Mainly focus on system level (Board Level) Component selection guide System circuit design Layout and placement considerations

1. GPS RF FRONT-END
CONSIDERATIONS
Component selection guide
System circuit design
Layout and placement considerations
Mainly focus on
system level
(Board Level)

2. Author : Criterion
Table of Contents
GPS Signal……………………………………………………1
SNR V.S. C/N0………………………………………………..4
Noise Figure…………………………………………………..6
Pre-SAW………………………………………………………9
Linearity………………………………………………………21
LNA…………………………………………………………...26
Mixer………………………………………………………….37
VCO…………………………………………………………..42
Layout and placement consideration……………………...52

3. Author : Criterion
GPS Signal
GPS signal is Direct-Sequence Spread Spectrum featuring these advantages that
allows many transmitters to share the same frequency band, and hard to jam[94].
Furthermore, GPS adopts Code Division Multiple Access (CDMA), and the
frequency spectrum of the signal is spread with a noise like code (sequence).
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4. Author : Criterion
Spreading codes have very low cross-correlation and are unique for every user
(low interference with other signals). Transmission bandwidth is much wider
than which of information. As shown below[94] :
The L1 signal is a BPSK signal modulated in phase by the C/A-code and the
information of the navigation message. The L1 frequency = 1575.42 MHz
= 154 x 10.23 MHz. And the chipping rate of C/A code is 1.023 Mcps[49].
2

5. Author : Criterion
Most commercial front-ends have a high gain exceeding 80 dB. The high gain is
achieved at a cost of either a high noise level or high power consumption[49]. For
example, the RTR6285A of Qualcomm has approximately 82 dB gain. Because
GPS has 43 dB processing gain, the GPS baseband signal has 123 dB (80 + 43 =
123) gain[49].
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6. Author : Criterion
SNR V.S. C/N0
SNR is usually expressed in terms of decibels(dB). It refers to the ratio of the
signal power and noise power in a given bandwidth[57].
C/N0, in contrast, is usually expressed in decibel-Hertz (dB-Hz) and refers to the
ratio of the carrier power and the noise power per unit bandwidth. Thus, we can
express C/N0 as follows:
In real GPS application, the C/N0 is usually 37 ~ 45 dB-Hz. During design stages,
the conducted C/N0 should be larger than 40 dB-Hz with -130 dBm GPS signal.
4

7. Author : Criterion
We assume that GPS receiver front-end bandwidth is 4 MHz, and the SNR should
be -29 ~ -21 dB. This is logical for the receiver front-end noise floor is about -110
dBm, and it’s impossible for real GPS signal to be larger than -110 dBm. Namely,
the GPS SNR must be negative until it enters the receiver’s base band processing
stages.
As shown above, we can see that before integration, the SNR is really negative.
But the SNR increases as the integration time increases. The SNR gain in this case
is also referred to as processing gain due to spread spectrum feature.
The SNR is most useful when considered within the baseband processing blocks
of a GPS receiver. In dealing with SNR, the bandwidth of interest needs to be
specified. A receiver’s front-end bandwidth determines the SNR seen by the input
side of the various baseband processing stages of the receiver. As we have seen,
the SNR in a GPS receiver is dependent on the receiver’s front-end bandwidth.
Referencing just the SNR value in a GPS receiver does not usually make sense
unless one also specifies the bandwidth and processing stage within the
receiver[57].
5

8. Author : Criterion
Noise Figure
GPS signals are transmitted by medium power satellites, with approximately
40dBm. When they reach the Earth, they are normally received by antennas with
a minimum power of approximately –131dBm. That is why it’s impossible for
real GPS signal to be larger than -110 dBm as mentioned earlier. The C/N0 is
related to the maximum bit error rate (BER) required for a GPS receiver at the
output, which is 10-5[49]. No doubt, with higher C/N0, comes better positioning
accuracy and TTFF[3].
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9. Author : Criterion
Also, as shown below, the higher C/N0 is, the higher detection probability and
the better sensitivity will be.
With lower NF(Noise Figure), comes higher C/N0. As shown below[58] :
That’s why this process will require a lower NF. Each dB decrease in noise figure
helps improve the sensitivity by a dB[33].In general, the sensitivity can be
-158 dBm ~ -160 dBm with 2.5 dB NF of overall receiver[4].
7

10. Author : Criterion
Another reason for requiring a lower NF lies in the increasing use of GPS
receivers in urban environments. The received signal power is reduced in places
with high buildings and narrow streets. In this case, the receiver is unable to
detect the satellite signal or provides an imprecise position. Moreover, another
factor to consider in urban environments is the presence of interference signals
that the receiver can capture, increasing CNR degradation, and all the
performance, such as TTFF, positioning accuracy, and sensitivity, will aggravate
as well[49].
8

11. Author : Criterion
Pre-SAW
As shown below[31],
These outband blockers may leak into the GPS receiver’s path and have a gigantic
impact on the receiver’s sensitivity by overloading the receiver’s LNA or
backend[31]. The stronger outband blocker is, the more degradation of SNR will
be[41].
9

12. Author : Criterion
As shown below, if these blockers are close to LNA’s P1dB, the gain will decline
due to saturation:
As shown below, when the blocker is larger than -10 dBm, the gain will drop
dramatically, whereas the NF will increase significantly[39].
10

13. Author : Criterion
Moreover, if these blockers are larger than LNA maximum input level, the gain
will be diminished to zero, and this circumstance makes LNA has large insertion
loss merely. As a result, instead of being amplified by LNA, the GPS signal will
be submerged in LNA’s noise floor[31].
This creates a big challenge to the handset designers. The designers need to
maintain the sensitivity of the GPS receiver for the weak incoming GPS signal
while there are strong blockers from the transmitting voice or data. This requires
a GPS receiver front end with very good blocking to these strong blockers[31].
In other words, a LNA with extremely excellent linearity is able to avoid being
overloaded by these strong blockers.
11

