RFIC Design - Mixers. Transceiver architectures Two ports Distortion Resonance and Matching networks Broadband amplifiers

Transcription

1 RFIC Design - Mixers Transceiver architectures Two ports Distortion Resonance and Matching networks Broadband amplifiers Noise and Low noise amplifiers (LNA) Mixer Oscillators Synthesizers Power amplifiers (PA) 1

2 Mixers Goal: frequency translation Not an LTI system Non-linear OR Time-invariant Upconversion mixer Downconversion mixer Multiplier as a mixer: Use filter to remove LO+RF or LO-RF: 2

3 RF = 1000MHz LO = 900MHz IF = 100MHz Unwanted signal: RF=800MHz also maps to IF=100MHz Problem: Both interferers and noise Image Problem Solution 1: Image Rejection using Filter Solution: multi-stage filtering using multi-if requires lower Q s (the reason to use superheterodyne arch.) Example: RF = 1000MHz, IF = 10MHz Using single IF stage: Q = 100 Using two IF stages (IF1 = 100MHz): Q1 = Q2 = 10 Restricts IF selection due to Q of filter. Example: for RF=1000MHz and IF=1MHz Fifth order Chebyshev filter 80db attenuation But Q of filter =1000 (to remove interferers) (Definition of Q of filter?) Requires components with Q>1000 expensive / bulky Solution 2: Direct Conversion (next slide) 3

4 Direct Conversion No image problem! But: several other issues Down converting RF + interferers requires highly linear LNA and mixer Even order distortions produce DC Flicker noise LO Leakage produces DC Problem: Phase of LO RF: LO: if Solutions: Solution 1: transmitting a pilot tone receiver locks to it. Solution 2: using I/Q mixer (next slide) 4

5 I/Q mixer Other benefits: PM (Phase Modulation) Digital modulations: Also AM modulation: BPSK (1b) QPSK (2b) 3 bits 5

6 I/Q Hartley Mixer Can be used: As a single sideband mixer For image rejection 90º shitf multiply Pos Freq by -j and Neg Freq by +j 6

7 Image Rejection Ratio: Image rejection ratio Example: Gain mismatch Phase mismatch IRR = 30 40dB, But we usually need 60 70dB ±45º Delay Element: For gain matching: ω=1/rc Narrowband Solution: Multi-stage polyphase Insertion loss I/Q LO: generated directly Divide by 2: two LO Quadrature oscillator: two LC tanks 7

8 Large Signal Drive Large signal drive in mixers small signal model is not accurate I n (b): modified Bessel functions I C waveform One period of I C for different b values 8

9 9

10 Differential Pair No even harmonics! 10

11 Mixer Analysis RF signal: small small signal model LO signal: large changes the small signal model A linear time varying periodic system Transconductance changes periodically First term: amplification, other terms: mixing 11

12 Harmonic Mixer We can use harmonics of LO to build a mixer. Example: LO=500MHz, RF=900MHz IF = 2LO RF = = 100MHz Advantage: using a lower LO separation between RF and LO Disadvantage: lower conversion gain higher noise g n : the n th harmonic transconductance For a BJT: 12

13 A BJT Mixer AC equivalent 13

14 Another BJT Mixer LO is capacitively coupled 14

15 BJT Mixer Analysis 15

16 LO and RF Signal Drive 16

17 BJT Mixer Analysis: General Approach For a weak RF signal: The LO and other unwanted terms are minimized using an IF filter. 17

18 Distortion in Mixers Insert a signal with two tones Generate IM products that could be translated to IF. 18

19 MOS Large Signal Drive Assume Vo is small enough to not take the device into cutoff No harmonics! ideal mixer 19

20 MOS Mixer Ignoring output resistance: Independent of V A Notes: - Transistor must remain active. - A real FET is not square law complicates analysis 20

21 Dual Gate Mixer (Cascode amplifier) LO RF No need to capacitive coupling or using a transformer Large LO pushes M1 into triode in part of a cycle Modulates g m1 Fourier decomposition gives g C 21

22 Mixer Analysis (Time Domain) Mixer has periodic transfer function:, 22

23 Mixer Analysis (Freq. Domain) Input spectrum is shifted by all LO harmonics. Any signal from an IF from any LO harmonic, is downconverted. Images are easy to remove, but noise will be folded onto IF. Note: antenna noise is filtered, but new noise is generated by filter itself. 23

24 Current Commuting Mixers Realization Note: For MOS version gc is smaller since gm is smaller Also MOS needs larger LO drive To linearize: add RF degeneration To match: add inductive degeneration 24

