Author : Criterion

Why do we need external filter

In most wireless radios a weak desired signal can be accompanied by interferers

that can be significantly stronger[1]. For example, as liiustrated below, if phone1

transmits maximum GSM power, i.e. 33 dBm. Let’s assume the antenna efficiency

of phone1 and phone2 are both 50%(i.e. 3dB loss). Consequently, the phone2

receiver circuit will suffer from 0 dBm interferer.

We usually call the strong interferer blocker or jammer.

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Author : Criterion

Blockers can drastically degrade the receiver performance through compressing

its gain and hence increasing its noise figure, as shown below[3] :

As shown below[8], stronger the blocker, more the gain reduction.

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Author : Criterion

According to cascade noise figure formula[4,6] :

In general, less the LNA gain, higher the noise figure and worse the sensitivity.

Take SKY74092-11 of SKYWORKS for example, its P1dB is -5 dBm, and may be

saturated by the 0 dBm blocker as mentioned above[2].

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Author : Criterion

In Zero-IF (ZIF) architecture receiver, RF downconverts to baseband directly.

And DC offset is one of nonlinear effects as well[2,3]. Thus, if the strong blocker

saturates receiver, the DC offset due to receiver nonlinearity also aggravates

sensitivity.

Besides, in ZIF architecture, even though a strong out-of-band blocker, once

amplified by the LNA, may find a path to the LO-input port of the mixer, thus once

again producing self-mixing, which is DC component at the mixer output and

aggravates sensitivity as well[5,8]. As shown below :

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Author : Criterion

In addition, if the blocker has high phase noise, which also contributes to the

overall noisefloor level[2].

Even though the blocker has low phase noise, if Rx LO has finite phase noise that

mixes with blocker, and creates reciprocal mixing at the mixer output, the

sensitivity also degrades, as illustrated in the figure below[2]:

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Author : Criterion

In Frequency-Division Duplexing(FDD) communication system, there is Tx

leakage[2].

Hence there are three tones at the Rx port : Tx signal, Rx signal, and blocker[2] :

6

Author : Criterion

Similarly, even though blocker has low phase noise, if the Tx signal has high

phase noise, i.e. adjacent channel leakage ratio(ACLR), which also contributes to

the overall noisefloor level[8].

Of course, if Rx LO has finite phase noise that mixes with Tx leakage, and creates

reciprocal mixing at the mixer output as well[8,11].

Tx leakage also causes gain reduction and DC offset, as mentioned earlier, if

receiver is not linear enough.

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Author : Criterion

If the blocker is near Rx signal, which and Tx leakage will cause 3 order

intermodulation(IMD3) and cross-modulation(XMD) simultaneously due to LNA

nonlinearity. Both IMD3 and XMD are near Rx signal, resulting in sensitivity

degradation[2].

Besides, if the blocker is near Tx signal in spectrum, LNA nonlinearity also causes

IMD2, which is like DC offset, and will aggravate sensitivity.

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Author : Criterion

Thus, the blocker and Tx leakage may affect the receiver by following

products[6] :

As mentioned above, we already know the effects from Tx leakage and blocker.

Higher the Tx leakage and blocker, higher the noise figure[7,14] :

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Author : Criterion

Especially, if the mobile station is at the edge of the cellular boundary and there

is a strong blocker presence, due to the power control, the mobile's transmitter

power is kept close to it's maximum level maintain communication quality[2].

And the mobile station receives extremely weak signal in this case[13].

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Author : Criterion

Since the desired signal is extremely weak, the LNA gain must be kept high to

lower cascade noise figure to achieve acceptable sensitivity[4].

But, higher the gain, worse the linearity and worse immunity to blocker[3]. And

higher the Tx leakage and blocker, higher the noise figure, as mentioned above.

That is to say, this is the worst case.

11

Author : Criterion

Hence in most standards, the receiver must satisfy a certain blocking template

defined at various blocker frequencies and levels, to insure sensitivity with

blocker. For instance, in the GSM standard, a desired signal only 3 dB above the

sensitivity could be accompanied by an out-of-band blocker as large as 0 dBm,

and as close as only 80 MHz to the edge of the PCS band[6,12].

Therefore, the blocker must be filtered out prior to reaching the LNA, especially

with high gain mode[1]. For these reasons, all existing receivers inevitably use an

external SAW filter at the LNA input to reject out-of-band blocker[7].

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Author : Criterion

Although WCDMA scheme proves to be more resilient to out-of-band blockers,

due to the spread-spectrum nature of the system[10].

