chapter 4 transceiver architectures - sjtu

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Chapter 4 Transceiver Architectures

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Sections to be covered

• 4.1 General Considerations• 4.2 Receiver Architectures• 4.3 Transmitter Architectures

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Chapter OutlineHeterodyne Receivers

Basic structure Problem of Image Sliding-IF RX

Transmitter Architecture

Direct-Conversion TX Heterodyne and Sliding-IF TX

Direct-ConversionReceivers

Basic structure

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Recall: Generic TX & RX

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General Considerations: Narrow Channel Bandwidth What is the impacts of narrow channel bandwidth?

The transmitter:a) employ narrowband modulation and amplificationb) avoid leakage to adjacent channels;

The receiver:a) process the desired channel b) sufficiently reject strong in-band and out-of-band interferers.

The linearity of a receiver must be high enough: alleviate compression or significant intermodulation.

Is it possible to design a powerful filter in RF side?

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Example of Interferer-Suppressing Filter

Solution:

A 900-MHz GSM receiver with 200-kHz channel spacing must tolerate an alternate adjacent channel blocker 20 dB higher than the desired signal. Calculate the Q of a second-order LC filter required to suppress this interferer by 35 dB.

The filter frequency response an attenuation of 35 dB at 400 kHz away from center frequency of 900MHz.

For a second-order RLC tank,Parallel RLC tank circuit

inI R C L outVFor an attenuation of 35 dB (=56.2) at 900.4 MHz,

2max

1 156.2

1 ( 2 )

63,200

pp

p

VV

Q

Q

ωω

= =+

⇒ =

pωω∆

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Can We Simply Filter the Interferers to Relax the Receiver Linearity Requirement?

Two issuse: First, the filter must provide a very high Q (interferer level is 20dB above

the desired signal lever) Second, the filter would need a variable, yet precise center frequency. This

is very difficult to implement.

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Basic Heterodyne (超外差) Receivers

Due to its high noise, the downconversion mixer is preceded by a low-noise amplifier

A Mixer performing downconversion. The center of the downconverted channel ω in - ω LO is intermediate frequency

(IF). “Heterodyne” receivers employ an LO frequency unequal to ω in

a nonzero IF.

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Constant LO: each RF channel is downconverted to a different IF channel

How Does a Heterodyne Receiver Cover a Given Frequency Band?

Constant IF: LO frequency is variable, all RF channels within the band of interest translated to a single value of IF. This case is more common as it simplifies the design of the IF path

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Basic Heterodyne Receivers: Problem of Image

Two spectra located symmetrically around ω LO are downconverted to the IF.

Due to this symmetry, the component at ωim is called the image of the desired signal.

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Observations What creates the image?

The numerous users in all standards (from police to WLAN bands) ;

If one interferer happens to fall at ωim = 2ωLO - ωin, then it corrupts the desired signal after downconversion.

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How to suppress image? “image-reject filter”

a relatively small loss in the desired band a large attenuation in the image band

A filter with high image rejection typically appears between the LNA and the mixer; the gain of the LNA lowers the filter’s contribution to the receiver noise

figure.

Image Rejection

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Channel Selection and Band Selection Most receiver front ends do incorporate a “band-select” filter:

rejects “out-of-band” interferers

selects the entire receive band

Where to put the channel selection filter?

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Condition 1: as 2ωIF increases (the center of the downconverted channel (ωIF)); Condition 2: the bandwidth of IF filter is given;

we need a higher Q in the IF filter (channel selection).

Image Rejection versus Channel Selection: trade off on ωIF

A low IF helps with the suppression of in-band interferers.

To maximize image rejection, it is desirable to choose a large value for 2ωIF. A high IF allows substantial rejection of the image.

Residual

No residual

high IF

low IF

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Dual Downconversion (Ⅰ)

The front-end filter selects the band while providing some image rejection as well (Point B)

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After amplification and image-reject filtering, spectrum of C obtained linear mixer translates desired channel and adjacent interferers to first IF

(Point D)

Partial channel selection BPF3 permits the use of a second mixer with reasonable linearity. (Point E)

Spectrum is translated to second IF. (Point F)

Dual Downconversion (Ⅱ)

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Dual Downconversion (Ⅲ)

BPF4 suppresses the interferers to acceptably low levels (Point G)

An optimum design scales both the noise figure and the IP3 of each stage according to the total gain preceding that stage.

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Another Example of Image

Solution:

Assuming low-side injection for both downconversion mixers in figure above, determine the image frequencies.

As shown below, the first image lies at 2ωLO1 -ωin.

The second image is located at 2ωLO2 - (ωin - ωLO1).

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Zero Second IF

To avoid secondary image, most modern heterodyne receivers employ a zero second IF.

