Multimode 2.4 GHz Front-End with Tunable g m -C Filter. Group 4: Nick Collins Trevor Hunter Joe Parent EECS 522 Winter 2010

Transcription

1 Multimode 2.4 GHz Front-End with Tunable g m -C Filter Group 4: Nick Collins Trevor Hunter Joe Parent EECS 522 Winter 2010

2 Overview Introduction Complete System LNA Mixer Gm-C filter Conclusion

3 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

4 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

5 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

6 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

7 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

8 LNA Challenges IIP3 optimization Matching to mixer s strange input impedance

9 LNA Challenges

10 LNA Challenges Z mixer = 15 j75 Ω

11 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

12 Mixer Design AC coupled Quick M1, M2 gate bias Square LO assumed No tail Current source Improves linearity Improves noise More Headroom

13 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

14 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

15 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

16 Noise Folding Noise adds at DC from odd harmonics of 2.4GHz 2.4GHz 7.2GHz 12GHz

17 Transient Example DC offset!

18 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 ( n) Primary goals are linearity, power, area C is big Assumes large gain in front (1/f noise, subthreshold)

19 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

20 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.

21 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

22 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

23 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

24 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

25 Bode Plot This is entire transconductor Shows Evidence of feedback loop issues

26 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

27 Transconductance Maximum value of μs Variation of g m less than 0.1% over 0.5V input range

28 IIP3

29 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

30 Thank you Mohammad Ghahramani Prof. Wentzloff db Café

31 Questions?