14. Author : Criterion
In terms of dynamic range, the low limit is sensitivity. In order to acquire the
extremely weak GPS signal, the sensitivity should be low enough. But, dynamic
range is finite, this feature indicates that the LNA’s P1dB will not be large. That is
to say, it’s impossible for GPS LNA to possess extremely excellent linearity.
Typically, the DCS1800 (1710MHz) PA output is taken as the primary concern of
the outband blockers, which is certainly much higher than GPS signal, directly
saturating the LNA without external SAW filters, then the SNR degrades[41,44].
12

15. Author : Criterion
As shown below, ALM-1912 has pre-SAW, but ALM_1612 doesn’t. If we fix on the
blocker strength(-10 dBm), it can be seen the NF of ALM_1612 increases much
more than which of ALM-1912. That’s why adding pre-SAW prior to LNA
for rejection of the jammer is necessary to achieve acceptable sensitivity with
strong outband blocker[55].
13

16. Author : Criterion
Furthermore, we need to put DC-block at LNA input to avoid making DC Offset
feed into LNA.
Nevertheless, the DC block at input is optional as it is usually provided by the
pre-filter before the LNA in many GPS applications[75].
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17. Author : Criterion
According to Friis formula :
The important variables contributing to system sensitivity are the pre-SAW’s
IL(insertion loss) and LNA’s NF, while LNA subsequent blocks have minimal
impact[28] :
15

18. Author : Criterion
Let’s illustrate the idea further[55].
As shown above, there are three types of GPS modules :
Type1 : pre-SAW + LNA + post-SAW
Type2 : LNA + post-SAW
Type3 : pre-SAW + LNA
Compared to type1 and type3 , type2 has lowest NF for it has no pre-SAW.
Besides, compared to type3, type1 has post-SAW. But NF of type1 is higher than
type3 merely 0.04 dB. It proves again that LNA subsequent blocks such as
post-SAW have minimal impact[28] :
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19. Author : Criterion
Thus, the pre-SAW must have low IL to posses lower NF. The post-SAW should be
selected to emphasize its outband rejection over IL, since its primary function is
to block any blocker amplified by LNA[28]. As shown below :
By the way, type2 has no pre-SAW but its linearity is not worse than type1 and
type3. That’s to say, instead of improving LNA’s linearity, pre-SAW can simply
help relax LNA’s linearity requirement.
17

20. Author : Criterion
As shown above, the input impedance of pre-SAW, including matching1 and
matching2, must be 50Ω to avoid degrading SAW filter performance due to
mismatched impedance. Although matching1 and matching2 belong to LNA
source-pull as well, matching3 influences LNA more than matching1 and
matching2 due to the fact it is closer to LNA. Thus, in terms of source-pull, we just
need to tune matching3 to lowest NF location on Smith Chart. As for placement,
please place pre-SAW as close to the LNA as possible, leaving only enough room
to place the matching components between them[54]. Otherwise, the outband
blocker may still feed into LNA.
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21. Author : Criterion
As shown above, the Pin2/3/5 are GND pins[101]. For a good outband noise
rejection, a low crosstalk is necessary. Low crosstalk can be realized with a good
RF layout. The major crosstalk mechanism is caused by the “ground-loop”
problem. Grounding loops are created if input-and output transducer GND are
connected on the top-side of the PCB and fed to the system grounding plane by a
common via hole. To avoid the common ground path, the ground pin of the input
and output transducer should be isolated from the top-side grounding plane.
Otherwise, the outband noise rejection degrades. In this PCB layout, the
grounding loops are minimized to realize good ultimate rejection[100]. As shown
below :
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22. Author : Criterion
Besides, the input and output grounding pins are isolated and connected to the
common ground by enough separated via holes. Plentiful GND via holes will
provide a more effective ground and better outband noise rejection for the
pre-SAW[55].
Besides 50Ω impedance, the variation in pre-SAW response is also dominated by
temperature drift, resulting in unacceptable interference and high IL[102].
As a result, numerous GND via holes will also help mitigate the thermal effects.
20

23. Author : Criterion
Linearity
As mentioned earlier, linearity requirements are imposed by receiver behavior to
external interferences. The linearity specifications are dictated by the required
system’s ability to perform in the presence of external interfering signals[49].
However, the GPS receiver does not have adjacent or alternate channel
interferers and for that reason only out-of-band desensitization performance is
of interest[46]. Thus, one of the most important characteristics of a GPS receiver
in a mobile terminal is how good the off-band linearity is[46].
But, as shown above, signals from close bands, or even inter-modulation(IMD)
products of other signals from other bands due to LNA’s nonlinearity, could
create a signal in the same band as the GPS signal. And these in-band interferers
can’t be filtered by any SAW filter posterior to LNA. For example, the IMD3
(1570MHz) consisting of DCS1800 and PCS1900 falls into GPS band, becoming an
in-band jammer and cannot be filtered by post-SAW[39, 42].
21

24. Author : Criterion
Although these in-band interferers will not saturate LNA subsequent blocks
necessarily. Nevertheless, in fact, jammers merely stronger than -110 dBm can
cause problems in GPS performance such as TTFF and accuracy, even if there is
processing gain for GPS baseband signal[3]. Because receiver can pick false
jammer peak instead of real GPS signal peak in case of multitone jammers, or
consume more time to find real GPS signal[3]. As shown below :
As a result, mixer is the biggest linearity decider in receiver chain, but if LNA’s
linearity is not good enough, those IMD products as a result of LNA’s nonlinearity
can’t be filtered and will aggravate sensitivity as well, even though the mixer has
excellent linearity. Namely, LNA’s linearity is equally as important as mixer’s.
22

