Lecture 7 Filters 2004 H K Page 14 Lowpass to Bandpass Transformation Table a B2 B1 1 a Q filter From Zverev Handbook of filter synthesis Wiley 1967 p 157 EECS 247 Lecture 7 Filters 2004 H K Page 15 Lowpass to Bandpass Transformation Lowpass Bandpass L2 Vo L2 Rs Vin C1 C2 Rs C3 RL Vin C1 L1 Vo C3 L3 RL Each capacitor replaced by parallel L C Each inductor replaced by series L C EECS 247 Lecture 7 Filters 2004 H K Page 16 Lowpass to Bandpass Transformation 1 R 0 C1 QC1 L1 1 R QC1 0 C2 1 1 QL 2 R 0 L2 QL 2 Vo Rs Vin C3 L1 C1 RL L3 R 0 C3 QC3 L3 C2 L2 1 R 0 Where C1 L2 C3 normalized lowpass values Q bandpass filter quality factor 0 filter center frequency 1 R QC3 0 EECS 247 Lecture 7 Filters 2004 H K Page 17 Signal Flowgraph 6th Order Bandpass Filter L2 C2 Vo Rs Vin Vin R Rs C1 1 C3 L1 1 1 R 1 s L1 sC1R R 1 RL L3 1 1 s C2 R s L2 R s L3 1 s C3R Vout R RL 1 Note each C L in the original lowpass prototype replaced by a resonator Substituting the bandpass L1 C1 by their normalized lowpass equivalent previous page The resulting SFG is EECS 247 Lecture 7 Filters 2004 H K Page 18 6th Signal Flowgraph Order Bandpass Filter 1 Vin R Rs 1 1 Q C1 0 s 0 0 s Q C1 s Q L 2 1 1 0 s Q C3 Q C3 0 Q L 2 0 s Vout R RL s 1 Note the integrators have different time constants Ratio of time constants for each resonator 1 Q2 typically requires high component ratios poor matching Desirable to convert SFG so that all integrators have equal time constants for optimum matching Scale nodes to obtain equal integrator time constant EECS 247 Lecture …
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