RF System Analysis

1

J.Dąbrowski, Intro to RF Front-End Design1

RF-System Analysisfrom system requirements to Rx/Tx specs

• Test environment of radio system specs- Receiver analysis- Transmitter analysis

• Distribution of specs over Rx blocks• Summary

Outline


Test environment of system specs

• Frequency bands /filtering• Reference sensitivity /NF• Blocking requirements /1dB point and DR• Intermodulation requirements / IP3, IP2 and

LO phase noise• Image rejection

• Transmitted power• Out-of channel emission /filtering and LO

phase noise• Spurious tones

2


Reference sensitivity

DemodulatorPart

Digital BasebandsignalReceiver

Front-EndPin,minSNRin SNRmin

ADC

sensitivity Pin,min

BERmax SNRmin

NF = SNRin – SNRmin= Pin,min– (-174dBm/Hz + 10logB) – SNRmin

For DECT std. Pin,min= -83dBm, B = 1.728 MHz

BERmax= 10-3 → SNRmin ≈ 10dB (also for N+I)

NF = -83 + 174 -10log(1.728×106)-10 ≈ 18dB Quite relaxed

But 3..4 dB would be sacrificed for loss in duplexer and RF filter


BER versus SNR in demodulator

10-3

SNR = Psig /N = (EbR)/(N0B)

SNR = Eb/N0 × R/B

R/B ≈ 0.5 … 1.5 (often)R – bit rateB – channel bandwidth

SNRmin = (Eb/N0)dB + (R/B)dB = 12 -1.8 = 10.2 dB

(R/B)DECT = 1152/1728 = -1.8 dB

GFSK synchronous differential demodulator in DECT

Ideal GFSK synchronous demodulator (for AWGN)

3


Comment on CDMA receivers

ff

EncodingBB signal BB signal after spreading

BBB

DecodingBB signal received

Processing gain:

GP = BSS /BBB ≅ Rchip /Rdata

GP = SNRBB /SNRSS

BSS

fBBB

SNRmin = (Eb/N0)dB + (R/B)dB ≈ (Eb/N0)dB - (GP)dB

For a given demodulation scheme

Example:

For WCDMA GP = 10lg (3.84Mcps/12.2kbps) = 25dB SNRmin = (Eb/N0)dB - (GP)dB = 7dB – 25dB = -18dB

QPSK


Blocking requirements

• Desensitization• Dynamic range • Reciprocal mixing and LO

phase noise• Band and channel filtering

4


Desensitizationfor DECT

Co-channel

( ) ( )

3

123

21

223

1

23

1

33.1

12

3

...cos2

)(3)(

αα

σσαα

α

ωα

α

=

<>

−

+

+=

IP

bl

SSbl

A

A

tAtA

ty

dBmPPIIP

AA

blbl

bl

IP

33,1

2log10

12

max3

2

23

−=−

+>

−>

σ

σ

This IP3 of the RF part only, after mixer inband blockers are suppressed

-24.7dBm0.53dB

-20.4dBm0.791dB

IIP3RFσGain drop

This is kind of AM that in GMSK


1dB compression point and DR

In-band interferers must not saturate the Rx

Pbl max = -33dBm and also Psig max = -33dBm

P1dB > -33dBm, DR = P1dB – Pin,min

DR > -33 – (-83) = 50dB

P1dB

G +Pin

1dB

Pin minSensitivity

dBIIP

dBPIIP

RRAIIP

RRAP

PdB

IP

dB

4.226.933

6.9

233.1

log102

log10

2145.0

log102

log10

point compresion 1dBcalled isblocker thedrop 1For

3

dB13

30

1

0

23

3

30

1

0

2dB1

dB1

1

−=+−>

=−

==

==

αα

αα

But in practice might be different

This DR refers to the whole Rx chain.The blocker will be suppressed in BB filter, but the max signal will not.