25. Author : Criterion
However, as mentioned earlier, the LNA’s linearity is not good enough due to
finite dynamic range, but pre-SAW can help relax LNA linearity requirement.
Thus, prior to LNA , we need to eliminate the outband blockers as much as
possible in advance to reduce the IMD products due to LNA’s nonlinearity for
weaker blockers result in weaker IMD products. It proves again that pre-SAW is
necessary.
According to Friis formula, for achieving the low noise requirements, quite large
gains are used, especially in the LNA, to detect lower power signals ,thereby
improving the sensitivity of the receiver[49]. But large gain can deteriorate the
receiver pass-band linearity as well. As shown below, with larger gain, comes
lower NF at first. But NF remains nearly constant when gain is larger than 12 dB.
In contrast, the IIP3 declines continually as gain increases.
Consequently, a compromise for the gain and linearity performance is
needed[46]. Larger gain is no guarantee of lower NF, and yet it is guaranteed that
worse linearity.
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26. Author : Criterion
Since those in-band interferers are almost IMD products, indicating the
worst-case scenarios for the linearity requirements with respect to IIP3 and
IIP2[46]. Those interferences will have a significant influence on the
performance of the signal processing, as they will not be filtered in the RF
front-end. Thus, the linearity of the GPS receiver is redefined as the limit of the
highest interference power that the receiver can handle before it begins to
perform incorrectly. That’s to say, the higher linearity is, the better immunity to
interferences will be. With better immunity to interferences, comes better
sensitivity.
There are several ways to meet the linearity requirement in the receiver. One way
would be by careful system design and partitioning the block gains appropriately
[46]. An additional LNA, which deteriorates the receiver linearity with the added
gain, would then be still needed to comply with the noise figure requirement.
It indicates again that the LNA gain is related to overall receiver linearity and
sensitivity. The LNA gain should not be too high to meet the linearity
requirement[46]. As mentioned earlier, with poor linearity, comes poor
sensitivity as well.
The other way would be to intensify the linearity of the mixer, which can be
passive type. As mentioned earlier, mixer is the biggest linearity decider in
receiver chain. Passive mixers do not consume static power, then they
introduce no Flicker noise (or 1/f noise) and DC Offset. They also show a good
nonlinearity behavior.
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27. Author : Criterion
In contrary, active mixers generate conversion gain (typically 10 dB) and show a
reasonable nonlinearity behavior but they introduce Flicker noise because of the
biasing current of the CMOS devices, thereby aggravating sensitivity[40].
As mentioned above, a high-gain LNA will help reduce NF by minimizing mixer
contribution, and yet at the expense of higher power consumption in this block
and worse linearity in the whole receiver. Take Qualcomm RTR6285A for
example, the receiver has high linearity mode and low power mode. As a
consequence, if we want to intensify the immunity to interferences further, we
are capable of choosing high linearity mode at the expense of higher power
consumption in this block[39].
As mentioned earlier, the outband rejection of pre-SAW is important as well. In
general, FBAR can have high outband rejection and low IL simultaneously, and
yet at the expense of cost[28].
25

28. Author : Criterion
LNA
As mentioned earlier, in urban environments, the received signal power is
reduced in places with high buildings and narrow streets. As mentioned earlier,
GPS signal is normally received by antennas with a minimum power of
approximately –131dBm[55]. But, in this case, the received GPS signal by
antennas may become -150 dBm[56], and the receiver is unable to detect the
satellite signal or provides an imprecise position[49,53]. Thus, we need external
LNA to improve sensitivity, TTFF, and position accuracy[33].
Even though the RF amplifier input signal is single-ended, a differential structure
has been selected based on its advantages that have better immunity to second
order distortion(IMD and harmonics) and common mode noise[41, 49]. The
advantage of this architecture is the high-linearity for blocker rejection from
other interferences[41]. Furthermore, CDMA operation requires high selectivity
to reject Tx leakage, thereby suppressing cross-modulation. The differential
configuration LNA is beneficial [17].
26

29. Author : Criterion
As shown below, the balanced signal is generated off-chip by a balun[46]. The
LNA output drives double-balanced passive I/Q mixers. The mixers are driven by
LO signal with 25% duty cycle[41].
But, a differential structure has disadvantages as well. The major disadvantage is
that balun adds extra loss, thereby deteriorating NF. Balanced type SAW filter has
larger IL than unbalanced type as a consequence of integrated balun[13].
The pros and cons of differential LNA are summarized as below[49] :
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30. Author : Criterion
Moreover, to tune the differential matching is more complex than single matching.
The detailed differential matching tuning procedure is described in[17]. By the
way, the L2 and L3 shown above should be placed in ways limiting mutual
coupling. Do not locate L2 and L3 too close to shield walls (this might cause EM
coupling and inductor de-Q)[51].
Since the major interference and blocker signals are out-of-band signals, some
additional filtering can be achieved by high-Q inductors in the matching[46]. Due
to the features that high-Q inductors have low loss and narrow bandwidth, these
advantages can help lower the NF and improve outband noise rejection further.
28

