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Architectures and RF System Design Issues for Integrated Receivers and Transmitters in 3rd Generation Wireless Handsets

10/26/01. Walid Y. Ali-Ahmad. 2. TALK OUTLINE. Introduction: Example of present level of integration in an RF chipset for CDMA cellular radioReceiver ArchitecturesHeterodyne ReceiverImage-Reject ReceiverDirect-Conversion ReceiverLow-IF ReceiverTransmitter ArchitecturesIF-Modulation / Up-conversion TransmitterDirect-Modulation TransmitterOffset-PLL TransmitterSummary.

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Architectures and RF System Design Issues for Integrated Receivers and Transmitters in 3rd Generation Wireless Handsets

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    1. 10/26/01 Walid Y. Ali-Ahmad 1 Architectures and RF System Design Issues for Integrated Receivers and Transmitters in 3rd Generation Wireless Handsets Walid Y. Ali-Ahmad Senior Member of Technical Staff Wireless Communications Group Maxim Integrated Products Sunnyvale, CA

    2. 10/26/01 Walid Y. Ali-Ahmad 2 TALK OUTLINE Introduction: Example of present level of integration in an RF chipset for CDMA cellular radio Receiver Architectures Heterodyne Receiver Image-Reject Receiver Direct-Conversion Receiver Low-IF Receiver Transmitter Architectures IF-Modulation / Up-conversion Transmitter Direct-Modulation Transmitter Offset-PLL Transmitter Summary

    3. 10/26/01 Walid Y. Ali-Ahmad 3 Example of Present Level of Integration in an RF chipset for CDMA Cellular Radio

    4. 10/26/01 Walid Y. Ali-Ahmad 4 Heterodyne Receiver Advantages: Down-conversion to baseband I&Q is done at an “Intermediate-Frequency” (IF) lower than RF. This results in superior I & Q matching. Its selectivity, which measures the receiver’s capability to process a desired small channel in the presence of close-in strong interferes, is done partly at IF using a highly selective SAW filter and at baseband I&Q using low-pass baseband filters. The use of the IF SAW filter relaxes the linearity requirements (IIP2, IIP3) of the succeeding IF and baseband stages. DC offsets at baseband I&Q do not limit its sensitivity because they are minimized by the fact that the first LO frequency is not equal to the input RF carrier frequency.

    5. 10/26/01 Walid Y. Ali-Ahmad 5 Heterodyne Receiver (cont’d) Design Issues: The need to use an an off-chip passive BPF for image rejection adds cost and board space requirements. Currently, due to technology limitations, this “image-reject” BPF and the IF SAW filter can not be integrated on-chip. The trade-off between image rejection and channel selection is key in determining the IF frequency. Low frequency IF SAW filters (40-150MHz) have high Q and provide high adjacent channel selectivity. However, These filters tend to be large. High frequency IF SAW filters (150-400MHz) have a relatively smaller physical size, but provide a lower adjacent channel selectivity. Good frequency planning is essential in order to minimize spurious responses generated in the receiver’s front-end (Fs = ±m?FRF ±n?FLO1 ±p?FLO2) . The “half-IF” spurious response at (FRF+ FLO)/2 can be a serious problem in the case of low IF frequency. The front-end mixer after LNA should have a low 2nd-order distortion and a high suppression of the (2FLO?2FRF) product.

    6. 10/26/01 Walid Y. Ali-Ahmad 6 Importance of IF Selectivity for Suppression of 3rd-order IM products Typical Two-stages cascaded IIP3 equation, in linear format:: ;“g1” is gain of stage 1; “il” is insertion loss of IF filter Equivalent IIP3 of IF block when including selectivity “S” (dB) ahead of IF stage: Generalized equation for overall IIP3 of a receiver chain with M cascaded stages:

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    12. 10/26/01 Walid Y. Ali-Ahmad 12 Design Issues: 1– DC offsets: Static offsets are caused by process mismatch and drift of analog circuitry that vary slowly vs. T?, aging, and current gain setting. Time-variant offsets are caused mainly by parasitic LO coupling to mixer RF port, LNA input port, and antenna port. LO “self-mixing” occurs in mixer and it produces a dc component at the mixers I & Q baseband outputs. Time-variant offsets can also be caused by a large interferer which can leak from LNA or mixer input to LO input port and self-mix with itself to produce a dc offset at mixers outputs. The time-variance is due to reflection of LO leakage against moving objects back to receiver and due to receiver movements. Maximum frequency content of time-variant DC offset due to Doppler shift = 2??max/?; where ?max: maximum moving object or car speed.

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    21. 10/26/01 Walid Y. Ali-Ahmad 21 Baseband I&Q signals undergo quadrature modulation at an intermediate IF frequency (wIF). The following IF filter (BPF1) rejects the harmonics of the IF signal. The IF modulated signal is then up-converted to (FIF ? FLO2). The unwanted sideband imposes tough rejection requirements on BPF2, typically 50-60dB, in order to meet transmitter’s spurious emissions levels imposed by standards. This topology does not allow full transmitter’s integration because of use of off-chip passive devices such BPF2 and BPF1. On-chip I and Q matching is superior since modulation is done at IF and not at RF. This will lead to better EVMs and lower cross-talk between I & Q channels. IF filtering reduces transmitted noise in RX band. Wide power control dynamic range because control it is distributed between RF and IF sections.

    22. 10/26/01 Walid Y. Ali-Ahmad 22 In direct-conversion transmitters, the baseband signal is directly modulated unto the RF carrier. The output carrier frequency is equal to the LO frequency at mixers inputs. This topology is attractive for full transmitter’s integration since it does not use an intermediate IF stage with upconversion and interstage IF filter. Its main disadvantage is the corruption through “injection pulling” of the VCO spectrum by the high level PA output. Isolation required is normally > 60dB. The isolation can be highly improved by “offsetting” the LO frequency by using 2xLO off-chip and dividing by 2 on-chip or by adding or subtracting another oscillator. The power control dynamic range is limited by the carrier feedthrough. A fully integrated differential transmitter architecture will minimize carrier feedthrough because of higher of common mode rejection (differential LO inputs and modulator output).

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