But as mentioned earlier, FDD communication system has Tx leakage issue. In

principle, the duplexer is effectively a dual-band highly selective filter, isolating

the RX and TX bands. However, cost and size constraints play the major role,

especially in multi-band applications, and limit the duplexer performance. As a

result, the duplexer isolation is finite, on the order of 45 to 50 dB, and the

receiver is plagued by the transmitter signal, as mentioned earlier. Less the

isolation, stronger the Tx leakage and worse the sensitivity. Generally speaking,

the duplexer isolation should be at least 55dB[2].

13

Author : Criterion

Thus, traditionally, at both the PA input and at the input of the receiver, external

SAW filters are used to alleviate these issues[8].

14

Author : Criterion

Why do we wanna remove external

filter

Nevertheless, since the external filters are typically not tunable, in multi-mode or

multi-band applications, each band requires a dedicated filter[8]. And this has

several obvious disadvantages.

First, it increases the cost, especially in multi-mode and multi-band applications,

where several of these filters are needed.

Second, according to cascade noise figure formula[4], the insertion loss of the

SAW filter, typically as high as 2 to 3 dB, degrades the receiver sensitivity

directly[4]. If the LNA is differential architecture that necessitates the use of a

off-chip balun with at least 1 dB of insertion loss that also adds directly to the

receiver’s noise figure[7,14].

That is to say, for a differential LNA, the sensitivity degrades more than

single-end architecture because of an external balun.

15

Author : Criterion

In order to overcome the loss of the external SAW filter and balun at the receiver

input, perhaps an external LNA is required[3], which helps to achieve a lower

noise figure. But an external LNA increases the cost as well, and it may degrade

the receiver’s linearity due to its inherent gain[3].

Third, the presence of these filters removes the flexibility of sharing the LNAs in

multi-mode or multi-band applications, particularly in software-defined

radios[8,9].

Therefore, it is highly desirable to eliminate these external filters[8]. As shown

below, if both the receiver and transmitter SAW filters could be eliminated, the

number of the external components would be reduced by a factor of 2, to 10.

This is a much more favorable situation, and not only reduces the cost but also

improves the PCB and package complexity.

16

Author : Criterion

The risk of removing external filter

However, eliminating RF filtering is challenging, this comes at the cost of more

stringent requirements of linearity for both receiver and transmitter[7,8]. As

indicated earlier, removing the receiver SAW filter reduces the receiver immunity

to strong out-of-band blockers that saturate LNA and aggravate sensitivity. In

FDD communication system, TX leakage mixes with an out-of-band blocker.

Due to the nonlinearity of the receiver, the IMD and XMD products will fall in the

desired receiver channel. These effects are typically relaxed by placing an

external SAW filter at the receiver to further attenuate the transmitter leakage

and out-of-band blockers. Otherwise, both the isolation of duplexer and the

nonlinearity requirements would be very stringent. Depending on the duplexer

isolation in the TX band, it can be shown that with typical blocker levels specified

in the 3GPP standard, this could lead to an IIP3 requirement of somewhere

around − 5 to 5 dBm[2,8].

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Author : Criterion

Desensitization of the receiver by the RX-band transmitter noise is another

notable consequence of finite isolation of the duplexer. In terms of Tx, the

third-order and fifth-order nonlinearity of the transceiver would cause spectral

regrowth, described by a metric called ACLR[17].

Because PA is the main nonlinearity contributor, that is to say, ACLR at PA output

is worse than which at PA input.

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Author : Criterion

As mentioned earlier, higher the Tx signal ACLR, higher the Rx band noise floor.

To understand the challenges, since the problem arises primarily from the noise

of the TX leakage appearing at the receiver band—we run some calculations[8].

Assuming an ideal PA, with a TX power of 27 dBm at the PA output (to allow for 3

dB loss of duplexer to ensure 24 dBm at the antenna) and a phase noise of − 158

dBc/Hz at the TX output, with 45 dB of duplexer isolation in the RX band, the

receiver noise floor due to the TX will be 27 dBm − 45 − 158 = − 180 dBc/Hz.

Thus, if the receiver chain noise figure alone is 3 dB, for instance, with the TX

turned on at the maximum gain, it degrades to 3.5 dB. With a SAW filter placed at

the PA input, the TX phase noise can be relaxed directly proportional to the SAW

rejection[8].

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Author : Criterion

However, in a SAW-less TX, depending on the duplexer isolation, a very

stringent phase noise of about − 158 to − 160 dBc/Hz at the TX-RX frequency

offset would be needed, which could lead to extra power consumption. Because

there is a compromise between linearity and power consumption. Thus, typically

use of a costly SAW filter before the PA is inevitable[8]. This method can help us

reduce ACLR at PA input in advance, then reduce ACLR at PA output further. With

a SAW filter placed at the PA input, the transceiver linearity can be relaxed

directly proportional to the SAW rejection.

