6. 776 High Speed Communication Circuits and Systems Lecture 15 VCO Examples
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6. 776 High Speed Communication Circuits and Systems Lecture 15 VCO Examples Mixers Massachusetts Institute of Technology March 31, 2005 Copyright © 2005 by Hae-Seung Lee and Michael H. Perrott Voltage Controlled Oscillators (VCO’s) L1 Vout Cvar L L2 Vout Vout M1 M2 Vs Vbias Cvar M1 C1 V1 Vcont Ibias Ibias Cvar Vcont Include a tuning element to adjust oscillation frequency Varactor replaces (part of) fixed capacitance - Typically use a variable capacitor (varactor) - Note that some fixed capacitance cannot be removed (transistor junctions, interconnect, etc.) Fixed cap lowers frequency tuning range H.-S. Lee & M.H. Perrott MIT OCW Model for Voltage to Frequency Mapping of VCO T=1/Fvco VCO frequency versus Vcont L1 L2 Vout Vout M1 Cvar M2 Vs Vcont Vin Ibias Vbias Fvco Cvar fo Fout slope=Kv Vin Vcont Vbias Model VCO in a small signal manner by looking at deviations in frequency about the bias point - Assume linear relationship between input voltage and output frequency H.-S. Lee & M.H. Perrott MIT OCW Model for Voltage to Phase Mapping of VCO Phase is more convenient than frequency for analysis Intuition of integral relationship between frequency and phase - The two are related through an integral relationship 1/Fvco= α out(t) out(t) 1/Fvco= α+ε H.-S. Lee & M.H. Perrott MIT OCW Frequency Domain Model of VCO Take Laplace Transform of phase relationship - Note that K v is in units of Hz/V T=1/Fvco L1 Vout Cvar Vout M1 M2 Vs Vcont Vin Frequency Domain VCO Model L2 Cvar vin 2πKv s Φout Ibias Vbias H.-S. Lee & M.H. Perrott MIT OCW Varactor Implementation – Diode Version Consists of a reverse biased diode junction - Variable capacitor formed by depletion capacitance - Capacitance drops as roughly the square root of the bias voltage Advantage – can be fully integrated in CMOS Disadvantages – low Q (often < 20), and low tuning range (± 20%) V+ V+ Cvar VP+ V- Depletion Region H.-S. Lee & M.H. Perrott N+ N- n-well P- substrate V+-V- MIT OCW A Recently Popular Approach – The MOS Varactor Consists of a MOS transistor (NMOS or PMOS) with drain and source connected together - Abrupt change in capacitance as inversion channel forms Advantage – easily integrated in CMOS Disadvantage – Q is relatively low in the transition region - Note that large signal is applied to varactor – transition region will be swept across each VCO cycle Watch out for gate-to-bulk capacitance! H.-S. Lee & M.H. Perrott MIT OCW A Recently Popular Approach – The MOS Varactor H.-S. Lee & M.H. Perrott MIT OCW A Method To Increase Q of MOS Varactor Cvar C 2C 4C W/L 2W/L 4W/L W/L LSB MSB Coarse Control 000 001 010 011 Coarse 100 Control 101 110 111 Overall Capacitance to VCO Vcontrol Fine Control Vcontrol Fine Control High Q metal caps are switched in to provide coarse tuning Low Q MOS varactor used to obtain fine tuning See Hegazi et. al., “A Filtering Technique to Lower LC Oscillator Phase Noise”, JSSC, Dec 2001, pp 1921-1930 H.-S. Lee & M.H. Perrott MIT OCW Supply Pulling and Pushing L1 Vout Cvar L L2 Vout Vout M1 M2 Vs Vbias Cvar M1 C1 V1 Vcont Ibias Ibias Cvar Vcont Supply voltage has an impact on the VCO frequency - Voltage across varactor will vary, thereby causing a shift in its capacitance - Voltage across transistor drain junctions will vary, thereby causing a shift in its depletion capacitance This problem is addressed by building a supply regulator specifically for the VCO H.-S. Lee & M.H. Perrott MIT OCW Injection Locking in Oscillators Recall Barkhausen’s Criteria x=0 e H(jw) y Barkhausen Criteria e(t) Closed loop transfer function Asin(wot) H(jwo) = 1 Self-sustaining oscillation at frequency ωo if H.-S. Lee & M.H. Perrott Asin(wot) y(t) MIT OCW Injection Locking Mechanism With the input x=0, the selfsustaining oscillation occurs at ωo because At frequency small deviation ∆ω away from ωo, the magnitude of G(jω) is still very large So, what if the input x is a nonzero signal at ωo+∆ω? If the circuit is purely linear, the output y will contain both the oscillation at ωo and the amplified input at ωo+∆ω (superposition) H.