31. Author : Criterion
In general, the wire-wound type inductor has higher Q value than multi-layer
type.
As for the LNA input matching, as mentioned earlier, the pre-SAW input
impedance must be 50Ω to avoid degrading SAW filter performance due to
mismatched impedance. But, the pre-SAW output matching should be designed
to achieve the minimum NF(not necessarily 50 Ohm).
As shown above, matching1 affects LNA input impedance as well. Nevertheless,
compared to matching1, matching2 affects more. Thus, matching1 must be 50Ω,
to achieve the best pre-SAW performance, minimum NF is just realized by
matching2(non-50Ω).
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32. Author : Criterion
Certainly, matching2 is pre-SAW output matching as well, a change of the output
impedance(non-50Ω) of the pre-SAW will also influence the frequency response.
As a consequence, you ought to do the real sensitivity measurement to decide the
matching2, minimum NF(non-50Ω) or 50Ω.
The LNA land pattern is shown as above[74], which should be grounded through
isolated area[55]. Especially, pre-SAW and LNA grounds are separated to avoid
any unwanted parasitic effect from deteriorating LNA RF performance. Besides,
put GND vias as many as possible to spread the heat to prevent from degrading
LNA performance due to high temperature[54]. As shown below :
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33. Author : Criterion
Let’s take AVAGO ALM-1912 for example, the schematic is shown above, L2 is a
RF choke isolating the GPS signal from the DC supply[55]. Moreover, the L2
biasing inductor together with the C2 bypass capacitor sets the output matching.
As a consequence, L2 has influence on LNA’s performance as well.
31

34. Author : Criterion
Input and output return loss :
Gain :
NF :
32

35. Author : Criterion
Linearity :
As summarized below :
As a result, in addition to matching, you can also adjust the L2 value to achieve
the best performance.
33

36. Author : Criterion
In gereral, with higher Vdd, comes better linearity. As shown below[55] :
If we fix on the Vdd(2.2 V), it can be seen that linearity varies for different Idd.
Thus, we are able to adjust R2 value for desired Idd current to achieve best
linearity[55].
In addition to linearity, other performance can also be obtained by increasing the
supply voltage[74].
Consequently, we should prevent supply voltage from IR drop to prevent from
deteriorating performance.
34

37. Author : Criterion
As shown in the noise circle below :
The high gain zone is high NF zone as well on Smith Chart. Namely, in terms of
source-pull, high gain also leads to high NF. There is always a trade-off between
gain and NF[49]. As mentioned earlier, choosing the gain level within the various
blocks of the receiver is always a trade-off. A high-gain LNA will help reduce NF
by minimizing mixer contribution. That is to say, even if we use a passive mixer,
which doesn't influence the cascade NF. But a high-gain LNA increases power
consumption and noise in this block[49]. A low-gain LNA may improve linearity
and power consumption, but would require a low-noise mixer. Such a mixer
would consume a lot of power. If you use a passive mixer with a low-gain LNA,
the receiver overall NF will be high. In other words, a low-gain LNA combined
with a low-noise mixer may not offer a significant advantage in total power
consumption over a high-gain LNA combined with a mixer with a higher NF.
Because mixer plays the major role in power consumption. Therefore, a receiver
configuration with accurate gain has been chosen.
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38. Author : Criterion
As mentioned earlier, poor linearity results in poor sensitivity, thereby increasing
the TTFF. As a result, we ought to select a LNA with relatively good linearity. With
the same level blocker, the better LNA’s linearity is, the shorter TTFF will be[25].
In general, the LNA P1dB should be at least -5 dBm, and IIP3 should be at least 5
dBm.
.
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39. Author : Criterion
Mixer
An active mixer has been selected because it presents a higher gain and
lower noise than the passive ones[49]. But, for the direct-conversion
receiver, the typical challenges are DC offsets, mixer second-order nonlinear
effects, and flicker noise. Because these interference is near the desired GPS
signal down-converted directly to baseband[39, 46].
Flicker noise is even larger than the down-converted Rx signal[69]. Active mixers
suffer from high 1/f noise and poor linearity, especially when the supply voltage
is low. In contrast, a current driven passive mixer can provide relatively good
linearity and inherent low 1/f noise performance due to the absence of DC
current[38, 69]. Although a passive mixer has larger NF than active ones, the NF
of LNA subsequent blocks contribute to sensitivity slightly. Thus, in general,
passive CMOS mixers are considered as the appropriate choice for
direct-conversion receivers for they do not contribute to 1/f noise[69].
37

40. Author : Criterion
Taking previous pre-SAW considerations into account, the input impedance of
the mixer has been set to optimize the power consumption, gain, NF, and
linearity of the entire RF mixer[49].
Nevertheless, as shown above, the modern GPS receiver almost has no external
matching components between LNA and mixer.
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41. Author : Criterion
But, some receivers may add external SAW filter between LNA and mixer. It
indicates that there will be external matching components at mixer input. As
mentioned earlier, the input impedance of the external SAW filter must be 50Ω to
avoid degrading SAW filter performance due to mismatched impedance. Similarly,
the input impedance of mixer must be also 50Ω to prevent from deteriorating
mixer performance(power consumption, gain, NF, and linearity) due to
mismatched impedance.
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42. Author : Criterion
In order to get low conversion loss from mixers, typically a high LO power is
needed[49, 69].
As shown above, the higher LO power is, the higher gain(the lower conversion
loss) of mixer will be. Especially for a passive mixer, which acts like switches
controlled by the signal from LO[38].The higher LO power is essential to achieve
the lower conversion loss, thereby being able to drive the ADC[40].
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43. Author : Criterion
But strong LO power may result in significant LO leakage due to the finite mixer
port to port isolation. LO leakage causes self-mixing, thereby generating a static
DC level aggravating sensitivity[78]. As shown below :
Therefore, as a result of high LO power value, the isolation between LO and mixer
must be high enough to avoid LO leakage[49]. Besides, in order to avoid
degrading the performance of the RF mixer, the LO power level must guarantee
the appropriate switching performance of the mixer core. The LO power level
should be between–10dBm and –3dBm.
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44. Author : Criterion
VCO
Voltage controlled oscillator, as the name implies, the output frequency is
dependent on control voltage[49] :
As a consequence, VCOs are very sensitive to noise on the supply sources[45]. To
achieve accurate frequency, the precise tuning voltage is essential[23].
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45. Author : Criterion
As shown above, the ideal spectrum of oscillator should has no “skirt”.
Nevertheless, in real world, it’s impossible. If the waveform of oscillator signal in
time domain has timing error, i.e., phase error or jitter, there will be phase noise
resulting in larger “skirt” , thereby aggravating sensitivity in spectrum. Besides
phase error, phase noise is generated by power supply input as well[45].
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46. Author : Criterion
So, we should care these power traces related to VCO, PLL, LO, and synthesizer
very much.
In general, there are three problems in power supply : high frequency noise,
ripple, and IR drop. As the following formula [48] :
IDD is total power supply current. According to ohm's law : I=V/R.
If the IR drop is too much, e.g. power layout trace is too long or too narrow, the
current declines as the resistance increases, then the phase noise raises. As
shown above, the maximum IR drop of Qualcomm WTR1605L is 20mV. Because
the IR drop for these power traces related to VCO, PLL, LO, and synthesizer must
be less than 20 mV, we should at least make their trace width meet the rule :
1A = 40mil.
44

