GNSS De-sense By IMT and PCS DA Output

1

Tx in-band noise effect on GNSS sensitivity

The driver amplifier (DA) output, in order to achieve the

best GNSS sensitivity, specifies a maximum noise over

1565 – 1607 MHz (GNSS frequency band) to be below

-153 dBm/Hz at the PA input[1].

Because the PCS band is closer to GNSS band, the

“skirt” of PCS band should be more sharp than IMT band to meet the specification.

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By Criterion

Transceiver

eLNA

-171 dBm/Hz


Assuming Noise Figure = 3 dB, so the GNSS path

effective noise power is:

-174 (Thermal Noise) + 3 = -171 dBm/Hz

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By Criterion


GNSS receivers specify a maximum GNSS band noise

power at the GNSS antenna of -184 dBm/Hz[1].

Transceiver

eLNA

-184 dBm/Hz

GNSS IMTPCS

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By Criterion


With GNSS band noise power, the total noise floor will

increase. The calculation is as below[1] :

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By Criterion


Therefore, with GNSS band noise power, the total noise

floor will increase 0.2 dB. At this level, the GNSS

receiver’s sensitivity is degraded by an acceptable 0.2 dB[2].

Transceiver

eLNA

GNSS IMTPCS-170.8 dBm/Hz

-171 dBm/Hz0.2 dB

-184 dBm/Hz

6

By Criterion


Besides, we also know that the GNSS band noise power

degrades sensitivity indeed. The higher the GNSS band

noise power is, the more degradation of sensitivity will

be. As shown below[1]:

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By Criterion

Tx architecture analysis

The Tx architecture is as below[1] :

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By Criterion

Tx architecture analysis

As mentioned above, GNSS band noise power at the

GNSS antenna should NOT be larger than -184 dBm/Hz.

Thus, in terms of Tx architecture, the specification can

be written as:

Thus, the required attenuation for Tx architecture is as

below[1]:

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By Criterion

DA Output Matching

For DA output, because the impedance between DA and

PA is the load-pull of DA. Thus, make the impedance be

closer to 50 Ohm to reduce the TX noise in GNSS band.

Transceiver

DA10

By Criterion

SAW Filter

For SAW filter, the Q-factor should be as large as

possible to posses higher noise rejection and lower

insertion loss.

Besides, the proper layout techniques are necessary as

well. Take the TDK SAW filter for example, its foot print

is as below[2] :

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By Criterion

SAW Filter

The proper layout is as below :

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By Criterion

SAW Filter

Keeping input circuits as far as possible from output

circuits[3].

All the GND Pins and GND plane of top layer should be

grounded together to enhance the isolation between

input and output.

Both input and output, the GND pin of shunt

component should be isolated from the GND plane.

Otherwise, the noise may leakage from input to output

through common GND. Noise

The impedance of input / output should be 50 Ohm to

avoid degrading the performances such as insertion

loss and rejection.

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By Criterion

Notch Filter

The necessary attenuation can be obtained using not

only a SAW filter, but also a discrete notch filter[1].

TransceiverDC Block

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By Criterion

Series Type Notch Filter

For series type notch filter, the larger the C value is, the

higher the insertion loss in IMT/PCS band will be.

GPS / GNSS

Band

IMT / PCS

Band C (pF) L (nH) Loss (dB)

Blue 0.3 34 0.1 ~ 0.2

Pink 1 10 1.2 ~ 2.3

Green 2 5.1 3.3 ~ 5.3 15

By Criterion


Because the C value depends on the inner layers. The

larger the C value is, the more the inner layers will be.

Every layer has inner resistance, and the total

resistance = R1 // R2 // R3 //……Rn. Thus, the more the inner layers are, the smaller the total resistance will be.

In other words, the larger the C value is, the lower the

ESR will be, thereby making insertion loss large due to

that more signal flows to GND.

R1

R2

R3

R4

C1

C2

C3

C4

RTotal = R1//R2//R3//R4……

Ctotal = C1//C2//C3//C4……

RTotal = R1//R2/R3//R4

CTotal = C1//C2/C3//C4

Low ESR

Signal

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By Criterion


Thus, the C value should be small while designing

series type notch filter.

Nevertheless, with the identical tolerance, smaller C

value leads to larger variation. For example, with ± 0.1

pF tolerance, the variation is 5% for 2 pF capacitor, but

the variation is 33% for 0.3 pF capacitor.

The larger the C value variation percentage is, the

larger the notch frequency variation will be. Let’s

illustrate the concept by following simulation.

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By Criterion


With 34nH L value, we modify the C value

(0.3 pF± 0.1 pF ), and the frequency response is as

below.

GPS / GNSS

Band

IMT / PCS

Band

C

(pF)

L

(nH)

Notch

Frequency

(MHz)

Blue 0.3 34 1565

Pink 0.2 34 1930

Green 0.4 34 1365

The largest notch frequency variation is 365 MHz (1930

MHz – 1565 MHz) while C value changes from 0.3 pF to

0.2 pF.

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By Criterion


With 5.1nH L value, we modify the C value

(2 pF± 0.1 pF ), and the frequency response is as below :

C

(pF)

L

(nH)

Notch

Frequency

(MHz)

Blue 2 5.1 1576

Pink 1.9 5.1 1617

Green 2.1 5.1 1538

GPS / GNSS

Band

IMT / PCS

Band

The largest notch frequency variation is merely 41 MHz

(1617 MHz – 1576 MHz) while C value changes from 2 pF

to 1.9 pF.