Thus, the Pros and Cons of external filter can be summarized as below:

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Author : Criterion

Blocker-Tolerant techniques

Although external filters can reject out-of-band blockers. Nevertheless, not all

blockers can be eliminated by external filter, explained briefly as follows.

As shown above, an external SAW filter can eliminate out-of-band blockers and

Tx leakage, but can’t eliminate in-band blockers, which is ultimately through

baseband filtering[8,11]. And the nonlinear effects aggravating sensitivity, e.g.

gain reduction, DC offset, self-mixing, phase noise, IMD, and reciprocal mixing,

can be caused by in-band blockers as well.

21

Author : Criterion

As mentioned above, eliminating external filters can contain the flexibility of

sharing the LNAs in multi-mode or multi-band applications. Take ALM-1106 of

AVAGO for example, it can implemented in GPS/ISM/Wimax applications

simultaneously. Due to its wideband characteristic (0.9-3.5 GHz frequency

range), it is suitable for wideband application[9].

But compared to narrowband design, a wideband receiver can’t employ external

RF filtering and, therefore, a large blocker will saturate a conventional front-end

design. Besides, since external filtering has been ruled out, a conventional

wideband design has no selectivity and amplifies both the wanted signal and any

blockers. Any blocker present (whether or not it causes gain compression) will

be downconverted along with the wanted signal.

Consequently, for wideband or SAW-less design, the receivers need to be

considered blocker-tolerant[7]. And there are mainly three ways :

(1). Improve the receiver and transmitter linearity

(2). On-chip filtering

(3). Improve the isolation of duplexer

22

Author : Criterion

In terms of Tx linearity, as mentioned above, lower the ACLR, better the

sensitivity. Generally speaking, there are nine solutions to improving ACLR[15].

In terms of Rx linearity, a simple way to improve the linearity of a LNA is to

increase the bias levels, which will improve the linearity of LNA at the expense of

current consumption[2].

23

Author : Criterion

In addition, as shown below, the IIP2 almost has its worst value for the best

IIP3 in some cases[20].

In order to keep the IIP2 performance and at the same time provide third-order

compensation, a differential structure is proposed. In the ideal case, the match of

transistor is perfect, so there should be no 2nd-order non-linearity at the biasing

point of the differential pair for differential output[20].

Especially, in ZIF receiver, the IIP2 is a critical performance parameter. It is a

measure of IMD2 and helps quantify the receiver’s susceptibility to single- and

2-tone interfering signals. As mentioned earlier, IMD2 is like DC offset, which

aggravates sensitivity.

24

Author : Criterion

As shown below, higher the IIP2, better the sensitivity[18]. It proves again that

poor linearity leads to poor sensitivity[3].

In the case where the interfering modulated signal is very large, very high IIP2

performance should be required to minimize SNR degradation. Non-idealities

that can degrade IIP2 performance include device mismatch (without mismatch

and ideal differential stages IIP approaches infinity) and layout asymmetry.

Part-to-part IIP2 performance can also change due to process variation and

temperature. Sufficient margin was allocated for IIP2 to account for part to part,

process and temperature variation. Because of this, it is important to design the

receiver with sufficient IIP2 margin to specification. Digitally assisted calibration

techniques can be employed, as shown below[18] :

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Author : Criterion

An automatic on chip IIP2 calibration routine is used to enhance IIP2 while also

accounting for interaction between the I and Q mixers[18]. Besides, large DC

offsets can degrade common-mode rejection ratio which can degrade IIP2 and

limit IIP2 calibration range. DC offsets generated from IIP2 calibration can limit

ADC headroom or require additional DC offset correction further down the

receiver chain. Changes in DC offset with IIP2 calibration are also undesirable

since a separate DC offset correction is required after each IIP2 calibration

adjustment. That’s the reason why an additional DC offset correction is built in

the circuit . The DC offset correction system is a critical component for ZIF

receiver design.

26

Author : Criterion

Although, for GSM requirements where the modulated blocker is at 6 MHz from

the receive band (AM suppression) the IIP2 requirement is not as stringent and

no IIP2 calibration is necessary[18]. Nevertheless, because in FDD

communication system adopting ZIF design, the sensitivity is highly dependent

on the IIP2 performance[2]. Consequently, IIP2 calibration is necessary for FDD

communication system adopting ZIF design. Take RTR6285A of Qualcomm for

example, there is IIP2 calibration for WCDMA.