25 Differential Output + RF Degeneration No DC term No RF feedthrough Differential output RF: degenerated Known as Gilbert cell Gain doubled (due to diff output) 25

26 Double Balanced Mixer Single balanced: solves RF feedthrough Double balanced: solves LO feedthrough ac operation R E : for linearization of RF transconductance stage To reject LO: matching of Q1 to Q4 is important Also rejects even order distortion processes half of the signal more linear more transistors more noise Even single ended output multiplies input by ±1 rejects RF 26

27 Voltage Switching Mixers Switches voltage instead of current Multiplies RF by ±1 Needs good switches: Low Ron, High Roff Need diff RF and diff LO Diff RF signal generated by balun or balanced LNA MOS switching (passive) mixer: Large transistors to get low Ron Limit: feed-through cap Transistors are on or off (no gm stage) very linear, but no power gain Need large LO MOS Model: ON OFF In off state S and D couple through overlap caps and substrate 27

28 MOS Ring Mixer LO > 0 IF = RF LO < 0 IF = -RF MOS resistance comes into play as a voltage divider. H-Bridge Ring Mixer model Thevinin equivalent voltage source: Used when PMOS is available. Use Fourier expansion to get g c In practice, C IF (cap at the IF port) complicates the analysis. 28

29 Passive Current Mixer G m stage Gilbert quad (DC is removed) Current to Voltage converter Advantages: No DC in quad: no flicker noise generated by them. Linearity is good (voltage swing does not limit linearity of the mixer (unlike Gilbert cell)) Opamp can be used as an IF filter. Disadvantages: Need larger drive compared to Gilbert cell. Additional noise and power due to op-amp. Op-amp needs CMFB circuit. 29

30 Ring or Quad Ring (passive) and Quad (active) mixers are similar. In Ring: transistors are biased in saturation (have DC current). In Quad: transistors are biased near the threshold. 30

31 Diode Mixer model Used when PMOS is not available, OR for high frequency signals. Diode is biased by choke. Two operation modes: - Diode ON/OFF: Mixing by a large LO that turns off the diode. - Diode non-linearity: Mixing by a smaller LO that does not turn the diode off) has less gain, and more distortion. 31

32 Diode Ring Mixer Positive LO Cycle D1/D2 and D3/D4 turn on alternatively and switch the polarity of RF. LO and RF are isolated by transformers. Balanced operation rejects RF and LO. Needs large LO to turn diodes on/off. No DC RF is rejected at IF port. The only feedthrough: diodes reverse isolation. Drawbacks: bulky transformers; large LO. Advantage: very linear. At microwave (THz) freq., couplers are used instead of transformers. 32

33 Passive Mixer LO Driver Generate large LO signal with large drive capability: (to drive large MOS transistors) Generates square Less power, but sinusoidal Tank resistance To lower power, device should be biased near thereshold Q P 33

34 Noise in Mixer SSB signals: noise from image band also falls into IF, so F=2. Using IR filter F=1. DSB signals: there is signal in the image band as well, so F=1. All image bands of LO harmonics folds onto IF. Consider the first three harmonics. Parseval s theorem Gilbert Cell: Gm stage: Generates noise that is downconverted. LO differential pair: Generates noise when both transistors are on (diff. amplifying mode). When one transistor is off, the noise is rejected, due to degeneration (similar to cascode). So if LO amplitude is increased, noise figure is improved. 34

35 Periodic Steady State (PSS) Simulation Mixer time domain simulation is slow. Example: For RF=1GHz and IF=MHz, to simulate the circuit for 10 IF cycles, we need RF cycles. (intermodulation calculation, like IM3, has a similar problem) We use PSS simulation instead. The simulator finds a periodic large signal solution. In a mixer this is done using LO signal alone. Usually converges in 4-5 cycles of LO simulation time. Circuit should not be chaotic (periodic input results in periodic output) No problem with high Q circuits, as they reach the steady state point after Q cycles. Also no problem with autonomous circuits (like oscillators). PSS options: - List of large signals (in a mixer: only LO). - Beat frequency or the frequency of the periodic operating point. For two large signals f1 and f2, this is f1 - f2 (for mixer, this is the LO frequency) - Time for stabilization (mixer does not need this). 35

36 PAC and PNoise Analysis PAC analysis: The circuit is linearized around the periodic operating point. For a given AC input, number of transfer functions needed equals number of LO harmonics Input frequency fi is translated to fi+kfo (fo is the beat frequency) PNoise analysis: An analysis that takes the frequency translation effects into account (suitable for mixer noise analysis) PAC Analysis PNoise Analysis 36