EE247 Lecture 7 Automatic on chip filter tuning continued from last lecture Continuous tuning Reference integrator locked to a reference frequency 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 filters Highpass filters Bandpass filters Lowpass to bandpass transformation Example 6th order bandpass filter Gm C BP filter using simple diff pair EECS 247 Lecture 7 Filters 2007 H K Page 1 Summary last lecture Continuous time filters Opamp MOSFET RC filters 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 Reference integrator locked to reference frequency EECS 247 Lecture 7 Filters 2007 H K Page 2 Summary Reference Integrator Locked to Reference Frequency Tuning error due to gmcell offset voltage resolved S2 Vref A C1 Advantage over previous schemes fclk can be chosen to be at much higher frequencies compared to filter bandwidth N 1 C2 Feedback forces Gm to vary so that i nt g C1 Gm N f cl k or 0int g Gm C1 f cl k N Feedthrough of clock attenuated by filter EECS 247 S3 Gm Lecture 7 Filters 2007 H K Page 3 DC Tuning of Resistive Timing Element Vtune Tuning circuit Gm replica of Gm used in filter I Gm Rext used to lock Gm to accurate off chip R Feedback forces Gm 1 Rext I Issues with DC offset Account for capacitor variations in this gm C implementation by trimming Rext 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 2007 H K Page 4 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 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 EECS 247 Lecture 7 Filters 2007 H K Page 5 Example Seventh Order Tunable Low Pass OpAmp RC Filter EECS 247 Lecture 7 Filters 2007 H K Page 6 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 source 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 2007 H K Page 7 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 2007 H K Page 8 Digitally Assisted Filter Tuning Concept VP AC VDC EECS 247 Lecture 7 Filters 2007 H K Page 9 Practical Implementation of Frequency Tuning AC Signal Generation From DC Source Vout Vout ClockB high Clock high Clock ClockB Vout EECS 247 2 0 Lecture 7 Filters Vout 2007 H K Page 10 FREQ CONT ter Fil TUNE CHOP 625kHz Practical Implementation of Frequency Tuning Digital Signal Processor DSP1616 40MHz Register Vref Vref A D 4bit 10MHz 2 AC Measurement DC Measurement EECS 247 Lecture 7 Filters 2007 H K Page 11 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 2007 H K Page 12 Simplified Frequency Tuning Flowchart EECS 247 Lecture 7 Filters 2007 H K Page 13 Digitally Assisted Offset Compensation EECS 247 Lecture 7 Filters 2007 H K Page 14 Filter Tuning Prototype Diagram EECS 247 Lecture 7 Filters 2007 H K Page 15 EECS 247 Lecture 7 Filters 2007 H K Page 16 Chip Photo EECS 247 Lecture 7 Filters 2007 H K Page 17 Measured Tuning Characteristics EECS 247 Lecture 7 Filters 2007 H K Page 18 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 2007 H K Page 19 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 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 2007 H K Page 20 Integrator Based High Pass Filters 1st Order Conversion of simple high pass RC filter to integrator based type by using signal flowgraph technique C Vin EECS 247 Vo s RC Vo Vin 1 s R C R Lecture 7 Filters
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