Place ωLO2 at the center of the first IF signal the output of the second mixer contains the desired channel with a zero center

frequency.

In this case, the image is the signal itself. The left part of the signal spectrum is the image of the right part and vice

versa.

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Example of Zero Second IFSuppose the desired signal is accompanied by an interferer in the adjacent channel. Plot the spectrum at the second IF if ωLO2 = ωIF1.

the desired channel appears at ±ωIF1

accompanied by the interferer. ① the spectrum at negative frequencies is convolved with the LO impulse at +ωLO2,

sliding to a zero center frequency for the desired channel. ② the spectrum at positive frequencies is convolved with the impulse at -ωLO2 and

shifted down to zero.

What happens if the signal becomes its own image?

The output consists of two copies of the desired channel surrounded by the interferer spectrum at both positive and negative frequencies.

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Symmetrically-modulated versus Asymmetrically-modulated Signal

AM signals are symmetric. FM signals are asymmetric. Most of today’s modulation schemes, e.g., FSK, QPSK, GMSK, and QAM,

exhibit asymmetric spectra around carrier frequency.

AM signal generation

A real baseband signal having a symmetric spectrum Sa( f ) is mixed with a carrier, thereby producing an output spectrum that remains symmetric with respect to fLO.

The information in the sideband below the carrier is different from that in the sideband above the carrier.

FM signal generation

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Corruption of the Asymmetric Signal Spectrum

Downconversion to a zero IF superimposes two copies of the signal. If the signal spectrum is asymmetric, the original signal spectrum will be

corrupted.

What happens if the signal becomes its own image?

Recall the example of Zero Second IF

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How to avoid Self-corruption?

Solution:

Downconversion to what minimum intermediate frequency avoids self-corruption of asymmetric signals?

To avoid self-corruption, the downconverted spectra must not overlap each other. the signal can be downconverted to an IF equal to half of the signal

bandwidth. An interferer may now become the image.

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How can downconversion to a zero IF avoid self-corruption?

By creating two versions of the downconverted signal that have a phase difference of 90。

Quadrature downconversion

Heterodyne RX with quadrature downconversion.

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Sliding-IF Receivers

Modern heterodyne receivers employ only one oscillator Oscillators put on the same chip suffer from unwanted coupling.

The second LO frequency is derived from the first by “frequency division”.

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Sliding-IF Receivers: Divide-by-2 Circuit

Divide-by-2 topology can produce quadrature output: rising edge of CLK drop edge

The second LO waveforms at a frequency of fLO1/2

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Direct-Conversion Receivers

Why it is a superior choice with respect to heterodyne receiver? Absence of an image greatly simplifies the design process; Channel selection is performed by on-chip low-pass filter (active circuit topologies

with relatively sharp cut-off characteristics); Mixing spurs are considerably reduced in number.

“direct-conversion,” “zero-IF,” or “homodyne” architecture. This type of receiver entails its own issues but has become popular in the past

decade.

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Transmitter Architecture: General Considerations

where

An RF transmitter performs modulation, upconversion, and power amplification.

In most of today’s systems, the data to be transmitted is provided in the form of quadrature baseband signals.

For example, the GMSK waveform in GSM can be expanded as

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Transmitter Architecture: generating GMSK waveform

where

cosΦ and sinΦ are produced from xBB(t) by the digital baseband processor; The input generates a sequence of address, producing a set of levels. Converted to analog form by D/A converters.

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Direct-Conversion Transmitters: OverviewThe above expression of a GMSK waveform can be generalized to any narrowband modulated signal:

Define the quadrature baseband signals as:

Called as “Direct-Conversion” transmitters: This topology directly translates the baseband spectrum to the RF carrier by means

of a “quadrature upconverter”. The upconverter is followed by a PA and a matching network, providing maximum

power delivery to the antenna and filter out-of-band components.

Notice: the baseband signal is produced

in the transmitter has a sufficiently large

amplitude (several hundred millivolts),

noise of the mixers is much less critical than in the receivers.

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Direct-Conversion Transmitters: other issues

The direct-conversion approach provides the most compact solutionand a relatively “clean” output.

The output spectrum contains: only the desired signal around the carrier frequency (and its harmonics) but no spurious components

An attribute similar to that of direct-conversion receivers.

Other issues: I/Q mismatch Carrier leakage Mixer linearity TX linearity Oscillator pulling

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Direct-Conversion Transmitters: I/Q Mismatch

For the four points in the constellation:

The I/Q mismatch in direct-conversion transmitter results in “cross-talk” between the quadrature baseband outputs distortion in the constellation.

Suppose a phase mismatch of Δθ and amplitude mismatch of ΔAc.

