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RF MICROELECTRONICS BEHZAD RAZAVI

RF MICROELECTRONICS BEHZAD RAZAVI. 2010.08.09 지능형 마이크로웨이브 시스템 연구실 박 종 훈. Contents. Ch.5 Transceiver Architecture 5.1 General Considerations 5.2 Receiver Architectures 5.2.1 Heterodyne Receivers 5.2.2 Homodyne Receivers 5.2.3 Image-Reject Receivers 5.2.4 Digital-IF Receivers

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RF MICROELECTRONICS BEHZAD RAZAVI

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  1. RF MICROELECTRONICSBEHZAD RAZAVI 2010.08.09 지능형 마이크로웨이브 시스템 연구실 박 종 훈

  2. Contents • Ch.5 Transceiver Architecture • 5.1 General Considerations • 5.2 Receiver Architectures • 5.2.1 Heterodyne Receivers • 5.2.2 Homodyne Receivers • 5.2.3 Image-Reject Receivers • 5.2.4 Digital-IF Receivers • 5.2.5 Subsampling Receivers • 5.3 Transmitter Architectures • 5.4 Transceiver Performance Tests • 5.5 Case Studies

  3. Ch.5 Transceiver Architecture • Primary criteria in selecting transceiver architectures • Complexity, cost, power dissipation, and the number of external components • IC technologies evolve the relative importance of each of these criteria changes

  4. 5.1 General Considerations • 1. Limited spectrum • E.g., 30kHz in IS-54 and 200kHz in GSM • Use of sophistiated techniques • Coding, compression, bandwidth-efficient modulation • 2. Narrow band

  5. 5.1 General Considerations • Example • Bandpass filter • Select a 30kHz channel while rejecting interfering channels 60kHz away • Second-order LC filter : Q is on the order of 107 • Difficult to achieve even in such filters as surface acoustic wave(SAW) devices

  6. 5.1 General Considerations • Tradeoff : loss & Q • Critical parameter : out-of-band rejection and in-band loss • 3. Band VS Channel • Band • Spectrum in which the users of a particular standard are allowed to communicate • Channel • Signal bandwidth of only one user in the system

  7. 5.1 General Considerations • 4. Transfer function of front-end filters • Out-of-band rejection at 20MHz offset • Receive band is approximately equal to 30dB

  8. 5.1 General Considerations • 5. Large in-band • Odd-order nonlinearities yield intermodulation(Ch.2) • Third-order distortion is usually dominant, the IP3 • Distorting amplitude • Zero-crossing points of the desired signal are corrupted • Important even if the signal carries information only in its phase or freq.

  9. 5.1 General Considerations • Indicate the importance of controlled spectral regrowth through proper choice of the modulation scheme and the power amplifier • 6. Dynamic range of the signals • With multipath fading and path loss, the required dynamic range for the received signal is typically greater than 100dB • Minimum detectable signal : microvolt range • Input noise and cross-talk become critical

  10. 5.1 General Considerations • Leakage to the receive path is on the order of 30mVPP(≈-26dBm) • LNA 1-dB compression point : -25dBm -> Desensitize the LNA • NADC, GSM : Offsetting the transmit and receive time slots • Analog FDD(AMPS) : Require high isolation

  11. 5.1 General Considerations • 7. Power Amplifier • PA turned on and off to save power • However, the large current drawn by the PA(several amperes) • Introduces tremendous noise in the supplies • Typical battery output impedances may change the battery voltage by several hundred millivolts

  12. 5.2 Receiver Architectures • 5.2.1 Heterodyne Receivers • 5.2.2 Homodyne Receivers • 5.2.3 Image-Reject Receivers • 5.2.4 Digital-IF Receivers • 5.2.5 Subsampling Receivers

  13. 5.2.1 Heterodyne Receivers • In heterodyne architectures, the signal band is translated to much lower frequencies so as to relax the Q required of the channel-select filter

  14. 5.2.1 Heterodyne Receivers • This operation is called downconversion mixing • Simply downconversion • The sinusoid is generated by a local oscillator • Intermediate frequency(IF) • The center of the downconverted band • Critical parameter • Bearing trade-offs with many other aspects of the performance