27

Author : Criterion

Non-Ideal 90 degree balance in the I/Q demodulator produces unwanted images

which can be close to the carrier[11,27]. As shown above, the figure shows the

passband gain and stop-band rejection versus the I-Q phase and gain imbalance.

Any mismatch between the gain and phase of the two paths results in less

stop-band rejection and passband gain[6]. In ZIF, the signal along with the

blocker downconverts to baseband directly, and the sensitivity becomes

poor[6,27]. But an automatic on chip IIP2 calibration routine is used to enhance

IIP2 while accounting for interaction between the I and Q mixers[18].

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Author : Criterion

Besides, as shown below, baseband Low-pass filter (LPF) can’t eliminate those

distortion products close to the downconverted signal[11].

Therefore, it proves again the IIP2 calibration and DC offset correction circuit are

necessary, especially in FDD communication system adopting ZIF architecture.

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Author : Criterion

In terms of on-chip filtering, a passive mixer driven by 50% duty-cycle clocks

constructs a built-in high-Q band-pass filter (BPF) through impedance

transformation[1,21]. As mentioned earlier, the blocker can be even 0 dBm. In

order to prevent saturation of the LNA input devices by the strong blocker, the

built-in high-Q BPF is placed in front of LNA. The figure below plots the

simulated gain of LNA when the built-in BPF is ON or OFF. As it is observed, when

the BPF is ON there is a sharp roll-off below and above passband, even sharper

than external SAW filter[6].

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Author : Criterion

There are some drawbacks in ZIF architecture receiver, and one of these is flicker

noise, a.k.a “1/f noise”[23]. Ficker noise degrades the total noise figure, which

results in the degradation of receiver sensitivity[24].

Ficker noise is even larger than the downconverted Rx signal.

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Author : Criterion

As mentioned in[3], the linearity of the mixer limits the whole linearity of the

receiver front-end and is effectively scaled down by the gain of LNA. Therefore, it

is essential to attain high linearity with low 1/f noise, which is difficult in active

mixers. Widely used active mixers suffer from high 1/f noise and poor linearity,

especially when the supply voltage is low. On the other hand, a current driven

passive mixer can provide relatively good linearity and inherent low 1/f noise

performance due to the absence of DC current[22]. Thus, in general, passive

CMOS mixers are considered as the appropriate choice for ZIF receivers because

they do not contribute to 1/f noise [24]. Furthermore, the passive mixer is

chosen not only for its well-known better 1/f noise and linearity over the active

mixer in low supply voltage applications, but also for its impedance

transformation property, which has been widely utilized to construct a built-in

high-Q BPF suppressing the transmitter leakage and other blockers[21].

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Author : Criterion

Nevertheless, in order to get low conversion loss from a passive mixer, typically a

high LO power is needed[2]. As shown below, higher the LO power, lower the

noise figure[25].

But strong LO power may result in significant LO leakage due to the finite mixer

port to port isolation[2]. LO leakage causes self-mixing, thereby generating a

static DC level aggravating sensitivity. Besides, with a non-zero time varying

drain current, even with zero mean, noise appears around zero frequency and

harmonics of the excitation. Also, if a dc offset is present with a large LO

excitation and large blocker, it seems possible a small amount of dc current could

exist in a passive mixer. When a very large RF blocking signal is present, such as

TX leakage in a full duplex system, there is potential for the large signal bias

point to shift exacerbating the problem further. The large signal can also

potentially increase DC offsets, due to imbalanced parasitic capacitance of

non-ideal layouts[18].

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Author : Criterion

It has been demonstrated that increasing AC current through a mixer device

channel with a large RF blocker can generate considerably more 1/f noise and

DC offset. This is a significant concern for SAW-less operation. In conventional

ZIF receiver architectures, with 50% duty-cycle LO waveforms generation, the

mixer devices are either on or off. However with non-ideal rise and fall times, it

causes the total noise referred to the output of the baseband amplifier to

increase. However, compared to 50% duty-cycle, the DC induced currents are

lower with 25% duty-cycle LO. And the 1/f noise will have less contribution to

overall noise after commutation for a 25% duty-cycle system, resulting in less

noise amplification at baseband and larger signal to noise(SNR). 25% duty-cycle

LO waveforms result in improved 1/f noise and DC offset performance even

though with large blocking signals as well[18].

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Author : Criterion

As shown above, simulated gain and 1/f noise versus LO duty cycle at high power

level of LO(-26 dBm). Even though with large blocker, 1/f noise is significantly

improved by 15 dB with 25% LO duty-cycle compared to 50% LO duty-cycle[18].

Besides, the LO waveform must ideally be a square wave to ensure abrupt

switching and hence maximum conversion gain [28].