Direct-Conversion Transmitters

I/Q mismatch Carrier leakage Mixer linearity TX linearity Oscillator pulling

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Carrier Leakage: Definition The analog baseband circuit producing the quadrature signals in the transmitter exhibits

dc offsets the baseband port of each upconversion mixer.

Effect: The upconverter output contains a fraction of the unmodulated carrier.

Called “carrier leakage,” and quantified as:

Carrier Leakage will lead to two adverse effects: distorting the signal constellation making it difficult for power control

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Effect of Carrier Leakage(Ⅰ)

First, it distorts the signal constellation.

For a QPSK signal:

The baseband quadrature outputs suffer from dc offsets horizontal and vertical shifts in the constellation.

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Effect of Carrier Leakage(Ⅱ) Difficult for power control:

the output power of the transmitter must be varied across a wide range

With a short distance between the base station and the mobile: The carrier power dominates, difficult to measure the actual signal power, the mobile output power can not be reduced to a correct level.

power control

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Reduction of Carrier Leakage

The baseband processor produces a zero output The detector measures only the leakage. The loop consisting of the TX, the power detector, and the DACs drives the

leakage toward zero. The final settings of the DACs are stored in the register.

Zero input



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Mixer Linearity

Suppose a GMSK signal the baseband I/Q inputs experience a nonlinearity given by α1x + α3x3. The upconverted signal assumes the form:

Upconversion mixers in a transmitter sense no interferers. However, excessive nonlinearity in the baseband port of upconversion mixers

will corrupt the signal raise the adjacent channel power

The second term also represents a GMSK signal but with a threefold modulation index, occupying a larger bandwidth.

For variable-envelope signals, A3(t) appears in both terms of equation above, making the effect even worse.



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TX Linearity The linearity of transmitters must be chosen according to

the spectral regrowth requirement the tolerable distortion of the signal to be transmitted.

The distortion of a variable-envelope signal is typically characterized by the compression that it experiences.

For example, the average power of the 64-QAM OFDM signal in 802.11a must remain about 8 dB below P1dB of a given circuit.

Discussed in chapter 2



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Oscillator Pulling The PA output exhibits very large swings (20Vpp for 1 W delivered to a 50-Ω

load), couple to various parts of the system Through silicon substrate, package parasitics, and traces on the printed-circuit

board. It is likely that an significant fraction of the PA output couples to the local

oscillator.

Consider an output spectrum centered at ωLO +Δωand model it by an impulse of equal energy.

What happens if a sinusoid at a frequency of ω1=ωLO+Δω is “injected” into an oscillator operating at a frequency of ωLO, where Δω<<ωLO?

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Effect of Oscillator Pulling

The output phase of the oscillator, Φout, is modulated periodically. Remains around 900, a rapid 3600 rotation.

In order to avoid injection pulling, the PA output frequency and the oscillator frequency must be made sufficiently different. Direct-conversion transmitter is not practical?

waveform

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Modern Direct-Conversion Transmitters Most of today’s direct-conversion transmitters avoid an oscillator frequency

equal to the PA output frequency. By frequency division and mixing

This architecture is popular for two reasons: injection pulling is greatly reduced the divider readily provides quadrature phases of the carrier

Is there any possibility for oscillator pulling? The PA nonlinearity produces a finite amount of power at the second

harmonic of the carrier; the LO may still be pulled.

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Is it Possible to Use a Frequency Doubler?

Is it possible to choose ωLO = ωc/2 and use a frequency doubler to generate ωc?

Solution: It is possible, but the doubler typically does not provide quadrature phases. Advantage:

No harmonic of the PA output can pull the LO.

Disadvantage: Doubler and the polyphase filter suffer from a high loss, Requiring the use of power-hungry buffers.

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Use Mixing to Derive Frequencies

The oscillator frequency is divided by 2 and the two outputs are mixed. The result contains components at ω 1 ± ω 1/2 with equal magnitudes.

Shall we keep both of the components? (1) half of the power delivered to the antenna is wasted.(2) the power transmitted at the unwanted carrier frequency corrupts

communication in other channels or bands.

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Heterodyne Transmitters Perform the signal upconversion in two steps; the LO frequency remains far from the PA output spectrum.

Advantage: the I/Q upconversion occurs at a significantly lower frequency than the

carrier,

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Sliding-IF TX Eliminate the first oscillator in TX;

Derive the required phases from the second oscillator.

We call the LO waveforms at ω1/2 and ω1 the first and second LOs, respectively.

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Carrier Leakage

The dc offsets in the baseband yield a component at ω1/2 at the output of the quadrature upconverter,

The dc offset at the input of the RF mixer produces another component at ω1 .

The former can be minimized as described before (zero input, power detector then zero output).

The latter, and the lower sideband at ω1/2, must be removed by filtering.

chapter 4 transceiver architectures - sjtu

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