-S. Lee & M.H. Perrott MIT OCW Injection Locking, Cnt’d Output Oscillator Adjustment of Gm Peak Detector Desired Peak Value In a real oscillator, the transfer function is non-linear to keep the amplitude constant (either by amplitude feedback or saturating Gm characteristic) But, let’s first look at what happens if the oscillator transfer function is linear and if a small amplitude signal is injected at the input x H.-S. Lee & M.H. Perrott MIT OCW Intuitive Look at Injection Locking, Linear Case Let’s conceptually make the oscillator transfer function linear by letting the output reach a desired amplitude (say 1V) and disengaging the amplitude feedback after sampling and holding the Gm adjustment voltage at that level The value of Gm is precisely that would make at that point Assuming nothing drifts, the output would be a constant amplitude oscillation at ωo Next, let’s see what happens if we inject a sinusoidal signal with a small amplitude, say 10mV, at ωo+∆ω at input x |G(jω)| is very large at this frequency – let’s say |G(jω)| =10,000 at ω=ωo+∆ω The output will be the superposition of 1V sinusoid at ωo and a 100V sinusoid at ωo+∆ω H.-S. Lee & M.H. Perrott MIT OCW Intuitive Look at Injection Locking, Nonlinear Case If the amplitude feedback is re-engaged, it will lower Gm to keep the total amplitude at the desired 1V level. This value of Gm would adjusted be far below what’s necessary to sustain oscillation at ωo Thus, only the sinusoid at ωo+∆ω will appear at the output with an amplitude of 1V. The VCO frequency is hence locked to the input frequency ωo+∆ω rather than oscillating at the free running frequency of ωo The injection locking phenomenon can be exploited as an alternative to phase-locked loops (See Tom Lee’s book, pp563-566, or p439, 1st, ed.) Otherwise, the injection locking is troublesome H.-S. Lee & M.H. Perrott MIT OCW Example of Undesired Injection Locking For homodyne systems, VCO frequency can be very close to that of interferers RF in(w) Interferer 0 Desired Narrowband Signal wint wo Mixer LNA RF in W LO signal LO frequency Vin - Injection locking can happen if inadequate isolation from mixer RF input to LO port Follow VCO with a buffer stage with high reverse isolation to alleviate this problem H.-S. Lee & M.H. Perrott MIT OCW Recent VCO Techniques Gm-boosted VCO for lower phase noise Recall gm-boosted LNA lowered noise factor: Vdd Ld C1 Out M2 C2 M1 In k LP T1 Ls VB Figure by MIT OCW. The apparent gm boost is is the result of the gate and source having 180o out-of-phase waveforms (it increases Vgs). H.-S. Lee & M.H. Perrott MIT OCW Gm-Boosted VCO Similar concept can be employed for VCO’s to lower phase noise. Transformer coupling is possible, but takes up area. Can boost gm just by feeding output back to source A B Vdd V+ C1 C2 VB L1 M1 L2 M2 IB IB basic concept Vdd Vctrl V- V+ V- C1 C1 C1 C2 C2 M1 IB M2 IB self-biased VCO C2 Figure by MIT OCW. See Xiaoyong Li et. al., “Low-Power gm-boosted LNA and VCO Circuits in 0.18µm CMOS” 2005 ISSCC Digest of Technical Papers pp. 534-353 H.-S. Lee & M.H. Perrott MIT OCW Wide Tune Range VCO Davide Guermandi , et. al “A 0.75 to 2.2GHz ContinuouslyTunable Quadrature VCO,” Digest of Technical Papers, 2005 ISSCC pp 536-537 SSBM Q QVCO Div2 I Fvco = [2.2 GHz - 3.3 GHz] + 20% 2.75 GHz - B I Qout Fout = [0.74 GHz - 2.2 GHz] + 50% 1.47 GHz - Q C Iout Div2 "1" "0" A => 1/3 Fvco [0.74 GHz - 1.1 GHz] A B => 1/2 Fvco [1.1 GHz - 1.65 GHz] C => 2/3 Fvco [1.49 GHz - 2.2 GHz] Figure by MIT OCW. H.-S. Lee & M.H. Perrott MIT OCW Wide Tune Range VCO, Continued VCO, Divider and SSBM Circuits Vdd Vdd L L MP L L C C C Vs1 Load C Vs2 Mc R1 R2 Q CK Vbias Quadrature VCO R2 D Mc MP R1 Q Wrw D Wck B1 Wrw Wck Io Io CK B2 Balanced Modulator Figure by MIT OCW. H.-S. Lee & M.H. Perrott MIT OCW Very High Frequency VCO λ/4 short stub for 114GHz Vpp(114GHz) VDD Vo- (57GHz) Vctrl Ld1 Ld2 Buffer Cvar2 Lg2 Cvar1 Ztank Zactive VG Rg Lg1 M2 Ls2 Ping Chen, et. al. “A 114GHz VCO in 0.13µm CMOS Technology,” 2005 ISSCC Digest of Technical Papers pp. 404-405 Vo+ (57GHz) Buffer M1 Cs2 Cs1 Ls1 Figure by MIT OCW. H.