GPS RF FRONT-END CONSIDERATIONS.pdf

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GPS RF FRONT-END

CONSIDERATIONS

Component selection guide

System circuit design

Layout and placement considerations

Mainly focus on

system level

(Board Level)



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



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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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].

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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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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.

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Author : Criterion

We assume that GPS receiver front-end bandwidth is 4MHz, 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].

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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

aminimum 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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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].

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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].

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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].

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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].

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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.

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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].

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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].

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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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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] :

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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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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.

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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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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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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 eff ective 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.

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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 amobile 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].

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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.

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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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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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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].

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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 aminimum 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].

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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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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.

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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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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 eff ect 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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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.

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Author : Criterion

Input and output return loss :

Gain :

NF :

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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.

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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.

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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 amixer

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 amixer 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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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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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].

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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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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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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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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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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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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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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.

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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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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.

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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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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 amajor 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].

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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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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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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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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.

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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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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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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 :

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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].

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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].

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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.

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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].

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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].

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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].

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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.

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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

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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 amass of ground vias and connect this isolated ground area to the

PCB main ground plane directly[103].

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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 awire-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].

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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.

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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.

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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].

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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.

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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.

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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].

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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].

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Author : Criterion

Thus, ametal 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].

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Author : Criterion

Certainly, if possible, keep FPC away from GPS antenna[106].

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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].

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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].

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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).

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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.

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Author : Criterion

There is often back light driver chip for LCM application. The SMPS trace

generates strong EMI noise, as shown below[106] :

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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].

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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.

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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].

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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,

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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].

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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.

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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.

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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.

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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.

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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] :

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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.

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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.

GPS RF FRONT-END CONSIDERATIONS.pdf

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Transcript of GPS RF FRONT-END CONSIDERATIONS.pdf