First estimatefor Rx IP3

5


Comment on QAM systemsBER must be maintained in presence of any blocker

( )

minsigsigP

WrefP

IPSigbl

blN

SNRPN

NFBNFNN

ARAAP

−=

++−=+=

=

=

log10174

33.1,22

3

min

3

1230

2

1

23

αα

αα

E.g. for any blocker BER=10-3 → SNRminbut we have Rx Noise + blocker noise

(AM at f0 and uncorrelated)

Allowednoise for signalduring the test

Inherent Rx noise

( ) dBPIIPPP

mWPPP

P

dBsigdBbldBblN

sigIP

blblN

62

4

3

23

2

++−=

=

dBPP

PIIP

NPN

blNsigbl

sigPblNP

323

min

+−

+=

=+

Rx noise + Noise inducedby blocker

It is the Rx total IIP3, since the noise induced by blocker would not be suppressed by BB filter

This is kind of AM can be removed in GMSK Rx by amplitude clipping


Reciprocal mixing and LO phase noise

f

∫ ==H

L

f

favnn BSdffSP )(

Noise imposed:

Typically we require SIR >15dB

SIR = Psig - 10log SavB= Psig - 10log Sav - 10logB

L (foff ) = 10lg Sav - Pint = Psig – Pint - SIR - 10logB

L (1.73MHz) = -73 + 58 - 15 - 10lg(1.73·106) = -93 dBc/HzL (3.46MHz) = -73 + 39 - 15 - 10lg(1.73·106) = -112 dBc/HzL (5.18MHz) = -73 + 33 - 15 - 10lg(1.73·106) = -118 dBc/Hz

fL fH

desired

Noisy interferer at IF Sn(f )

foff

Sav

LO Phase Noise limits receiver selectivity

6


Blocking requirements (cont’d)

10-3 BER must be maintained

The Rx filters must suppress the blockers so that SNR = C/(N+I) = 10dB

Depending on Rx architecture we may have a few filters but at least the band-select and IF filter. We need an effective attenuation of at least 65..70dB provided by RF filter, LNA and BB filter

With an RF filter of 30..40dB we need another 30..40dB from LNA and BB

Attenuationrequired


Intermodulation requirements

∆P

IIP3 = ∆P/2 + Pin

Output

IIP3

Fundamental

3rd order IM

Two-tone test For DECT 2 × (Pint= -46dBm) each, and Psig= -80dBm

and still BER=10-3 → but we have Noise + IM3 (uncorrelated)

N = -83dBm - SNRmin = -93dBm (input referred thermal noise)

How much IM distortions can we allow ? → SNRmin = C/(N+I ) = 10dB

Psig,out

7


Intermodulation requirements (cont’d)

(N + PIM3 )|dBm = Psig - SNRmin = -80dBm -10dB = -90dBm ( all input referred )

(N + PIM3 )|dBm - N |dBm = -90dBm + 93dBm = 3 dB

(N + PIM3 ) / N = 2 → PIM3 = N

Hence:IIP3 = Pin + ∆P/2 ≈ - 46 + (-46+93)/2 = -22.5dBm (which is practically the same as obtained

before from the 1dB compression point )However, in zero-IF the IP2, LF feedthrough in mixer, and LO leakage would contribute as well.

RF Filter LNA LP

Filter ADC

LF feedthrough

HD2 at BB

LO LeakageWe focus on IP2 of mixer, LPF stops the interferers, and IP2 of LNA less critical


IP3 and IP2 requirementsWith this correction we have:(N + PIM3 + PIM2)|dBm = Psig - SNRmin = -80dBm -10dB = -90dBm ( all input referred )

(N + PIM3 + PIM2)|dBm - N |dBm = -90dBm + 93dBm = 3 dB

(N + PIM3 + PIM2) / N = 2 → PIM3 + PIM2 = N We allow less PIM3 to keep N

( ) ( )

mW10,5.0,12

11also and,4

42

2and,

6.4

3

2

23

3

33332

2

23

3

2

2

0

222

223

3

3

−=≈=+

++≈=+

=

==

int

intint

intint

intintint

PGNP

PG

P

P

PG

PG

PPN

PG

PG

P

P

PG

PGGR

AGGP

P

PP

FIP

F

IP

BBIP

RF

mixIP

LNA

LNAIPIPmixIPmix

RF

IP

mixIPmix

RF

RFmixRFmix

IMIP

IM

β

α

Assume all blocks contribute equally to the total IP3 and PIP2 mix = β PIP3 mix

Gain of RF filter and duplexer

8


IP3 and IP2 requirements (cont’d)

-24.7dBm0.53dB

-20.4dBm0.791dB

IIP3RFσGain drop

-2.7-22.425

-2.5-22.320

-2.1-21.915

-0.8-20.510

*) IIP3mix [dBm]IIP3 [dBm]β [dB]

*) For GLNA = 15 dB

Intermodulation requirements Single blocker requirements

Smaller β → IP2 of mixer has more impact on total (N+I) → then larger total IP3 required

How can we use the estimate for IP3RF ?

