mixers: reciprocity n-path designs - university of … t t v t m t v t lo lo lo if rf z z z if / s...

Copyright Mark Rodwell, 2016

Mixers:

Reciprocity

N-path designs

Mark Rodwell, University of California, Santa Barbara

ece145c lecture notes


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Citation: TUTORIAL 5594: System Noise-Figure Analysis for Modern Radio Receivers By: Charles Razzell,Maxim Integrated Products, Inc.

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N-path mixers: Motivation

Why use LNAs in superheterodyne receivers ?

...because mixer has high noise figure.

LNAs degrade overall receiver overload/jammer tolerance:

-Receiver IIP3 set by LNA IIP3 & (mixer IIP3)/(LNA gain).

-Fixed-tuned RF filter can't protect LNA & mixer

from interferers within receiver overall tuning bandwidth.

If we could make a very low noise figure mixer….

LNA would not be needed--> improved receiver IP3.

Out of band IM3 from mixer alone.

Low receiver noise (IF amp can have low Fmin).


N-path mixers: Motivation

Why do mixers have high noise figure ?

noise partly from component noise & attenuation.

noise partly from mixer image & harmonic responses.

Can we make mixers with very low noise figure ?

good devices→ low component noise.

subsampling method→ low/zero image responses.


Where are the harmonic responses coming from ?

sinewave. a...not

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elementsnonlinear signal-small

not switches, usingmix wish to weYet,

? thisdo might we How


Array of sample-hold gates:

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mixer. quadrature

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selected becan harmonic desired The


Selecting Harmonics:

trainpulseagainst

mixing :S/H One

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Selecting

NfLO /2

Selecting


N-path Filter Has No Image Responses

Array has N non-overlapping switches

unfiltered outputs: positive & negative IF images

unfiltered outputs: responses for all LO harmonics

sine/cosine weighting: selects particular LO response

sine / cosine quadrature summing suppresses IF image

Input RF filter suppresses responses beyond N*fLO.


What is the bandwidth ?

frequency signal baseband are / and

:Assume

Nff LOsignal

0impedance Equivalent

/1 cycleduty Switch

:theorycapacitor -switched use

NZ

N

CNZ )(1/2Bandwidth 0


N-path filter tuning

The receiver signal frequency is tuned by adjusting:

The LO (sampling) frequency (fsig = 2 fLO + fIF )

The selected LO harmonic (cos/sin weighting)

Wide tuning range is easily achieved


N-path filter with a nonzero IF Frequency

Input gates are sample-and-filter, not sample-and-hold

No in-band resonant enhancement of voltage or current

Out-of-band voltage is suppressed at switch

→ System IM3 set by switch stage, no Q:1 degradation in dynamic range

As N → ∞ perfect conversion, image response → 0, NF →0


With nonzero IF output frequency

phase relative 90at outputs Q and I the

summingby obtained then is mixing SSB

0


Again, what is the bandwidth ?

frequency signal baseband are / and

:Assume

Nff LOsignal

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/1 cycleduty Switch

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Simulations: Late 2008, early 2009

Simulation example showing the 8 sampling phases, and the sine and cosine weighted outputs both before and

after 90 degree phase-shifting


Simulation

CMOS switch array responses with 1st and 3rd LO harmonic tuning.

The IF bandwidth is set at 30 MHz equivalent to a Q of 100


Simulations

File:nf-vs-N-ssampRLC

ADS Sim mark4

0

1

2

3

No

ise

Fig

ure

(d

B)

2 4 6 8 10 12 14 16

Number of Samplers

65nm CMOS (digital)

35nm Std InP HEMT

35nm Low Noise InP HEMT

device model: 35 nm InP/InGaAs HEMT, 400-500 GHz ft , fmaxf compared to 65nm NMOS (state of art at time of simulation)

figure. noise reduced

content harmonic LO reduced

sinewave ion toapproximatbetter

: Increased

N


Dynamic Range

Input Capacitor C is large i.e. Z0C >> TON

Within passband, C charges to RF signal,

Current→ 0, distortion is minimized, IF amp determines IP3

Outside passband, C discharged

Current ~ Isig distortion is from current in switch element

No resonant enhancement in either case

No 3dB noise from image/harmonic response in limit as n→∞

No RF filtering needed since no LNA and mixer has high IIP3


Simulationsdevice model: 35 nm InP/InGaAs HEMT, 400-500 GHz ft , fmaxfcompared to 65nm NMOS (state of art at time of simulation)


65nm MOSFET Simulations – 8 element array

Simulation of in-band IM3, showing 12 dBm IIP3. Simulations showed 1.7 dB NF at 1.2 GHz12.5 dBm IIP3 is obtained with mixing against the 2nd LO harmonic (2.2 GHz inputs).

Out-of-band IM3 simulation ~0 dBm equivalent IIP3.Signals selected so that the IM3 response in-band but carriers are not.IM3 products are 20 dB stronger, so the "equivalent" IP3 has dropped 10 dB


65nm MOSFET Simulations – 8 element array

• Simulation of in-band IM3, showing 12

dBm IIP3.

• Using the 65 nm MOS model, simulations

showed 1.7 dB NF at 1.2 GHz

• 12.5 dBm IIP3 is obtained with mixing

against the 2nd LO harmonic (2.2 GHz

inputs).

• Out-of-band IM3 simulation indicating ~0

dBm equivalent IIP3.

• Signals selected so that the IM3 response

in-band but carriers are not.

• IM3 products are 20 dB stronger, so the

"equivalent" IP3 has dropped 10 dB


N-path: first demonstrated early 2009

mixers: reciprocity n-path designs - university of … t t v t m t v t lo lo lo if rf z z z if / s...

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