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Chapter 4 Transceiver Architectures

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Chapter 4 Transceiver Architectures Chapter 4 Transceiver Architectures 1 Sections to be covered • 4.1 General Considerations • 4.2 Receiver Architectures • 4.3 Transmitter Architectures 2 Chapter Outline Heterodyne Receivers Basic structure Problem of Image Sliding-IF RX Direct-Conversion Receivers Basic structure Transmitter Architecture Direct-Conversion TX Heterodyne and Sliding-IF TX 3 Recall: Generic TX & RX 4 General Considerations: Narrow Channel Bandwidth What is the impacts of narrow channel bandwidth? The transmitter: a) employ narrowband modulation and amplification b) 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? 5 Example of Interferer-Suppressing Filter 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. Solution: 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 For an attenuation of 35 dB (=56.2) at 900.4 MHz, I C in R L Vout V 11 = = Vmax ω 2 56.2 1(+ Qp 2 ) ω p ⇒=Qp 63,200 ∆ω ω p 6 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. 7 Basic Heterodyne (超外差) Receivers 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. Due to its high noise, the downconversion mixer is preceded by a low-noise8 amplifier How Does a Heterodyne Receiver Cover a Given Frequency Band? Constant LO: each RF channel is downconverted to a different IF channel Constant IF: LO frequency is variable, all RF channels within the band of interest translated to a single value of IF. 9 This case is more common as it simplifies the design of the IF path Basic Heterodyne Receivers: Problem of Image Due to this symmetry, the component at ωim is called the image of the desired signal. Two spectra located symmetrically around ω LO are downconverted to the IF. 10 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. 11 Image Rejection 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. 12 Channel Selection and Band Selection Most receiver front ends do incorporate a “band-select” filter: selects the entire receive band rejects “out-of-band” interferers Where to put the channel selection filter? 13 Image Rejection versus Channel Selection: trade off on ωIF To maximize image rejection, it is desirable to choose a large value for 2ωIF. A high IF allows substantial rejection of the image. Residual high IF 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). A low IF helps with No residual the suppression of low IF in-band interferers. 14 Dual Downconversion (Ⅰ) The front-end filter selects the band while providing some image rejection as well (Point B) 15 Dual Downconversion (Ⅱ) 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) 16 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. 17 Another Example of Image Assuming low-side injection for both downconversion mixers in figure above, determine the image frequencies. Solution: As shown below, the first image lies at 2ωLO1 -ωin. The second image is located at 2ωLO2 - (ωin - ωLO1). 18 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. 19 Example of Zero Second IF Suppose 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. The output consists of two copies of the desired channel surrounded by the interferer spectrum at both positive and negative frequencies. What happens if the signal becomes its own image? 20 Symmetrically-modulated versus Asymmetrically- modulated Signal AM signal generation FM signal generation A real baseband signal having a symmetric The information in the sideband below spectrum Sa( f ) is mixed with a carrier, the carrier is different from that in the thereby producing an output spectrum that sideband above the carrier. remains symmetric with respect to fLO. 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. 21 Corruption of the Asymmetric Signal Spectrum What happens if the signal becomes its own image? Recall the example of Zero Second IF Downconversion to a zero IF superimposes two copies of the signal. If the signal spectrum is asymmetric, the original signal spectrum will be 22 corrupted. How to avoid Self-corruption? Downconversion to what minimum intermediate frequency avoids self-corruption of asymmetric signals? Solution: 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. 23 An interferer may now become the image. How can downconversion to a zero IF avoid self- corruption? Quadrature downconversion Heterodyne RX with quadrature downconversion. By creating two versions of the downconverted signal that have a phase 。 difference of 90 24 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”. 25 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 26 Direct-Conversion Receivers “direct-conversion,” “zero-IF,” or “homodyne” architecture. This type of receiver entails its own issues but has become popular in the past decade. 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. 27 Transmitter Architecture: General Considerations 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 where 28 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. 29 Direct-Conversion Transmitters: Overview The above expression of a GMSK waveform can be generalized to any narrowband modulated signal: Notice: Define the quadrature baseband signals as: 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. 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 maximum30 power delivery to the antenna and filter out-of-band components. Direct-Conversion Transmitters: other issues The direct-conversion approach provides the most compact solution and a relatively “clean” output. The output spectrum contains: only the desired signal around the carrier frequency (and
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