EE247 Lecture 7 Automatic on chip filter tuning continued from last lecture Continuous tuning continued DC tuning of resistive timing element Periodic digitally assisted filter tuning Systems where filter is followed by ADC DSP existing hardware can be used to periodically update filter freq response Continuous time filter design considerations Monolithic highpass filters Active bandpass filter design Lowpass to bandpass transformation Example 6th order bandpass filter Gm C bandpass filter using simple diff pair Various Gm C filter implementations EECS 247 Lecture 7 Filters 2009 H K Page 1 Summary last lecture Continuous time filters continued Gm C filters Frequency tuning for continuous time filters Trimming via fuses or laser Automatic on chip filter tuning Continuous tuning Utilizing VCF built with replica integrators Use of VCO built with replica integrators Replica single integrator in a feedback loop locked to a reference frequency EECS 247 Lecture 7 Filters 2009 H K Page 2 DC Tuning of Resistive Timing Element Tuning circuit Gm replica of Gm used in filter Vtune I Rext used to lock Gm to accurate off chip R Gm Feedback forces IxRext Gm cell input Current flowing in Gm Cell I Gm 1 Rext I Issues with DC offset Rext Account for capacitor variations in this Gm C implementation by trimming C in the factory Ref C Laber and P R Gray A 20MHz 6th Order BiCMOS 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 2009 H K Page 3 Digitally Assisted Frequency Tuning Example Wireless Receiver Baseband Filters A D Digital Signal Processor DSP Osc RF Amp IF Stage 0 to 2 2 A D Systems where filter is followed by ADC DSP Take advantage of existing digital signal processor capabilities to periodically test if needed update the filter critical frequency Filter tuned only at the outset of each data transmission session offline periodic tuning can be fine tuned during times data is not transmitted or received EECS 247 Lecture 7 Filters 2009 H K Page 4 Example Seventh Order Tunable Low Pass OpAmp RC Filter EECS 247 Lecture 7 Filters 2009 H K Page 5 Digitally Assisted Filter Tuning Concept Assumptions System allows a period of time for the filter to undergo tuning e g for a wireless transceiver during idle VP AC VDC periods An AC e g a sinusoid signal can be generated on chip whose amplitude is a function of an on chip DC voltage AC signal generator outputs a sinusoid with peak voltage equal to the DC signal source AC Signal Power 1 2 DC signal power the input of the filter EECS 247 Lecture 7 Filters 2009 H K Page 6 Digitally Assisted Filter Tuning Concept VP AC VDC AC signal a frequency on the roll off of the desired filter frequency response e g 3dB frequency VAC VDC s i n 2 f desired t 3dB Provision can be made during the tuning cycle the input of the filter is disconnected from the previous stage e g mixer and connected to 1 DC source 2 AC source under the control of the DSP EECS 247 Lecture 7 Filters 2009 H K Page 7 Digitally Assisted Filter Tuning Concept VP AC VDC EECS 247 Lecture 7 Filters 2009 H K Page 8 Practical Implementation of Frequency Tuning AC Signal Generation From DC Source Vout Vout ClockB high Clock high Clock ClockB Vout Vout 0 2 Square waveform generated 2 peak to peak magnitude and frequency fclock EECS 247 Lecture 7 Filters 2009 H K Page 9 FREQ CONT ter Fil TUNE CHOP 625kHz Practical Implementation of Frequency Tuning Digital Signal Processor DSP1616 40MHz Register Vref Vref A D 4bit 10MHz AC Measurement DC Measurement EECS 247 2 Lecture 7 Filters 2009 H K Page 10 Practical Implementation of Frequency Tuning Effect of Using a Square Waveform AC Measurement Vout t 4 sin t 1 2 2 Vin t 4 sin n t n 1 3 5 n Input signal chosen to be a square wave due to ease of generation Filter input signal comprises a sinusoidal waveform the fundamental frequency its odd harmonics Key Point The filter itself attenuates unwanted odd harmonics Inaccuracy incurred by the harmonics negligible EECS 247 Lecture 7 Filters 2009 H K Page 11 Simplified Frequency Tuning Flowchart EECS 247 Lecture 7 Filters 2009 H K Page 12 Digitally Assisted Offset Compensation EECS 247 Lecture 7 Filters 2009 H K Page 13 Filter Tuning Prototype Diagram EECS 247 Lecture 7 Filters 2009 H K Page 14 EECS 247 Lecture 7 Filters 2009 H K Page 15 Measured Tuning Characteristics EECS 247 Lecture 7 Filters 2009 H K Page 16 Off line Digitally Assisted Tuning Advantages No reference signal feedthrough since tuning does not take place during data transmission off line Minimal additional hardware Small amount of programming Disadvantages If acute temperature change during data transmission filter may slip out of tune Can add fine tuning cycles during periods of data is not transmitted or received Ref H Khorramabadi M Tarsia and N Woo Baseband Filters for IS 95 CDMA Receiver Applications Featuring Digital Automatic Frequency Tuning 1996 International Solid State Circuits Conference pp 172 173 EECS 247 Lecture 7 Filters 2009 H K Page 17 Summary Continuous Time Filter Frequency Tuning Trimming Expensive does not account for temperature and supply etc variations Automatic frequency tuning Continuous tuning Master VCF used in tuning loop same tuning signal used to tune the slave main filter 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 DC locking of a replica of the integrator to an external resistor DC offset issues does not account for integrating capacitor variations Periodic digitally assisted tuning Requires digital capability minimal additional hardware Advantage of no reference signal feedthrough since tuning performed off line EECS 247 Lecture 7 Filters 2009 H K Page 18 RLC Highpass Filters Any RLC lowpass can be converted to highpass by Replacing all Cs by Ls and LNormHP 1 CNormLP Replacing all Ls by Cs and CNormHP 1 LNormLP LHP Lr CNormLP CHP Cr LNormLP where Lr Rr r and Cr 1 Rr r L4 C4 C2 L2 Rs Vin C1 Rs Vin C3 Lowpass EECS 247 L3 L1 Highpass Lecture 7 Filters 2009 H K Page 19 Integrator Based High Pass Filters 1st Order Conversion of simple high pass RC
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