From 1dB point we also have IP3 > -22.4dBmso we conclude that the impact of mixer IP2 must be small (e.g. β = 25dB or different balance between IP3 components is needed)

We see that the demands for the IIP3RF (-24.7dBm →3dB gain drop) are too relaxed. If we only allow 1dB:

dBGIIPPG

PG

GGPG

PP

dBRFBBBBIPRF

BBIP

RF

RFRFBBIP

RF

RFIPIP

2238.151

79.01010

,11

3

3

04.224.2

*

3

*

33

−≅→≈

+≈

=+≈ σ

For GRF = 15..20 dB we obtain practical values of IIP3BB = -7dBm … -2dBm

IIP3 = -22.4dBm

Large GRF is prohibitive


Image rejection in Rx

• Homodyne (zero-IF) overcomes problems of heterodyne (esp. image problem) but suffers from DC-offset, 1/f noise, LO leakage, self-mixing, even order distortions and IQ mismatch. For WCDMA systems DC band close to 0 can be sacrificed

• Low-IF overcomes DC-offset and 1/f noise but suffers from close-image problem and even order distortions. Image-reject mixer must be used like in zero-IF, but here the IQ requirements are much tougher. BPF at IF must be used (requires 2x more poles/zeros than LPF). For narrow-band systems better than zero-IF.

4)(

)()( 22 θ+∆

==AA

PPPP

IRRinsigim

outsigim

Amplitude and phase mismatch in IQ paths

In practice IQ IRR > -40dB but -60..-70dBare sometimes needed, so filters must help,such as polyphase filters in low-IF

9


Required image rejection

for DECT

fLO for Low-IF Rx

IR = Pim in – (Psig – 15dB)

= -58 – (-73 -15) = 30dB

IR = -73 – (-73 -15) = 15dB

Low-IF Rx,

zero-IF Rx, - More relaxed requirements


Channel Filter and ADC

BPF/LPF

Pbl –A(fbl)

ADC

f0

IF×

Usually, tradeoff between ADC DR and filter order

Attenuation ↔ (foffset, filter order)

In-band blockers suppressed

LPF for zero-IF

∆fch /2 f

12 dB/oct / 2 ord

24 dB/oct / 4 ord

∆fch

36 dB/oct / 6 ordA6(∆fch )

DRADC = (Pbl – A) – (Pin min – 15dB)

PmaxQuantization noise

/ input referred

3∆fch /2

Wantedchannel for 0-IF

Adjacentchannel

GLPF

Channel selection can be completed in BB proc. but ADC must maintain the blockers

15 dB below sensitivity is a rule of thumb if 10dB for SNR is required

10


Channel Filter and ADC (cont’d)

For DECT max signal = max blocker so LP Filter playsdifferent role → mainly band limitation (Nyquist criteria)

If fS high then the requirements for the filter relaxed ( minimum is 1.8 MHz )

We have: DRADC = Pin max – (Pin min– 15dB ) = -33 –(-83-15) = 65 dB

DRADC = 6.02N + 1.76dB → N = 11 bits and if this is too large we can use VGA

VGA ADCLPF

fS

ctrl

BBproc.

