GNSS De-sense By IMT and PCS DA Output

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.
2

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
3

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
4

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/Hz
0.2 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
DA
10

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.
13

By Criterion
Notch Filter
 The necessary attenuation can be obtained using not
only a SAW filter, but also a discrete notch filter[1].
Transceiver
DC Block
14

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
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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
16

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.
17

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.
18

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.
19

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
Inductance
21

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
22

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.
23

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.
24

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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By Criterion
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.
27

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.
28

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
29

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.
32

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
33

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
34

GNSS De-sense By IMT and PCS DA Output

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