IN5240 Fundamentals of RF Circuit Design Part 2

Institutt for Informatikk

IN5240 Fundamentals of RF Circuit Design

Part 2

Sumit Bagga* and Dag T. Wisland***Staff IC Design Engineer, Novelda AS

**CTO, Novelda AS


Outline

• Wireless communication systems• Performance metrics of a wireless receiver• RF building blocks

IN5240: Design of CMOS RF-Integrated Circuits, Dag T. Wisland and Sumit Bagga


2 May 16, 2008

7.1 Mixers characteristics

Frequency conversion Frequency Conversion

• RF wanted signal is down-converted by a mixer i.e., multiplication with a local oscillator (LO), 𝑓!" in time domain

• Multiplication in time domain à convolution in frequency domain (shift of RF signal)



Image

Image is the unwanted signal that lies symmetrically to the RF signal of interest with respect to the 𝑓!"

[Liscidini, ISSCC, 2015]



Hartley Receiver

Spectrum of sine and cosine are asymmetrical à image




Noise Mixing

• Receive mixer down converts wanted and the image bands to IF frequency à folding of noise at image frequency on top of wanted band at IF, and is: – Noise at desired and image RF bands down converted à

IF – Added noise from mixer circuit

• If the mixer is noiseless, SSB NF is 3 dB because of the image noise folding



SSB and DSB Noise

• SSB NF assumes no signal at the image frequency except source noise• DSB NF assumes image band w/ noise and an image signal equal to

the wanted signal


RFThermal Noise

�f

�LO

Image

IFThermal Noise


RF MixersPerformance Metrics

• Noise• Linearity: P1dB, input inferred intercept points (IIP3, IIP2)

– OP1dB = IP1dB + (𝐺 − 1)– IP1dB + 10.6 dB = IIP3

• Voltage conversion gain/loss• Port-to-port isolation (LO-RF, RF-LO and LO-IF)

– Leakage from a port to another is undesirable• Supply voltage• Power dissipation



Passive and Active Mixers

• Current and voltage mixers à transistors are switches• What is the ideal LO waveform?

– RF signal is multiplied by square wave not sinusoidal


EE215C B. Razavi Win. 13 HO #2

56

RF Mixers (I) General Considerations x Performance Parameters - Noise - Linearity: IP3, IP2 - Voltage Conversion Gain - Supply Voltage - Power Dissipation x Passive and active Mixers

x SSB and DSB Noise Figures

[Razavi, EE215C]


Single and Double Balanced Mixers


EE215C B. Razavi Win. 13 HO #2

57

x Port-to-Port Isolation The leakage from each port to the other may degrade the performance: - LO-RF Feedthrough - RF-LO Feedthrough - LO-IF Feedthrough And all other combinations … x Single-Balanced and Double-Balanced Mixers

What is the ideal LO waveform?

[Razavi, EE215C]


Passive Voltage Mixer

• Active devices (transistors) operate in triode• Large signals at input/output à difficult to completely

turn on/off transistorsIN5240: Design of CMOS RF-Integrated Circuits, Dag T. Wisland and Sumit Bagga



Passive Current Mixer

• Active devices (transistors) operate in triode• Low input impedance of transimpedance amplifier

input à small voltage swings at source/drain




Active Current Mixer

• Transconductor stage à input voltage to current• Switches (transistors) operate in saturation (i.e.,

cascodes coupling/de-coupling RF to IF




Mixer Comparison




LNA Performance Metrics

• ‘Dominant’ input device suppresses noise contributed subsequent blocks à ↑ gain– Trade-off gain for linearity

• Optimize input device for lowest noise figure– NF < 2 dB à CS-stage w/ 𝑔# ≫ $

%&Ω and minimum

gate resistance, 𝑅'• Cover bandwidth specified by standard• Conjugate matching at the input



LNA Stability

• Design for unconditional stability stable across any source (antenna radiation) impedance, 𝑍#– 𝐾 = $(|*!!|"(|*""|"+|∆|"

%|*!!||*""|

– ∆ = |𝑆$$𝑆%% − 𝑆$%𝑆%$|

– 𝜇 = $(|*!!|"

|*""(∆*!!∗ |+|*!"*"!|

• Measure two-port stability– 𝐾 > 1 and Δ < 1 or 𝝁 > 𝟏



LNA Topologies


Common-Source (CS) Common-Gate (CG) Broadband• +CG (Cascode)

• Resistive feedback

• Inductive load

• Inductive degeneration

• +CG

• Inductive load

• Feedback

• Feedforward (Boosted CG)

• Noise-cancelling

• Reactive cancelling


Typical CMOS IC Process

p-sub

n-wellp-well p+ n+

thick metal

vias metal

NMOS(not isolated)

PMOSNMOS

For passive devices, the most important things are (in order ofimportance):

Metal conductivity, distance to substrate, substrateconductivityOther important considerations include a triple-well (or deepnwell) for isolation

Top one or two layers are usually thicker

Niknejad Advanced IC’s for Comm

CMOS Process

• Active devices: triple-well (or deep n-well) for isolation

• Passive devices: metal conductivity, substrate resistivity and distance to substrate– Use top 1 or 2 metal layers à ‘thick’ Cu/Al (2-4 µm)