47. Author : Criterion
As shown below, if the waveform is single sinusoidal source, there is no spurs in
spectrum[48].
The real measurement in spectrum[49] :
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48. Author : Criterion
Nevertheless, single sinusoidal waveform combined with a square wave will
generate a lot of spurs, as shown below[48] :
As a result, we should keep these traces with high frequency noise away from
VCO power supply.
46

49. Author : Criterion
Let’s take Qualcomm RTR6285A for example, the ADC integrated in PMIC
converts the 19.2 MHz single sinusoidal waveform into the digital reference
frequency. That’s to say, the XO_OUT trace, which is rich in harmonics, should be
kept away from VCO power supply and well isolated to prevent from aggravating
VCO phase noise. And the XO_OUT trace should also be kept away from GPS signal
due to its 82 order harmonics (19.2 MHz x 82 = 1574.4 MHz)[51, 56].
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50. Author : Criterion
By the way, in some platforms, such as Qualcomm WTR2965, the ADC is
integrated in transceiver. A series DC block capacitor is required[59].
Besides, since power supply ripple can be a major contributor to VCO spurs[43],
decoupling capacitors are used on the VCO supply to improve the ripple. As
shown below, with larger decoupling capacitor, comes lower phase noise[98].
48

51. Author : Criterion
Consequently, take Qualcomm WTR1605L for example, we must add decoupling
capacitors on power traces related to VCO, PLL, LO, and synthesizer to reduce XO
or LO spurs[97].
Now that we already know that the poor power supply results in large phase
noise and spurs. As mentioned above, the phase noise and spurs of VCO
deteriorates sensitivity, if we suspect that the poor sensitivity is because of poor
VCO power supply, we can use a set of batteries to provide the external clean
power supply to minimize the possible noise introduced by the original power
supply system[49].
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52. Author : Criterion
As shown below, with the external clean power supply, the phase noise and spurs
improve much[99]. It indicates that there are some problems at the original
power supply system input.
In general, the VCO phase noise for GPS application should be less than
-92 dBc/Hz @ 100-KHz[44].
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53. Author : Criterion
As mentioned earlier, the VCO output frequency is dependent on control voltage.
As shown above, Kvco is the slope. We should make Kvco low to reduce VCO
modulation sensitivity to keep the oscillating frequency as stable as possible. In
general, the Kvco for GPS application should be less than 50 MHz/V[44].
Besides phase noise, the VCO should also has low frequency drift. As shown
below, the XO is the source of VCO.
As a result, you should select the XO with low frequency drift. In general, in GPS
application, the frequency drift should be within ±5 ppm (±7.877 kHz for GPS
and ±8.028 kHz for GLONASS)[23]
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54. Author : Criterion
Layout and placement Consideration
As shown above, with the the same impedance, the more distance between signal
trace and reference GND is, the wider signal trace width and the less IL will be.
52

55. Author : Criterion
Furthermore, if the signal trace is too close to adjacent layer, the IL may increase
as a result of parasitic capacitance.
Let’s suppose the parasitic capacitance is a 0201 size, 750 ff capacitor shunting
to GND.
As shown above, there is 0.16 dB additional IL at GPS frequency. Although the
assumption is not correct certainly, it is doubtless that parasitic capacitance
contributes to additional IL.
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56. Author : Criterion
As a result, if dielectric thickness is not sufficient, the next layer may need to be
cleared to mitigate parasitic capacitance to reduce IL[17].
As mentioned earlier, the cascaded NF is primarily dependent on the first LNA’s
NF and gain in the chain as well as any losses incurred prior to the LNA (e.g.,
caused by pre-SAW) for losses incurred posterior to LNA will be attenuated by
the reciprocal of the LNA’s gain. If we are able to reduce the IL of RF trace prior to
the LNA, the sensitivity will improve further[57].
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57. Author : Criterion
As shown above, a large transient current flows from the external supply to the
PA, creating IR drops across trace resistances RT1 and RT4. The good case keeps
the RT4 component out of the PMIC’s supply input, whereas the bad case places
the RT4 trace resistance in the PMIC’s path. Since the PMIC provides receiver
supply voltages, any transients on its input supply voltage may leak onto the
supplies of sensitive circuits such as VCO, PLL, LO, and synthesizer. This will
aggravate the XO or LO spurs and phase noise, thereby degrading the sensitivity.
As shown below :
55

