Parasitic Coupling in RF Circuits- A Real-Life Experience

Parasitic Coupling in RF Circuits: a Real-life experience Frank Op ‘t Eynde Central R&D Director Alcatel Microelectronics

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Content ▼

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Increasing the degree of integration is increasing the risk of parasitic feedback Feedback on practical RF circuit design experience: ● Parasitic coupling related to RX ● Parasitic coupling related to Synthesiser ● Parasitic coupling related to TX

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PA

BATMAN

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R F

L F

A N A L O G

A N A L O G

VCO

BASEBAND PROCESSOR

µPROCESSOR

RAM

ROM

DATA PORT

LNA

AUDIO PORT

Degree of Integration

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RF Frontend I(t)

A/D

LNA

D E M O D U L A T O R

26 dB Receiver digital part Q(t)

A/D

0,12,24 dB

26 dB

sin

-6, 0,6,...,48 dB +2dB option

cos

2*fLO

Serial Program m ing Interface

÷N

÷2

Control Block

Filter Calibration sin

P ower down control

I(t)

M O D U L A T O R

D/A

PPA Ref Offset Q(t)

Iref generator Vref generator

FO 4

RSSIA NC REFCLK

φ

VCO

cos

D/A

÷ 4 /÷ 8

÷2

analog test buss output drivers

8

RXDATA RXCLK

D/A

Analog test buss

D/A

4

D/A

D/A

4

8

TXDATA

Serial Test Mode interface

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DECT RF Frontend

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Bluetooth RF Frontend

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RADIO RAM

DCXO

RAM

FLASH

PLL

LOGIC

ARM7

RAM

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Parasitic coupling related to RX: Sensitivity reduction

-86

-94

0

1

2

3

4

5

6

7

8

9 Channel Nr.

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Sensitivity reduction (2) LNA

ADC

ADC

/2

PD /32/33

PA

DAC

DAC

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• PLL reference frequency = 2*channel spacing • /32/33 generates tones at all multiples of Fref • parasitic coupling to antenna input, via - VDD/VSS bondwires - substrate • at even channel numbers, one tone coincides with input signal frequency Alcatel Microelectronics

Sensitivity reduction (3) Solution: 1. Decouple Prescaler supply with 100 pF on-chip 2. Isolate Prescaler digital CMOS from substrate: pMOS

N-well

nMOS

P-well

sinker

P-epi NBL P-sub FO 9

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Parasitic coupling related to RX: Interferer measurements Receiver Selectivity

Useful Signal

Interferer

RX channel

0

1

2

3

4

5

6

7

8

9

Ch 0 Ch 5 Ch 9

-81 -22 -19.5

-54 -36.5 -35.5

-27.5 -25 -20

-24 -28.5 -21.5

-33.5 -54 -22

-22.5 -81 -35

-22 -53.5 -25

-21.5 -28 -28.5

-34.5 -24 -54

-22 -33.5 -80.5

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RX Interferer measurements (2) ∆f Receiver Selectivity

ADC

ADC

LNA

/2

Useful Signal

∆f FO 11

Interferer

PD /32/33

∆f Alcatel Microelectronics

RX Interferer Measurements (3) Clock

Running in parallel over > 1 mm

1.2 Volt

LNA

Clock Generator Voltage Reference

ADC

ADC

/2

PD /32/33

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Parasitic coupling related to Synthesiser Fref

FLO /2

P D

/ 32 / 33

• PLL spurs cause interferers to mix into signal band

/N

• Spurs are caused by: - feedthrough of Fref - parasitic coupling

Freq. FO 13

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PLL pulling by digital input pins Fref

500 kHz on TXin UNCAL

Marker 1 [T1]

Ref Lvl 0 dBm

RBW

-32.02 dBm

VBW

2.40300000 GHz

SWT

30 kHz

RF Att

10 dB

UNCAL

3 kHz 100 ms

Marker

Ref Lvl

Unit

dBm

0

0

dBm

1

[T1]

RBW

-26.29

dBm

VBW

2.40300000

GHz

SWT

30 kHz

RF Att

10

dB

3 kHz 100 ms

Unit

dBm

0

B -10

-20

1

-30

1MA -40

Bad

-50

-60

B

Transmit Spectra

-10

-20

1 -30 1MA -40

-50

-70

-80

-80

-90

-90

-100

-100

Center 2.403 GHz Date:

3.SEP.2001

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500 kHz/ 14:50:08

Good

-60

-70

Span 5 MHz

Center Date:

2.403

GHz

3.SEP.2001

500

kHz/

Span

5 MHz

14:50:50

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Parasitic coupling related to TX: TX Spectrum Fundamental

??? 3rd Harmonic Image

Continuous “1” transmission

Unknown spur at LO + 3BB

LO

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TX Spectrum Fundamental A

fo+δf fo

Spur a fo

fo+∆f

a fo+∆f A

Freq.

amplitude(δf) = ∆f * a/A FO 16

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TX Eye diagram

100 kHz

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PLL Pulling by Transmitter (1) VDD

/2

PD /32/33

PA

DAC

DAC

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PLL pulling by Transmitter (2) LNA

ADC

ADC

Isolation < 30 dB !!!

PA

/2

PD /32/33

PPA

DAC

DAC

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TX intermodulation due to LNA nonlinear input impedance LNA

ADC

ADC

/2

PD /32/33

PA

PPA

DAC

DAC

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Solution LNA

ADC

ADC

/2

TX-ENABLE

PD /32/33

PA

PPA

DAC

DAC

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Conclusions ▼

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Parasitic crosstalk is a risk when large and small signals are present on the same circuit RF circuits are vulnerable due to the presence of extremely small signals and extremely sensitive nodes Very little software support to predict the problems Root cause finding is cumbersome because incircuit measurements are difficult Alcatel Microelectronics