Circuits from RF to Bits EE 194 Brad

Circuits from RF to Bits EE 194 Brad Wheeler

Block Diagram • RF gain – Noise reasons • Down-conversion – Easier to build low frequency circuits • Signal processing – More gain, filtering • Digitization/demodulation – Extract the information

To LNA or Not to LNA? • Low Noise Amplifier (LNA) provides RF gain • LNA Design Recipe – Start with common source/gate – Add inductors – Burn lots of current • Pros – Lower Noise Figure – Better sensitivity • Cons – Power consumption Razavi, Behzad, and Razavi Behzad. RF microelectronics. Vol. 2. New Jersey: Prentice Hall, 1998.

Matching Networks • Achieve RF gain using LC resonance instead of active elements • Topologies – L, Pi, tapped element • Metrics – Gain • Must account for loading – Input impedance – Bandwidth • Tuning – Noise Cr: Prof Niknejad, EE 142

Matching Network Gain •

Matching Network Gain Unloaded

Matching Network Gain Unloaded Loaded, Q=15

Down-Conversion Mixers • Move signal from RF (GHz) to Intermediate Frequency (MHz) • Metrics – – Input impedance Gain Noise Linearity • Passive vs Active Razavi, Behzad, and Razavi Behzad. RF microelectronics. Vol. 2 New Jersey: Prentice Hall, 1998.

Passive Mixers • Input impedance • Noise • Gain – Sinc(Duty Cycle) Razavi, Behzad, and Razavi Behzad. RF microelectronics. Vol. 2. New Jersey: Prentice Hall, 1998.

Passive Mixers • Transparency – N-path filter – Limited by Ron/Rs • Andrews/Molnar – Hard switching, multiple phases • Cook/Berny – Resonant, sinusoidal drive

Hard Switching • Rail to rail switch drivers • Multiple clock phases, non-overlapping inputs • Higher performance, higher power to drive Andrews, Caroline, and Alyosha C. Molnar. "Implications of passive mixer transparency for impedance matching and noise figure in passive mixer-first receivers. " IEEE Transactions on Circuits and Systems I: Regular Papers 57. 12 (2010): 3092 -3103.

Sinusoidal Drive • Can also resonate out switch gate capacitance in LC tank • Sinusoidal waveform yields approximately square conduction cycle • Lower LO power, but fewer phases available Cook, Ben W. , et al. "Low-power 2. 4 -GHz transceiver with passive RX front-end and 400 m. V supply. " IEEE Journal of Solid-State Circuits 41. 12 (2006): 2757 -2766.

Receiver Components ü RF gain – Noise reasons ü Down-conversion – Easier to build low frequency circuits q Signal processing – More gain, filtering q Digitization/demodulation – Extract the information

IF Amplifiers • Direct conversion vs heterodyne – DC offsets, flicker noise – We need amplifiers/filters in the MHz range • First active amplifier becomes the noise vs power limiting factor

Filters • Why filter? – Interference – Noise • More options than Baskin Robbins • Discrete time (DT) vs continuous time (CT) – Aliasing • Active vs passive

CT Filters Tow-Thomas Biquad Active Ladder Filter Passive Ladder Filter Gm-C Filter Cr: Prof Boser, EE 240 C

Filter Frequency Ranges Cr: Prof Boser, EE 240 C

(Active) Switched Capacitor • Transfer function set by clocks, capacitor ratios – Very precise control • Active = Charge being pushed around by opamps Cr: Prof Boser, EE 240 C

Passive Switched Capacitor • Transconductor converts input to charge • Passive charge sharing defines filtering – Poles determined by capacitor ratios Continuous time gain and anti-alias filtering Discrete time filtering Tohidian, Massoud, Iman Madadi, and Robert Bogdan Staszewski. "Analysis and design of a high-order discrete-time passive IIR low-pass filter. " IEEE Journal of Solid-State Circuits 49. 11 (2014): 2575 -2587.

High Order Poles • One transconductor, many switches and caps • Can only implement poles on the real axis with this topology Tohidian, Massoud, Iman Madadi, and Robert Bogdan Staszewski. "Analysis and design of a high-order discrete-time passive IIR low-pass filter. " IEEE Journal of Solid-State Circuits 49. 11 (2014): 2575 -2587.

Complex Conjugate Poles • Negative feedback allows to move poles off real axis • Limited to low quality factor Invert Polarity Lulec, Sevil Zeynep, David A. Johns, and Antonio Liscidini. "A simplified model for passive-switched-capacitor filters with complex poles. " IEEE Transactions on Circuits and Systems II: Express Briefs 63. 6 (2016): 513 -517.

Complex Filters • By combining charge from In-phase and Quadrature samples, can make filters asymmetric about 0 Hz Madadi, Iman, Massoud Tohidian, and Robert Bogdan Staszewski. "Analysis and design of I/Q charge-sharing band-pass-filter for superheterodyne receivers. " IEEE Transactions on Circuits and Systems I: Regular Papers 62. 8 (2015): 2114 -2121.

Passive FIR Cook, Benjamin W. , and Axel D. Berny. "Passive discrete time analog filter. " U. S. Patent No. 8, 849, 886. 30 Sep. 2014.

Previous Filter Design • Cascade of IIR & FIR blocks • 4 th order filter centered at 2. 5 MHz with Fs = 100 MHz • 1 -bit zero crossing demodulator – High sampling frequency – Gain requirements – Offset cancellation (Simulated)

ADC/Comparator • Sample rate, number of bits, etc driven by system level modeling – Filter type/passband – Demodulator • Simplest ADC is just a comparator (1 -bit) – Demodulate FSK by counting time between zero crossings of square wave

Clocked Comparators Strongarm Razavi, Behzad. "The Strong. ARM Latch [A Circuit for All Seasons]. " Solid-State Circuits Magazine, IEEE 7. 2 (2015): 12 -17. Double Tail Schinkel, Daniel, et al. "A double-tail latch-type voltage sense amplifier with 18 ps setup+ hold time. " Solid-State Circuits Conference, 2007. ISSCC 2007. Digest of Technical Papers. IEEE International. IEEE, 2007.

Design Considerations • Power, offset, speed, noise • Lower offset either by up-sizing input devices, or by adding correction – Both cost more power – Pelgrom, Monte Carlo • Offset calibration schemes – Current tuning – Capacitor DAC – Body bias

Simulation Issues • Switched capacitor circuits require special simulation techniques • Periodic Steady State (PSS) – Compute a solution around a time varying operating point • Periodic Versions of all the normal analysis – PAC – PNoise