-S. Lee & M.H. Perrott MIT OCW Recent VCO Techniques R. Aparicio and A. Hajimiri, “Circular Geometry Oscillators,” ISSCC 2004 Digest of Technical Papers, pp378-379 VDD VDD L + - C + P2 P1 L L L VDD Oscillator core Oscillator core Extra inductance and loss + VDD VDD + L •Slab inductors offer higher Q than spiral/circular inductors due to Figure by MIT OCW. less current crowding and less substrate loss •In a conventional oscillator topology, the interconnect adds undesired inductance with loss •Circular geometry oscillator removes this problem H.-S. Lee & M.H. Perrott MIT OCW Circular Oscillator Implementation VDD Virtual Ground VDD L + - L/2 + + Virtual Ground Point - L + VDD VDD + Oscillator core + - L/2 D L' L/2 - V D D - D L' L/2 L/2 L' + L/2 L V L' DD L + - L/2 + V + DD + V - L/2 + - Figure by MIT OCW. Shorts the outputs at DC to remove stable DC operating point Shorts outputs at even harmonics to suppress undesired modes H.-S. Lee & M.H. Perrott MIT OCW Die Photo and Measured Results 0.9 mm Pick up loop SC Circular-Geometry Oscillator O SC O Single Frequency Technology Cross SiGe 7HP (CMOS transistors only) SC O O SC I mm Channel Length 0.18µm Center Frequency 5.35GHz 5.36GHz Tuning Range ---- 8.3% Output Power Buffer BIAS Output H.-S. Lee & M.H. Perrott VCO 1dBm Vdd 1.4V 1.8V Ibias 10mA 12mA BIAS Figure by MIT OCW. MIT OCW Circular Standing Wave Oscillator D. Ham and W. Andress ISSCC 2004 Digest of Technical Papers, pp380-381 H.-S. Lee & M.H. Perrott MIT OCW Standing Wave Oscillators Energy Injection λ/4 standing wave oscillator (SWO)** Short Reflection Reflective boundaries Reflection Short Short Reflection λ/2 SWO [4][5] Reflection Reflection Short Short Figure by MIT OCW. H.-S. Lee & M.H. Perrott MIT OCW Ring Transmission Line Principle ∆E (energy injection) Wave superposition V(φ) ~ e-jβrφ + ejβrφ ~ cos(βrφ) e-jβrφ ejβrφ r φ+2π φ Standing wave formation Periodic boundary condition V(φ) = V(φ+2π) Standing wave modes l = nλ (n = 1, 2, 3,....) Differential ring t-line H.-S. Lee & M.H. Perrott Figure by MIT OCW. MIT OCW Circular Standing Wave Oscillator (CSWO) T1 CSWO PSD at port T1-T2 L T2 L1 L2 Q Q R1 R2 2ω0 ω0 3ω0 ω After even-mode suppression B2 L B1 L: Loud Q: Quiet Even-mode suppression connection ω0 2ω0 3ω0 ω KEY H.-S. Lee & M.H. Perrott Figure by MIT OCW. MIT OCW Die Photo and Measurement Set up Agilent E4448A Spectrum Analyzer Vdd L RF probing Q Bias-Tee 50 Ω RF probing Q L Open-collector buffer S+ 2.1 mm G S- CSWO core H.-S. Lee & M.H. Perrott Figure by MIT OCW. MIT OCW Mixers H.-S. Lee & M.H. Perrott MIT OCW Mixer Design for Wireless Systems Zin From Antenna and Bandpass Filter PC board trace Zo Mixer Package Interface Design Issues RF in IF out To Filter LNA Local Oscillator Output - Noise Figure – impacts receiver sensitivity - Linearity (IIP3) – impacts receiver blocking performance - Conversion gain – lowers noise impact of following stages - Power match – want max voltage gain rather than power match for integrated designs - Power – want low power dissipation - Isolation – want to minimize interaction between the RF, IF, and LO ports - Sensitivity to process/temp variations – need to make it manufacturable in high volume H.-S. Lee & M.H. Perrott MIT OCW Ideal Mixer Behavior Desired channel RF in(f) 0 -fo 1 0 fo f IF out Local Oscillator Output = 2cos(2πfot) Undesired component LO out(f) -fo RF in f fo ∆f 1 Channel Filter -fo Undesired component IF out(f) -∆f 0 ∆f f fo RF spectrum converted to a lower IF center frequency - IF stands for intermediate frequency If IF frequency is nonzero – heterodyne or low IF receiver If IF frequency is zero – homodyne receiver Use a filter at the IF output to remove undesired high frequency components H.