EECS 247 Lecture 7: Filters © 2007 H.K. Page 1EE247 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 hardwarecan be used to periodically update filter freq. response• Continuous-time filters – Highpass filters– Bandpass filters• Lowpass to bandpass transformation•Example: 6thorder bandpass filter• Gm-C BP filter using simple diff. pairEECS 247 Lecture 7: Filters © 2007 H.K. Page 2Summary 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 frequencyEECS 247 Lecture 7: Filters © 2007 H.K. Page 3SummaryReference Integrator Locked to Reference FrequencyFeedback forces Gm to vary so that :S2S3GmC1C2VrefAintgintg0C1N/fclkGmorGmfclk / NC1τω====Tuning error due to gm-cell offset voltage resolvedAdvantage over previous schemes:Æfclkcan be chosen to be at much higher frequencies compared to filter bandwidth (N >1)Æ Feedthrough of clock attenuated by filterEECS 247 Lecture 7: Filters © 2007 H.K. Page 4DC Tuning of Resistive Timing ElementVtune Tuning circuit Gm Æ replica of Gm used in filterRext used to lock Gm to accurate off-chip RFeedback forces Gm=1/RextIssues with DC offsetAccount for capacitor variations in this gm-C implementation by trimmingRext.-+-+IIGmRef: 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 1993EECS 247 Lecture 7: Filters © 2007 H.K. Page 5Digitally Assisted Frequency Tuning Example:Wireless Receiver Baseband Filters• 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 (off-line/periodic tuning) – can be fine tuned during times data is not transmittedRF AmpOsc.A/D Digital Signal Processor (DSP)A/D π2IF Stage ( 0 to 2 ) EECS 247 Lecture 7: Filters © 2007 H.K. Page 6Example: Seventh Order Tunable Low-Pass OpAmp-RC FilterEECS 247 Lecture 7: Filters © 2007 H.K. Page 7Digitally Assisted Filter Tuning ConceptAssumptions:– System allows a period of time for the filter to undergo tuning (e.g. for a wireless transceiver during idle 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 filterVPAC=VDCEECS 247 Lecture 7: Filters © 2007 H.K. Page 8Digitally Assisted Filter Tuning ConceptVPAC=VDCAC signal @ a frequency on the roll-off of the desired filter frequency response(e.g. -3dB frequency)Provision can be made Æ during the tuning cycle, the input of the filter is disconnected from the previous stage (e.g. mixer) and connected to:1. DC source2. AC source under the control of the DSP ()desiredAC DC3dBVVsin2f tπ−=×EECS 247 Lecture 7: Filters © 2007 H.K. Page 9Digitally Assisted Filter Tuning ConceptVPAC=VDCEECS 247 Lecture 7: Filters © 2007 H.K. Page 102ΔΔPractical Implementation of Frequency TuningAC Signal Generation From DC SourceVoutClockClockBVout0+Δ−ΔΔVout=Clock=high+ΔΔVout=−ΔClockB=highEECS 247 Lecture 7: Filters © 2007 H.K. Page 11Δ2ΔΔDC MeasurementAC MeasurementA/D 4bit 10MHzDigital Signal ProcessorDSP1616 40MHzVref+Vref-FilterRegisterCHOPTUNEFREQ.CONT.625kHzPractical Implementation of Frequency TuningEECS 247 Lecture 7: Filters © 2007 H.K. Page 122ΔΔAC MeasurementPractical Implementation of Frequency TuningEffect of Using a Square Waveform()()n 1,3 ,5 ,..4Vin sinntntπω=∞Δ=∑• 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 ÆInaccuracy incurred by the harmonics negligible()()41Vout sint2tπωΔ=×EECS 247 Lecture 7: Filters © 2007 H.K. Page 13Simplified Frequency Tuning FlowchartEECS 247 Lecture 7: Filters © 2007 H.K. Page 14Digitally Assisted Offset CompensationEECS 247 Lecture 7: Filters © 2007 H.K. Page 15Filter Tuning Prototype DiagramEECS 247 Lecture 7: Filters © 2007 H.K. Page 16EECS 247 Lecture 7: Filters © 2007 H.K. Page 17Chip PhotoEECS 247 Lecture 7: Filters © 2007 H.K. Page 18Measured Tuning CharacteristicsEECS 247 Lecture 7: Filters © 2007 H.K. Page 19Off-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 receivedRef: H. Khorramabadi, M. Tarsia and N.Woo, “Baseband Filters for IS-95
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