35

Author : Criterion

Thus, in a ZIF receiver where the passive I and Q mixer are clocked with a 25%

duty-cycle LO, resulting in higher receiver gain relative to a 50% duty-cycle

implementation, a 3 dB improvement in gain by using 25% duty-cycle, as shown

above[18,28]. Besides, as mentioned earlier, I/Q imbalance results in image issue,

aggravating sensitivity. But with the 25% duty-cycle mixer design, the image

problem is eliminated[17,21]. On the other hand, as mentioned earlier, the IIP2

almost has its worst value for the best IIP3 in some cases. But a significant

advantage of using a passive mixer with 25% duty-cycle is that it is not sensitive

to increased IMD distortion when a large blocker is present. The results show

that the receiver IIP3 performance is improved significantly with 25% duty-cycle

compared to 50% duty-cycle. That is to say, with the 25% duty-cycle LO injection

into a passive mixer, which results in 3 dB additional gain; as well as lower noise

figure, IMD distortion and 1/f noise. Of course, with IIP2 calibration, these

improve more[18].

Consequently, if passive I and Q mixer are clocked with a 25% duty-cycle LO,

there are some advantages, as summaried below :

Construct a built-in high-Q BPF

High linearity

High immunity to strong blocker

Low 1/f noise and DC offset

higher receiver gain relative to a 50% duty-cycle, 3 dB improvement

Less image(undesired sideband) issue

36

Author : Criterion

As for duplexer, in addition to selecting a high isolation duplexer, the layout is

also critical. As shown below, we need to enhance isolation by means of good

layout.

We take SAYRF1G95HQ0F0A of Murata for example, because Tx port is far away

from Rx port. That is to say, there is good isolation between the layout traces

from the two ports.

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Author : Criterion

But for Tx and ANT port, we had better make ANT port trace be away from Rx

port trace, and make Tx port trace be away from ANT port trace.

Besides, if we don’t put multiple ground vias on ground pad, the Tx signal may

couple to ground pad firstly, then couple to Rx port further.

38

Author : Criterion

Consequently, as shown below, the correct PCB layout for duplexer should be like

this.

39

Author : Criterion

Reference

[1] A 65 nm CMOS Quad-Band SAW-Less Receiver SoC for GSM/GPRS/EDGE,

IEEE

[2] CDMA Zero-IF Receiver Consideration, Slideshare

[3] Sensitivity or selectivity -- How does eLNA impact the receriver performance,

Slideshare

[4] System(Board level) Noise Figure analysis and optimization,Slideshare

[5] On the Direct Conversion Receiver -- A Tutorial

[6] A Blocker Filtering Technique for SAW-Less Wireless Receivers

[7] A Blocker-Tolerant, Noise-Cancelling Receiver Suitable for Wideband

Wireless Applications

[8] Multi-Mode Multi-Band RF Transceivers for Wireless Communications

[9] ALM-1106 GPS Low Noise amplifier with Variable bias current and Shutdown

function, AVAGO

[10] Performance of spread spectrum system

[11] Wireless Technologies: Circuits, Systems, and Devices

[12] An Integrated GSM/DECT Receiver: Design Specifications

[13] Interference Mitigation Techniques for SAW-less CDMA Receivers

[14] HHM17147A1 for 673~2700MHz, MULTILAYER BALUN SPECIFICATION ,

TDK

[15] How to solve ACLR issue, Slideshare

[16] Design of a Direct Conversion Transmitter to Resist Combined Effects of

Power Amplifier Distortion and Local Oscillator Pulling

[17] Wireless Transceiver Circuits System Perspectives and Design Aspects

40

Author : Criterion

[18] A Single–Chip 10-Band WCDMA/HSDPA 4-Band GSM/EDGE SAW-less CMOS

Receiver With DigRF 3G Interface and +90 dBm IIP2

[19] A Comparative Study of Linearization Techniques of CMOS LNA

[20] A LINEARIZATION TECHNIQUE FOR RF LOW NOISE AMPLIFIER

[21] Analysis and design of a high-linearity receiver RF front-end with an

improved 25%-duty-cycle LO generator for WCDMA/GSM applications

[22] Image-Reject Mixer Arms Direct-Conversion Receivers

[23] Study of Flicker Noise for Zero-IF Receiver

[24] Design and Analysis of Low Flicker-Noise CMOS Mixers for

Direct-Conversion Receivers

[25] Passive Mixers in Downconverter Applications

[26] Understanding IP2 and IP3 Issues in Direct Conversion Receivers for

WCDMA Wide Area Basestations

[27] Introduction to I/Q signal, Slideshare

[28] RF MICROELECTRONICS—Mixer

41