  15. 5.2.1 Heterodyne Receivers • 1. Problem of Image • How are the LO frequency and the IF chosen? • The principal consideration is the image frequency

  16. 5.2.1 Heterodyne Receivers • Image-reject filter • Placed before mixer • Small loss & Large attenuation in the image band -> large 2WIF

  17. 5.2.1 Heterodyne Receivers • How large 2WIF can be? • Translate the center frequency to a sufficiently low value • However, requiring a higher Q in the IF filter • Trade-off : Image rejection & Channel selection • (Sensitivity & Selectivity)

  18. 5.2.1 Heterodyne Receivers • Two Other factors influence the choice of the IF • Availability and the physical size of filters for different frequencies • SAW or crystal devices : used at certain frequencies • E.g., 10.7MHz(in FM radios), 71MHz, etc., • Less costly • Reduce the form factor of portable systems • Drawback • Image reject filter -> passive, external component • LNA -> Drive the 50Ω input impedance of the filter • Trade-off • gain, noise figure, stability, power dissipation in the amplifier

  19. 5.2.1 Heterodyne Receivers • 2. Problem of Half IF • Second-order distortion in the RF and IF paths must be minimized • 50% LO duty cycle must be maintained • Image-reject filter achieve sufficient attenuation at (Win + WLO)/2

  20. 5.2.1 Heterodyne Receivers • 3. Dual-IF Topology • To solve the issue that Sensitivity & Selectivity • Heterodyning can be extended to multiple downcon-versions, each followed by filtering and amplification

  21. 5.2.1 Heterodyne Receivers • Most of today’s RF receivers employ two stages • Second IF • Narrow channel standard(IS-54) : 455kHz • Wide channel standard(DECT) : Several Megahertz • Analog FM systems : Demodulation is performed at this frequency • Digital modulation systems • Generates both in-phase(I) and quadrature(Q) components • Difficult to achieve • High linearity with reasonable noise, power dissipation, and gain • Despite the complexity and the need for a large number of external components, heterodyning is still viewed as the most reliable reception technique

  22. 5.2.2 Homodyne Receivers • Translated to the baseband in the first downconversion • Homodyne, direct-conversion, zero-IF architecture • Low pass filter with sharp cutoff characteristics • Double-sideband AM – Overlap positive and negative parts • Frequency and phase modulation – Avoid loss information

  23. 5.2.2 Homodyne Receivers • Advantage over a heterodyne • No image filter is required, and the LNA need not drive a 50Ω load • IF SAW filter and subsequent downconversion stage are replaced with low-pass filters and baseband amplifiers that are amenable to monolithic integration

  24. 5.2.2 Homodyne Receivers • 1. Channel Selection • Active low-pass filter is more difficult than passive filter • Exhibit much more severe noise-linearity-power trade-offs

  25. 5.2.2 Homodyne Receivers • (a) • Filter suppresses out-of-channel interferers • Nonlinear, high-gain amplifier • ADC to have a moderate dynamic range • However, filter impose tight noise-linearity trade-offs • (b) • Relaxes the LPF noise requirement • (c) • ADC must both achieve a high linearity and noise fllor well below the signal level

  26. 5.2.2 Homodyne Receivers • 2. DC Offsets • This effect arises from capacitive and substrate coupling and, if the LO signal is provided externally, bond wire coupling

  27. 5.2.2 Homodyne Receivers • Self-mixing • The leakage signal appearing at the inputs of the LNA and the mixer is now mixed with the LO signal, thus producing a DC component • If a large interferer leaks from the LNA or mixer input to the LO port and is multiplied by itself • Total gain from the antenna : 80 to 100dB • LNA/mixer combination : 25 to 30dB • Self-mixing varies with time • Leaks to the antenna and radiated and subsequently reflected from moving objects back to the receiver

  28. 5.2.2 Homodyne Receivers • Offset cancellation • Method1 • Baseband signal in the transmitter can be encoded • DC-free coding • Suited to wideband channels(e.g.,DECT) • Method2 • Exploit the idle time intervals in digital wireless standards to carry out offset cancellation

  29. 5.2.2 Homodyne Receivers • 3. I/Q Mismatch • Homodyne receiver must incorporate quadrature mixing • Shifting RF signal or the LO output by 90⁰

