Multimode 2.4 GHz Front-End with Tunable g m -C Filter Group 4: Nick Collins Trevor Hunter Joe Parent EECS 522 Winter 2010
Overview Introduction Complete System LNA Mixer Gm-C filter Conclusion
Introduction More and more devices becoming equipped with wireless technologies Bluetooth, WiFi, ZigBee all operate at a 2.4GHz carrier frequency Wireless front-ends are imperative for RF communication
System Level Receiver LNA Amplifies antenna signal and designed to minimize noise at the carrier frequency Mixer Demodulates signal, but adds unwanted harmonic frequencies Gm-C filter LPF removes unwanted harmonics, tunable for different channel bandwidths Direct Conversion RF and LO frequencies are the same
Low Noise Amplifier Necessary for low noise figure of system Low noise factor directly added to total noise factor High gain reduces noise contributed by following stages Noise factor predicted by the Friis equation F F LNA + F G Mix LNA 1 Fgm c 1 + G G LNA Mix
Low Noise Amplifier Inductively degenerated cascode topology Input matched to 50Ω Load Inductor resonates with output capacitance at 2.4 GHz Transistor lengths chosen based on α and f t for different current densities as practiced in class
Single-Differential Circuit Creates Differential Signal needed for mixer Cascoded CS differential amp w/ AC ground input Half-circuit technique not accurate, but decent assumption to start design
LNA Challenges IIP3 optimization Matching to mixer s strange input impedance
LNA Challenges
LNA Challenges Z mixer = 15 j75 Ω
Mixer Traditional Gilbert cell variant High port-port isolation Active, providing gain Direct conversion Less folded noise in downconversion No image rejection filter needed Flicker noise present at low frequencies DC offset present
Mixer Design AC coupled Quick M1, M2 gate bias Square LO assumed No tail Current source Improves linearity Improves noise More Headroom
Mixer Operation RF signal converted to current M1, M2 linear region Assumed LO signal & perfect switching f RF = f LO Square Wave from 0-1 requires manipulation of 2 1 Fourier series equation for analysis -1
Mixer Operation RF signal converted to current M1, M2 linear region Assumed LO signal & perfect switching f RF = f LO Square Wave from 0-1 requires manipulation of 2 Fourier series equation for analysis 1 Add 1-1
Mixer Operation RF signal converted to current M1, M2 linear region Assumed LO signal & perfect switching f RF = f LO Square Wave from 0-1 requires manipulation of 2 Fourier series equation for analysis 1 Add 1 Divide by 2-1
Noise Folding Noise adds at DC from odd harmonics of 2.4GHz 2.4GHz 7.2GHz 12GHz
Transient Example DC offset!
g m -C Filter Channel selection for direct conversion receivers 3 rd order Butterworth filter (60 db/decade) Amount of g m (2 100 μs) adjust BW between 600 khz (Bluetooth) 20 MHz (802.11.n) Primary goals are linearity, power, area C is big Assumes large gain in front (1/f noise, subthreshold)
Transconductor V DD I bias 2 I tune Translinear loop V inp V inm M 1 M 2 M 4 M 3 Ioutp I outm I 1 I 2 R 1 I tune V SS I tune R 2
Issues With Subthreshold Low Vth and rolloff for standard device This figure is Vth vs. L for standard pfet, LP (high Vth / low leakage), and 3.3V pfet.
Subthreshold Slopes For reasonable bias current, area unreasonably large I DS = µ C eff ox W L q( V V mkt ) kt gs t qvds ( m 1) exp 1 exp q 2 kt
Subthreshold Slopes For reasonable bias current, area unreasonably large I DS = µ C eff ox W L q( V V mkt ) kt gs t qvds ( m 1) exp 1 exp q 2 kt
Feedback Issues C GD of M 1 (& M 3 ) creates a right half plane zero that extends gain while phase crosses 180 sooner. I bias V DD 2 I tune The sizes of M 1 required for subthreshold operation with R 1 less than 1 MΩ required more than 3pF of compensation V inp M 1 M 2 I outp I outm I 1 R 1 I tune V SS
Feedback Issues C GD of M 1 (& M 3 ) creates a right half plane zero that extends gain while phase crosses 180 sooner. I bias V DD 2 I tune The sizes of M 1 required for subthreshold operation with R 1 less than 1 MΩ required more than 3pF of compensation V inp M 1 M 2 I outp I outm I 1 R 1 I tune V SS
Bode Plot This is entire transconductor Shows Evidence of feedback loop issues
g m -C Filter Abandoned sub-threshold approach. In saturation, output series expansion is below: g m 1/R Without short channel effects and channel length modulation, h.o.t. still appear
Transconductance Maximum value of 206.2 μs Variation of g m less than 0.1% over 0.5V input range
IIP3
Conclusion / Future Work We enjoyed our first large RF design project We learned a lot confronting challenges of RF analog design Subthreshold design will be an increasingly useful tool
Thank you Mohammad Ghahramani Prof. Wentzloff db Café
Questions?