58. Author : Criterion
Try to position the devices and route their supplies so that the PMIC and PA
routing goes in opposite directions from the primary power node, and keep
sensitive circuits far from the PA’s return current path, including the decoupling
capacitors on PA’s power supply. Route the PA supply traces and return paths far
from sensitive circuits that might pick up transient energy from the switching
current[17].
56

59. Author : Criterion
Care needs to be taken in the layout to isolate the noise-generating pins from the
noise-sensitive pins. Take Qualcomm RTR860x for example, these pins are
summarized as below[17] :
Particularly, all PLL supplies and Tx/Rx oscillators are noisy plus noise-
sensitive[17].
57

60. Author : Criterion
Instead of sharing a common ground flood with all RF transceiver ground pins,
the opposite grounding method is to keep multiple subgroupings separate from
each other until they converge on the main PCB ground plane[17].
The method can help isolate these noise-generating ground pins from those
noise-sensitive ground pins.
58

61. Author : Criterion
Since I and Q baseband outputs are sensitive analog signals, route them carefully
to prevent from desense. Technologies using narrower Rx channel bandwidths
will be influenced more. The worst cases are
GSM/LTE 1.4 MHz/CDMA/WCDMA/GPS[50,51].
Avoid routing RX_I/Q near or directly under PMIC SMPS(Switching Mode Power
Supply) PCB areas, for PMIC SMPS switching noise will couple magnetically and
electrically to RX_I/Q traces[50]. Besides, avoid overlapping receiver and PMIC
SMPS areas on double-sided board designs because even using a ground plane to
separate the PMIC SMPS areas from RX_I/Q traces will not block magnetic
coupling[50].
59

62. Author : Criterion
As shown below, the case where the I/Q are routed on a different layer vs. the
PMIC switching node with ground plane separation; magnetic coupling could
still cause desense[50].
60

63. Author : Criterion
Placing the PMIC between the receiver and baseband chips will put RX_I/Q lines
in direct path of noisy SMPS currents. If such a placement is inevitable, at least,
I/Q traces should be routed away from PMIC and SMPS; no overlapping
occurs[50].
Nevertheless, long RX_I/Q traces are more susceptible to noise coupling. If
possible, place receiver close to the baseband chip to shorten whose length[50].
At least, the long RX I/Q traces should be routed in inner layer for stripline
provides higher isolation as a result of the fact it is surrounded by ground
planes[51].
61

64. Author : Criterion
Because of limited areas on double-sided board, the baseband chip, PMIC, and
receiver are located on different layers. The recommended placement is as
shown below :
Baseband chip and receiver are away from PMIC and there are no overlapped
areas. Baseband chip is close to receiver to possess short RX_I/Q lines. But, in
that way, the power traces between PMIC and receiver will be long. Consequently,
it should be routed in inner layer to avoid radiating EMI noise outside. Moreover,
care needs to be taken for IR drop issue.
62

65. Author : Criterion
As for PMIC, key aggressors are listed as below:
l SMPS VSW node: VSW_Sx
l SMPS switching inductors
l SMPS ground: GND_Sx
l SMPS input capacitors
l VREG_S
l Other noisy digital signals, or switching power supply rails
63

66. Author : Criterion
As for SMPS input capacitors, which should be as close to PMIC as possible, or
the transient current from PMIC may couple to other traces, and the transient
current from other chip may leak into PMIC as well, thereby aggravating GPS
performance.
Furthermore, SMPS input capacitors should be grounded through isolated area,
or the transient current from PMIC may leak into other chips through common
GND, and the transient current from other chips may leak into PMIC through
common GND as well, thereby deteriorating GPS performance.
Besides, add a mass of ground vias and connect this isolated ground area to the
PCB main ground plane directly[103].
64

67. Author : Criterion
As for SMPS switching inductors,
With larger inductor value, comes less ripple and EMI noise. But, larger inductor
value results in more turns, thereby increasing DCR(DC resistance) and IR drop
issue.
By the way, with the same inductor value, the larger size is, the less DCR will be.
So, the inductor size should not be too small. And we should select the power
inductor with less DCR.
65

68. Author : Criterion
With less inductor value and less EPC(Equivalent Parallel Capacitance), comes
higher SRF and wider inducible range.
Because the SRF should be at least
(DC-DC Switching Frequency) * 10
For example, switching frequency is 1.2MHz, the SRF should be at least 12 MHz.
That’s the reason why we need wide inducible range.
Thus, the inductor value is neither the larger the better nor the less the better. It
is the more precise the better. Certainly, in terms of inducible range, the EPC
should be as small as possible.
66

69. Author : Criterion
Isat(saturation current) is the current level causing power inductor value to drop
30%. If the current goes above the rated Isat, the ripple aggravates. As shown
below :
That is, the Isat of power inductor is the larger the better.
67

70. Author : Criterion
In general, with the same inductor value, multi-layer type has higher transfer
efficiency than wire-wound type(due to low DCR ). But multi-layer has lower
saturation current than wire-wound type.
DCR (Ohm) Isat (mA)
Wirewound 0.26 0.68
Multilayer 0.14 0.28
The power inductor selection guide is listed as below :
l Low EPC
l Low DCR
l High Isat
l Accurate value
68

71. Author : Criterion
Moreover, the power inductor and decoupling capacitor should both
be as close to PMIC as possible to shrink the switching noise loop area.
Otherwise, the switching noise loop area enlarges.
69

72. Author : Criterion
In that way, the waveform distorts and EMI noise aggravates.
Also, the GPS antenna may pick up radiated EMI noise, then desense issue occurs.
70