  30. 5.2.2 Homodyne Receivers • Phase and Gain Mismatch • Suppose received signal xin(t) = acosWCt + bsinWCt

  31. 5.2.2 Homodyne Receivers

  32. 5.2.2 Homodyne Receivers • In practice, it is desirable to maintain the amplitude mismatch below 1dB and phase error below 5⁰ • Heterodyne architectures are much more relaxed • Frequency lower -> Less sensitive to mismatches in parastics • In IC design, the lower freqeuncy allows the use of large devices to improve the matching without excessive power dissipation • Amplified by 50 to 60dB before I/Q separation • Less stage -> Less mismatch • Can perform the I/Q separation in the digital domain to avoid mismatch issues • Homodyne receivers cannot

  33. 5.2.2 Homodyne Receivers • 4. Even-Order Distortion

  34. 5.2.2 Homodyne Receivers • Feedthrough • Mixers typically suffer from some asymmetry and their operation can be viewed as vRF(t)(a+AcosWLOt), where a is a constant • Attenuated by only 30 to 40dB as it couples to the output • IP2 • Second-order distortion can be characterized using the second-order intercept point • Plotting the beat signal power versus the input power and extrapolating the results yield the IP2

  35. 5.2.2 Homodyne Receivers • 5. Flicker Noise • High gain in the RF range • Active mixers • 6. LO Leakage • Leakage of the LO signal to the antenna and radiation therefrom creates interference in the band of other receivers using the same wireless standard • FCC(Federal Communications Commission) • In-band LO radiation : -50 and 80dBm

  36. 5.2.3 Image-Reject Receivers • 1. Shift-by-90⁰ operation

  37. 5.2.3 Image-Reject Receivers • RC-CR network

  38. 5.2.3 Image-Reject Receivers • 2. Hartley Architecture • The RF input with the quadrature phases of the local oscillator, sinWLOt and cosWLOt, low-pass filters the resulting signals, and shifts one by 90⁰ before adding them together

  39. 5.2.3 Image-Reject Receivers

  40. 5.2.3 Image-Reject Receivers

  41. 5.2.3 Image-Reject Receivers • Drawback • Sensitivity to mismatches

  42. 5.2.3 Image-Reject Receivers • : image-to-signal ratio at the input • (Image Rejection Ratio)

  43. 5.2.3 Image-Reject Receivers • 3. Weaver Architecture • Quadraturedownconversion followed by a 90⁰ phase shift produces in the two paths the same polarities for the desired signal and opposite polarities for the image

  44. 5.2.3 Image-Reject Receivers • We assume

  45. 5.2.3 Image-Reject Receivers • Secondary image • Interferer is not canceled because it is originally on the same side of WLO1 as the desired signal • Low-pass filters must be replaced with bandpass filters to suppress the secondary image

  46. 5.2.3 Image-Reject Receivers • Hartley and Weaver architectures • Incomplete image rejection due to gain and phase mismatch • Weaver circuit is free from the gain imbalance • But it suffers from the secondary image

  47. 5.2.4 Digital-IF Receivers • Low-frequency operations such as the second set of mixing and filtering can be performed more efficiently in the digital domain • Avoids the problem of I and Q mismatch

  48. 5.2.4 Digital-IF Receivers • A/D converter • Typically no higher than a few hundred microvolts(point A) • Quantization and thermal noise of the ADC must not exceed a few tens of microvolts • ADC must be sufficiently small to minimize corruption of the signal by intermodulation, if the first IF bandpass filter cannot adequately suppress adjacent interferers • ADC dynamic range must be wide enough to accommodate variations in the signal level due to path loss and multipath fading • ADC must achieve an input bandwidth commensurate with the value of IF while consuming a reasonable amount of power

  49. 5.2.4 Digital-IF Receivers • The above requirements make it difficult to employ a Nyquist-rate ADC in the digital-IF architecture • Cannot be obtained in today’s A/D converters • Sampling IF • ADC samples the signal at a rate slightly below fIF • The spectrum of the downconverted, digitized signal thus lies around fIF-fS

  50. 5.2.4 Digital-IF Receivers • High speed and high linearity • Utilized in base station • Digital IF and sampling IF architectures have not been used in portable terminals because of ADC performance limitations

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