-S. Lee & M.H. Perrott MIT OCW The Issue of Image Aliasing RF in(f) Image Desired Interferer channel 0 -fo fo -∆f ∆f RF in f Local Oscillator Output = 2cos(2πfot) LO out(f) 1 IF out IF out(f) 1 -fo 0 fo f -fo -∆f 0 ∆f f fo When the IF frequency is nonzero, there is an image band for a given desired channel band - Frequency content in image band will combine with that of the desired channel at the IF output - The impact of the image interference cannot be removed through filtering at the IF output! H.-S. Lee & M.H. Perrott MIT OCW LO Feedthrough RF in(f) Image Desired Interferer channel 0 -fo fo -∆f ∆f RF in LO feedthrough f Local Oscillator Output = 2cos(2πfot) LO out(f) 1 IF out(f) 1 -fo IF out 0 LO feedthrough fo f -fo -∆f 0 ∆f fo f LO feedthrough will occur from the LO port to IF output port due to parasitic capacitance, power supply coupling, etc. - Often significant since LO output much higher than RF signal If large, can potentially desensitize the receiver due to the extra dynamic range consumed at the IF output If small, can generally be removed by filter at IF output H.-S. Lee & M.H. Perrott MIT OCW Reverse LO Feedthrough RF in(f) Reverse LO feedthrough Image Desired Interferer channel 0 -fo fo -∆f ∆f f RF in Reverse LO feedthrough LO feedthrough Local Oscillator Output = 2cos(2πfot) LO out(f) 1 IF out IF out(f) 1 0 -fo LO feedthrough fo f -fo -∆f 0 ∆f f fo Reverse LO feedthrough will occur from the LO port to RF input port due to parasitic capacitance, etc. - If large, and LNA doesn’t provide adequate isolation, - then LO energy can leak out of antenna and violate emission standards for radio Must insure that isolate to antenna is adequate H.-S. Lee & M.H. Perrott MIT OCW Self-Mixing of Reverse LO Feedthrough RF in(f) Reverse LO feedthrough Image Desired Interferer channel 0 -fo fo -∆f ∆f f RF in Reverse LO feedthrough LO feedthrough Local Oscillator Output = 2cos(2πfot) LO out(f) 1 IF out IF out(f) 1 -fo 0 Self-mixing of reverse LO feedthrough LO feedthrough fo f -fo -∆f 0 ∆f f fo LO component in the RF input can pass back through the mixer and be modulated by the LO signal - DC and 2f component created at IF output - Of no consequence for a heterodyne system, but can o cause problems for homodyne systems (i.e., zero IF) H.-S. Lee & M.H. Perrott MIT OCW Removal of Image Interference – Solution 1 RF in(f) Reverse LO feedthrough Image Desired Interferer channel 0 -fo fo Image Rejection Filter IF out RF in f Local Oscillator Output = 2cos(2πfot) -∆f ∆f LO out(f) 1 IF out(f) 1 -fo 0 Self-mixing of reverse LO feedthrough LO feedthrough fo f -fo -∆f 0 ∆f fo f An image reject filter can be used before the mixer to prevent the image content from aliasing into the desired channel at the IF output Issue – must have a high IF frequency - Filter bandwidth must be large enough to pass all channels Filter Q cannot be arbitrarily large (low IF requires high Q) H.-S. Lee & M.H. Perrott MIT OCW Removal of Image Interference – Solution 2 RF in(f) Reverse LO feedthrough Desired channel 0 -fo fo ∆f=0 LO out(f) 1 -fo RF in f Reverse LO feedthrough IF out LO feedthrough Local Oscillator Output = 2cos(2πfot) IF out(f) 1 0 Self-mixing of reverse LO feedthrough LO feedthrough fo f -fo 0 fo f Mix directly down to baseband (i.e., homodyne approach) Issues – many! - With an IF frequency of zero, there is no image band - DC term of LO feedthrough can corrupt signal if time-varying - DC offsets can swamp out dynamic range at IF output 1/f noise, back radiation through antenna H.-S. Lee & M.H. Perrott MIT OCW Removal of Image Interference – Solution 3, Image Reject Mixer RF r(t) y(t) Balanced modulator Local Oscillator + -90o phase shifter IF Output -90o phase shifter Balanced modulator z(t) LPF or BPF ^z(t) Image rejected by similar method to SSB generation Image rejection limited by amplitude and phase matching of RF and LO paths. 40 dB image suppression is typical RF filter can reduce the image further if necessary, otherwise the RF image reject filter can be omitted. H.-S. Lee & M.H. Perrott MIT OCW Frequency Domain View of Image Reject Mixer H.-S. Lee & M.H. Perrott MIT OCW Frequency Domain View of Image Reject Mixer, Cnt’d It can be shown that image is rejected regardless of the RF input phase H.-S. Lee & M.H. Perrott MIT OCW