73. Author : Criterion
Furthermore, the switching noise may couple to PA power supply input, thereby
mixing with RF signal to produce IMD products near RF signal.
In that way, ACLR deteriorates, thereby increasing the GPS noise floor and
degrading sensitivity.
71

74. Author : Criterion
Besides, as mentioned earlier, SMPS input capacitors should be grounded
through isolated area. But, by doing this, it seems to be difficult to add numerous
GND vias in the ground island due to its limited area. Thus, extend the ground
island to the area underneath the power inductor to enlarge the ground island
area to add GND vias as many as possible. Also, the method is capable of
generating the minimum switching noise loop area.
72

75. Author : Criterion
If the GND vias are not many enough, the impedance of the ground island may be
not low enough as well as the switching noise may couple to other ground area ,
thereby leaking into other chips through ground. In that way, the ground island is
not able to isolate the switching noise thoroughly.
Nevertheless, do not extend the ground island to input capacitors area. Although
this method can also enlarge the ground island area to add plentiful GND vias,
the switching noise loop area enlarges even though SMPS power inductor and
input capacitors are extremely close to PMIC.
73

76. Author : Criterion
In layer2, the area below ground island on top layer should be ground island as
well. Also, there should be no traces on these ground islands.
Besides the inductor value, placement, and layout, its orientation has a large
impact on sensitivity too[50].
As shown above, the power inductor is a wire-wound type with exposed wire
ends on one side. Exposed wires facing PMIC result in radiated noise picked up
by PMIC, thereby aggravating GPS performance[50].
74

77. Author : Criterion
Furthermore, the power trace from SMPS power inductor and input capacitor
may be provided for other chips. Do not use daisy-chain configuration with
shared power traces from capacitor to multiple chips that are BGA package
type[51]. Since the transient current or noise from upriver pins may leak into
downriver pins, as shown below :
Also, the power trace length for downriver pins will be very long, thereby
deteriorating EMI noise and IR drop.
75

78. Author : Criterion
Instead, use star configuration with dedicated traces from capacitor to each chip
pin.
In that way, even though there is transient current or noise from upriver pins,
which will be filtered by capacitor rather than leaking into downriver pins. Thus,
the power trace length for downriver pins will not be too long, thereby mitigating
EMI noise and IR drop.
76

79. Author : Criterion
Nevertheless, branch at capacitor only instead of other places away from
capacitor[51].
Otherwise, the transient current or noise from upriver pins may still leak into
downriver pins[51].
77

80. Author : Criterion
In addition to RF trace, I/Q signal, and power, care needs to be taken for high
speed digital signal as well. Such as SSBI(Single-line Serial Bus Interface),
which is clocked at 19.2 MHz (reference clock frequency) and should be well
isolated, so good layout techniques are extremely important[51].
DDR(Double-Data-Rate) clock, such as 50 MHz, 100 MHz, 200 MHz, 400 MHz, and
533 MHz, the harmonics may generate wideband jammer radiating into GPS
antenna through likely radiation path such as power/GND[10]. For example, the
fourth order harmonics of 400 MHz DDR clock causes 15 dB desense to
GLONASS( 1600 MHz)[10]. Changing the clock frequency is a possible method to
mitigate the issue.
78

81. Author : Criterion
Good placement is shown as below[104] :
Almost all the components with high speed digital noise are far away from GPS
antenna, and their distance are listed as below[104] :
Bad placement is shown as below[104] :
The sub camera is too close to GPS antenna, and the desense is roughly 10 dB.
79

82. Author : Criterion
As for micro SD card, the bad sockets are shown as below[104] :
Their shielding effect is very bad, and desense issue may occur. Besides, the
related traces of micro SD card should be short and routed in inner layer for
stripline surrounded by ground planes provides higher isolation than microstrip
line. Furthermore, we are able to add absobers on EMI noisy chips to mitigate the
desense issue[106].
80

83. Author : Criterion
As for FPC(Flexible Printed Circuit), take LCD for example,
Since there is a link between LCD and PCB through FPC. Namely, regardless of
noise source, which is from LCD or PCB, unnecessary radiating noises are
produced from the FPC acting as an antenna. GPS antenna may pick up the
radiating noises and desense issue occurs[105].
81

84. Author : Criterion
Thus, a metal foil should be attached to the FPC for shielding unnecessary
radiating noises[105], as shown below :
In addition to covering the FPC with the shielding layer, the shielding layer
should electrically be stuck on the metal frame rather than being floating. Since
the metal frame has a large metal surface area working as a stable ground and
the radiating noises are effectively reduced. Otherwise, the unnecessary radiating
noises are not effectively reduced[105].
82

85. Author : Criterion
Certainly, if possible, keep FPC away from GPS antenna[106].
83

86. Author : Criterion
Let’s intensify the importance of grounding further. If we fix on the clock rate of
LVDS(Low-voltage differential signaling) of LCM(LCD Module) , 60 MHz. As
shown below, there will be spurs appearing at regular intervals(60 MHz) due to
harmonics in spectrum. The 26th order harmonics is near GPS signal
(60 MHz * 26 = 1560 MHz), and then approximately 10 dB desense occurs[104].
84

87. Author : Criterion
Consequently, we need to add gasket on PCB top side to intensify the grounding
between PCB and LCM metal frame(marked as A), and so does PCB bottom side
to intensify the grounding between PCB and metal back cover(marked as B), as
shown above[104].
Certainly, you can modify the clock rate as well to prevent GPS signal from being
interfered by harmonics[104].
85

88. Author : Criterion
Besides gasket, during the initial design stages, add ground vias on all the the
shielding frame pads as many as possible to intensify the grounding.
Otherwise, the EMI noise from noisy chip may radiate to GPS antenna through
shielding can acting as radiator(due to cavity resonator mechanism).
86