ADC quantization noise,

Pq = (∆2 /12)/(R0 p) ×1000 mW p – oversampling factor = 2fS /BW for zero-IF

= VFS2 2-2NBW / (24R0 fS) ×1000 mW, ∆ - ADC resolution = VFS 2-N


ADC and Front-end gain

11bits usingwhen 6.266.9202.62) ing(oversampl 1 and ,1 Take

8.0log10log2002.6 and,4.93

1017.21010510

101010510

105 valuepracticalfor so

25

10101010

91.113.81.113.8

1.113.8min

min

min

==+−=

===

−−+−=+=

⋅=⋅⋅−

=⋅⋅−

=⇒=

=<

⋅−=

−=

+=

−−−−

−−

NdBdBNGBfVV

dBBfVNPdBPG

PPP

GNF

NFNF

NFP

NSNRNFPPSNR

G

NGNFPPGSNR

dBFE

wSFS

wSFSdBmqdBmqdBFE

qqq

FEFE

RxFE

FE

q

inFEin

qFE

inFEFEq

inFE

This are rather mild design requirements

Sensitivity and input reference noise

Very relaxed

11


Other specs of Rx / Noise floor

wHzdBmindBmBNF log10

/+=

....1

log10

1042901038.1

14

/

2123

2

==

⋅=⋅⋅

⋅==

×

+

=

=

−−

=

mWkTN

HzWK

HzKWkTN

RRRRkTRN

RinRsHzdBmin

RinRsin

inSin

inSHzWin

RS

- +

2RSV

Nin

Rin

dBm

HzdBmFdBm

6.1114.62174

10728.1log10/174 6

−=+−=

⋅+−=

RF Filter

RF Amplifier

RS = RinNoise from antenna like resistor noise. If pointed at horizon Teq=290K

For DECT Bw=1728 kHz

Matching for power


Other specs of Rx / Spurious Free Dynamic Range (SFDR)

minminmax

3max

minmin

3)3(2

when)y sensitivitRx (

SNRFIIPPPSFDR

GFPPSNRFP

inin

IMin

in

−−

=−=

+=

+=

∆P

Output

IIP3Pin,max

Pout

F+G

F

fundamental

IM3

Pin

PIM3= F+G

Pin,min

SNRmin

PIM3

Max Pin is so that PIM3 < noise floor

SFDR = 0.67(-22.4+111.6)-(10+18)= 31.8 dB required for our DECT

SFDR is a combined performance measure of Rx for noise and linearity

12


Transmitter requirements

• Max power and emission mask- noise- spur tones

• Modulation quality- SNR - constellations and EVM/ BER


Phase noise

PAMatching

& BP FilterBP

Filter

FrequencySynthesizer

f0

RF×

Modulated signal

• Close-in phase noise reduces SNR

• Far-away phase noise creates spectral emission outside the channel

• Harmonics must be avoided

• PA provides spectral regrowth due to its nonlinearities

f0

Close-in PN

Far-away PN

fRF

Transmitted signal

IF

13


Emission mask and PN• Maximum output power 250mW (24 dBm)• Maximum radiated power

M0

foff

M1

M2

M3

Channel spacing

L (foff ) = 10log Soff - P0 = Moff -10log B - M0

L (1.73MHz) = -8 - 62 - 24 = -94dBc/HzL (3.46MHz) = -30 - 62 - 24 = -116dBc/HzL (5.18MHz) = -44 - 62 - 24 = -130dBc/Hz

More stringent than for the receiver

Estimate of LO phase noise:


Distribution of specs over Rx blocks

RFFilter

LNA

LO

I Q

LPF&VGA ADC

Duplexer

Transmitterpart

For each Rx block: Gi , NFi , IP3i For LO: phase noise & spur tones

How do we distribute Rx specs over the blocks ?

14


Distribution of specs (cont’d)

...

...333

13

1

...11

321

3

21

2

1

1

21

3

1

21

×××=

+++≈

+−

+−

+=

GGGGIP

GGIPG

IPIP

GGNF

GNFNFNF

Only approximate formula for IP3

If the blocks are not impedance matched, corrections needed

Observe that distribution of gain to maintain low NF and high IP3 (or IP2) is contradictory.

Largest signal or a blocker must not saturate LNA, mixer or IF filter.Often LNA requires gain control as well to avoid saturation of the following stages. Using VGA before ADC is typical.


Summary

• First order specs can be retrieved from system requirements

• Simulation models needed for verification• In case of integrated TRx’s the blocking and

intermodulation requirements are more stringent than for Rx /leakage, substrate coupling, and radiation of Tx power/ Digital part even more noisy

• Distribution of specs depended on the available RF blocks and architecture, many constraints, little degree of freedom usually

RF System Analysis

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