BEOL

• IC fabrication step after FEOL (front-end-of-line) is BEOL (back-end-of-line) à on-wafer interconnection of devices with metal wiring

• Modern sub-micron CMOS (< 90 nm) processes àtypically +9 metal and re-distribution (RDL) layers

• Dielectrics à complex stack-up of low and high-dielectric materials



Passive Devices

• RF components– Inductors/transformers– Capacitors – Transmission lines – Varactors (MOS devices)

• Design considerations– Accurate EM modeling to create SPICE (lumped

element) models– Density fill à



MIM & MOM Capacitors

• MIM (metal-insulator-metal): parallel-plate capacitor à two planes of metal (or polysilicon) separated by a thin oxide w/ high dielectric constant

• MOM (metal-oxide-metal)– Multi-finger inter-digitated capacitor à vertical BEOL

metal stack and inter-metal dielectrics



MOM vs MIM

• High density à MIM (unit capacitance of MOM < MIM)

• High quality factor à MIM• Cost à MOM (MIM require extra mask)

Niknejad, EECS 105



Inductors

• Deep sub-micron processes à low 𝐾 (thin) metals (7-9) à ↓ 𝑄 inductors/T-lines– Reduce cost; alternative à thick/ultra-thick metals

• Uses include filtering, impedance matching (tuning out capacitance (w/ series or parallel resonance)), high gain and power efficiency and low noise figure (e.g., w/ reactive negative feedback), high linearity



Inductor Types

Monolithic Transformers for Silicon RF IC Design, Long, JSSC, 2000



Inductor Topologies and Metrics

• Self-inductance, Q-factor, SRF (self-resonance frequency)

• Geometric considerations– Rectangular/square/octagonal/circular/F8– Spiral or symmetrical– Center-tap – Shielding (floating or grounded)– Tapered

• Physical parameters– Outer to inner area, number of turns, metal width,

spacing; tapering?



Substrate, Skin and Proximity Effects

• Current flows through a loop (or spiral) àmagnetic fields pass through substrate à induce eddy currents in opposite direction (Lenz’ Law)– ↓ w/ high resistivity substrate (luxury!)

• ↑AC resistance and current flows near surface à↑ distance – Skin depth Cu (5.8x107 S/m) is 0.66 at 10 GHz

• Proximity of adjacent conductor à ‘current crowding’



Passive Filters

• Butterworth, Chebyshev, Elliptic, Bessel, …• Design methodology

– Prototype low pass filter design– Transformation from LPF to high pass, band pass and

band stop – Frequency translation

• Impedance transformation



Filters Comparison



LPF Design (rfcooltools.com)

• Normalized capacitors and inductors (𝑔$, 𝑔%, …𝑔&) à denormalized by:

𝐶 = 4!567"8

& 𝐿 = 9!8567"

• 𝑔$ and 𝑔&'$ denote source and load impedances and are equal to 1

• 𝐶, 𝐿 and 𝑔 (normalized values) are obtained from a look-up table



Transformation from LPP


[Kim, EEE 194]


T and π Filter Networks (rfcec.com)



What is an Oscillator?

• Converts dc power à sinusoidal waveform• High-Q LC tank or a resonator (crystal, cavity, …)

– Lossy LC-tank à amplitude of the oscillator decays

• Oscillation frequency, power, phase noise/jitter, stability, tuning range



Oscillator Design

• Amplitude and frequency stability • Concept of negative resistance • Oscillator topologies (Colpitts, Hartley, Clapp,

Cross-coupled, …)• Injection locked oscillators

– Locking range, injection pulling,



Positive Feedback

• Oscillators à feedback systems• Fraction of the output signal is fed back to sustain

oscillations à ‘injected’ energy required to compensate for lossy tank

Feedback Perspective

vo

−vo

n

gmvin : 1

Many oscillators can be viewed as feedback systems.The oscillation is sustained by feeding back a fraction ofthe output signal, using an amplifier to gain the signal,and then injecting the energy back into the tank. Thetransistor “pushes” the LC tank with just about enoughenergy to compensate for the loss.

A.M. Niknejad University of California, Berkeley EECS 142 Lecture 21 p. 6/25 – p. 6/25


[Niknejad, EECS 242]


Barkhausen’s CriterionLoop Gain ( 𝐴𝛽 )

• Magnitude of the product of open loop gain and the magnitude of the feedback factor of the amplifier is unity– 𝐴𝛽 = 1

• System poles are on jω-axis à constant amplitude oscillations– 𝐴𝛽 < 1à decay– 𝐴𝛽 > 1à amplitude increases exponential to steady-state

• Phase shift around the loop is 0 or integral multiples of 2𝜋



Phase Noise

• Phase noise spectral density (PN) units à dBc/Hz and measured at ∆𝑓 from the 𝑓(

• Low spectral purity à convolution of blocker (∆𝑓) & 𝑓!" à noise contribution in RF BW (reciprocal mixing)




Key References

1. A. M. Niknejad, EECS 142, 242 and 1052. A. Liscidini, “Fundamentals of Modern RF

Receivers,” ISSCC 20153. E. Kim, EEE 1944. B. Razavi, EE215C


Documents

IN5240 Fundamentals of RF Circuit Design Part 2