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By Criterion


With the identical tolerance(± 0.1 pF), the notch

frequency variation of (0.3 pF± 0.1 pF) is larger than

(2 pF± 0.1 pF ). It proves again that the larger the C

value variation percentage is, the larger the notch

frequency variation will be.

Thus, for series type notch filter, the C value is a

compromise between insertion loss and notch

frequency variation. It is neither the larger the better nor

the smaller the better.

20

By Criterion


Besides, for series type notch filter, the GND pin should

be isolated from other GND. Otherwise, the notch

frequency will drift due to additional parasitic

inductance.

Main GND Main GND

Parasitic

Inductance21

By Criterion

Parallel Type Notch Filter

For parallel type notch filter, the larger the L value is,

the higher the insertion loss in IMT/PCS band will be.

GPS / GNSS

Band

IMT / PCS

Band

C (pF) L (nH) Loss (dB)

Blue 15 0.7 0.1 ~ 0.16

Pink 10 1 0.2 ~ 0.5

Green 5.1 2 0.7 ~ 1.4

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By Criterion


Because the larger the L value is, the more the turns

will be, thereby increasing ESR.

With the identical parallel notch resonant frequency, the

larger the L value is, the smaller the C value will be. As

mentioned above, smaller C value leads to larger ESR.

Thus, both larger L and smaller C contribute to larger

ESR, thereby increasing insertion loss.

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By Criterion


Thus, the L value should be small while designing

parallel type notch filter.

Nevertheless, with the identical tolerance, smaller L

value leads to larger variation. For example, with ± 0.1

nH tolerance, the variation is 5% for 2 nH inductor, but

the variation is 14.3% for 0.7 nH inductor.

The larger the L value variation percentage is, the larger

the notch frequency variation will be. Let’s illustrate the concept by following simulation.

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By Criterion


GPS / GNSS

Band

IMT / PCS

Band

C

(pF)

L

(nH)

Notch

Frequency

(MHz)

Blue 15 0.7 1553

Pink 15 0.6 1678

Green 15 0.8 1453

With 15pF C value, we modify the L value

(0.7 nH ± 0.1 nH ), and the frequency response is as

below.


MHz – 1553 MHz) while L value changes from 0.7 nH to

0.6 nH.

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C

(pF)

L

(nH)

Notch

Frequency

(MHz)

Blue 5.1 2 1576

Pink 5.1 1.9 1617

Green 5.1 2.1 1538

With 5.1 pF C value, we modify the L value

(2 nH ± 0.1 nH ), and the frequency response is as below.


MHz – 1538 MHz) while L value changes from 2 nH to

2.1 nH.

GPS / GNSS

Band

IMT / PCS

Band 26

By Criterion


With the identical tolerance(± 0.1 nH), the notch

frequency variation of (0.7 nH ± 0.1 nH ) is larger than

(2 nH ± 0.1 nH ). It proves again that the larger the L

value variation percentage is, the larger the notch

frequency variation will be.

Thus, for parallel type notch filter, the L value is a

compromise between insertion loss and notch

frequency variation. It is neither the larger the better nor

the smaller the better.

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By Criterion

Notch Filter Placement

We combine the series type notch filter with parallel

type one, and the noise rejection and insertion loss are

acceptable.

GPS / GNSS

Band

IMT / PCS

Band

0.3 pF

34 nH

0.7 nH

15 pF

From Smith Chart, it illustrates the impedance shifts

from 50 Ohm a bit.

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By Criterion


As mentioned above, the impedance between DA and

PA should be closer to 50 Ohm to reduce the TX noise

in GNSS band.

Thus, we need to place matching networks in front of

DC block and notch filter.

By doing this, we can regard (DC Block + Notch Filter)

as ZL and make ZS = ZL by means of matching networks.

Matching

Network

ZS

ZL

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By Criterion


As mentioned above, there is already a Frond-End

component including a duplexer posterior to PA.

If the rejection of duplexer is at least 45 dB, and the

ANT-to-ANT isolation is at least 10 dB, it is NOT

necessary to put notch filter or SAW filter posterior to

PA to suppress noise further.

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By Criterion


In addition, the insertion loss of the notch or SAW will

increase PA Post-Loss by placing notch filter or SAW

filter posterior to PA.

The larger the post-loss is, the larger PA output will be,

thereby aggravating GNSS band noise from PA due to

nonlinear effect.

Post-Loss

PA output

In general, the insertion loss of notch ought to be kept

below 1.5 dB, and which of SAW filter ought to be kept

below 3 dB.

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By Criterion

DC Block Consideration

According to the capacitive reactance formula,

as long as a series capacitor can block DC regardless

of its C value.

As mentioned above, larger C value results in lower

ESR, thereby reducing insertion loss. So the C value

should be large to posses lower loss.

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By Criterion

ACLR / ACPR

For GNSS band noise, we ought to care not only DA

output, but also PA output.

Thus, the ACLR / ACPR should meet specification[1,4].

GNSS IMTPCS

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Reference

[1] GNSS Desense by IMT/PCS DA Output, Qualcomm

[2] SAW TX Filter PCS / WCDMA Band II, TDK

[3] SAW Filter PCB Layout

[4] How to solve ACLR issue, Slideshare

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Date post:	19-Jan-2017
Category:	Engineering
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GNSS De-sense By IMT and PCS DA Output

Engineering