89. Author : Criterion
Also, this method is able to provide the RF block with more effective shielding
effect and prevent RF block from being interfered by outside noise.
As for shielding can, you cannot strengthen its grounding too much. In wireless
test, sometimes, poor grounding is worse than no grounding for shielding can
will act as radiator as a result of cavity resonator mechanism.
87

90. Author : Criterion
There is often back light driver chip for LCM application. The SMPS trace
generates strong EMI noise, as shown below[106] :
88

91. Author : Criterion
Since power inductor is used to stabilize the switching transient current and
reduce the ripple. In other words, the current between back light driver chip and
power inductor is extremely unstable, which is a terribly strong aggressor, as
shown below[106] :
Consequently, the trace between back light driver chip and power inductor must
be short, and the trace posterior to power inductor should be routed in inner
layer as a result of better isolation[106].
89

92. Author : Criterion
Besides, the shielding can ought to be also well designed, especially for noisy
chips. Take MMD(Memory Module design) for example[104] :
As shown above, the MMD shielding can with gaps is very close to antenna feed
point(1 cm), and the EMI noise may leak into GPS antenna through these gaps.
Initially, the desense was approximately 10 dB. After sealing these gaps with
copper foil,
The desense improved about 5 dB. Thus, in addition to keeping the noisy chips
area(including shielding can and connector) away from GPS antenna, sealing
these shielding can gaps is important as well.
90

93. Author : Criterion
Moreover, perhaps there are numerous test points on these high speed digital
signal, especially interface such as I2S, PCIE, etc.. You ought to think of them as
noise-generating aggressors for they are a portion of these high speed digital
signal as well. Therefore, all of them must be taken away from GPS antenna, or
desense issue will occur as well[106].
Even ground pad, you should regard it as a portion of these high speed digital
signal as well, and try to keep it away from GPS antenna[106].
91

94. Author : Criterion
MIPI(Mobile Industry Processor Interface) is often used as LCM interface, which
is rich in harmonics[106].
Some users may use dynamic wallpaper for their cellphones,
92

95. Author : Criterion
The dynamic wallpaper may cause serious desense issue due to MIPI of
LCM[106]. There is usually EMI filter on the MIPI path of LCD.
For those differential high speed digital signals, we often focus on the common
mode noise rejection. As a result, select the EMI filter with better common mode
noise rejection.
Maintain a solid ground with no breaks in the plane reference for MIPI clocks to
provide a low-impedance path for return currents[103]. If the MIPI trace is long,
route it in inner layer for stripline surrounded by ground planes provides higher
isolation than microstrip line[106].
93

96. Author : Criterion
Moreover, SSC(spread spectrum clock) is also a technique to mitigate desense
issue.
As shown above, the technique is able to spread the MIPI clock energy with wider
bandwidth, and then avoid all the energy is concentrated in GPS channel. The real
spectrum is as shown below[106] :
Surely, to modify the MIPI clock frequency is also a technique to mitigate desense
issue.
94

97. Author : Criterion
Besides FPC, connector is the connection between PCB and LCM as well. In other
words, the noise, which is from LCM or PCB, must go through connector. Thus, we
have to keep in mind that connector is an aggressor rich in noise. Let’s analyze
the following case[106] :
The noise couples to camera data bus from LCM connector, and the noise flows
into camera module through connector and FPC. Since the FPC is near GPS
antenna and acts as an antenna, which radiates noise to GPS antenna and causes
desense issue. Consequently, avoid making LCM connector being close to traces
of the components near GPS antenna.
95

98. Author : Criterion
Let’s analyze the following similar case[106] :
As a result of the fault that memory traces overlap micro SD card traces, and the
micro SD card connector is near GPS antenna. The noise from memory traces
couples to micro SD card traces, flowing into micro SD card connector
, and then the radiating noise from micro SD card connector acting as a radiator
is picked up by GPS antenna. Therefore, desense issue occurs.
As mentioned earlier, if possible, keep any FPC and connector away from GPS
antenna.
96

99. Author : Criterion
As for SIM card,
As shown above, the SIM card traces on top layer were too long, which caused
approximately 5 dB desense[106]. After covering the top layer traces with copper
foil and intensifying the grounding of SIM card connector, the sensitivity
improved roughly 3 dB[106]. Consequently, as mentioned earlier, the noisy traces
should be short and routed in inner layer as a result of better isolation. Moreover,
as shielding can, you cannot strengthen the grounding of connector too much for
poor grounding is worse than no grounding in wireless test due to the fact that
connector will act as radiator because of cavity resonator mechanism.
97

100. Author : Criterion
As for LNA, we ought to pay attention to not only RF trace, but also to power and
enable trace[104].
As shown above, the LNA enable trace is surrounded with LCM RGB
(Red, Greem Blue) traces, and the noise may leak into LNA through enable trace,
thereby causing 10 dB desense issue in conducted test. As shown below[104] :
98

101. Author : Criterion
Furthermore, care needs to be taken in the thermal placement[103]:
Due to thermal noise, with higher temperature, comes worse sensitivity. Some
considerations are listed as below :
l Keep the PA away from the other heat sources.
l Keep very hot components away from the battery.
l Keep the PMIC away from the baseband chipset.
l Keep the XOs away from the heat sources/gradients.
99

102. Author : Criterion
As for heat flow under the thermal components, firstly, it is important to fill the
heat source mount side with copper. A higher copper density or a large amount
of copper provides better thermal relief and heat transport[103].
Secondly, add ample GND vias under or near the hot spots, and connect them
directly to main ground plane. Vias on the PA ground pad are very important, and
should be as many as possible.
100