Microwave Engineering Europe - June 2008 - (Page 34)

34 RECEIVER ARCHITECTURE single signal will be centered about DC and rejected by the demodulator. If there are two unwanted signals in the receive band (1960 MHz, for example), then a second order product will be generated by the LNA at the difference frequency. This signal will be demodulated and appear as a baseband artifact at the A/D converter. We need not address this condition, however, because out of band signals emerging from the front-end diplexer will not be strong enough to create distortion products of any importance. Consider ﬁrst a single unmodulated tone entering the receiver (see Figure 2). As detailed above, this tone will give rise to a DC offset at the output of the demodulator. If the baseband cascade following the demodulator is DC-coupled, this offset will be applied to the A/D converter and reduce its dynamic range. The WCDMA speciﬁcation (3GPP TS 25104.740) calls out an out-of-band tone at -15 dBm, located 20 MHz or more from either receive band edge (section 7.5.1). Compute the DC offset generated in the I/Q demodulator: • Tone entering receive antenna port: -15 dBm • Diplexer rejection at 20 MHz offset: 0 dB • Bandpass rejection at 20 MHz offset: 2 dB • RF gain preceding LT5575: 20 dB • Tone entering LT5575: +3 dBm • LT5575 IIP2, two tone: +60 dBm • LT5575 a2: 0.00317 • DC offset at LT5575 output: 0.32 mV • Baseband voltage gain: 31.6 • DC offset at A/D input: 10 mV Single WCDMA carriers can also serve as interferers, as detailed in section 7.5.1. In one case, this carrier is offset by at least 10 MHz from the desired carrier, but is still in the receive band. The power level is -40 dBm, and the receiver must meet a sensitivity of -115 dBm for a 12.2 kbps signal at a BER of 0.1 percent. Here are the details: • Signal entering receive antenna port: -40 dBm • RF gain preceding LT5575: 20 dB • Signal entering LT5575: -20 dBm • LT5575 IIP2, two tone: +60 dBm • LT5575 a2: 0.00317 A MATLAB simulation performed using a pseudo-random channel predicts the following: • Distortion at LT5575 output: -98.7 dBm This result agrees well with that given by equation 6, which predicts a distortion power of -98.2 dBm. The baseband product that appears at the LT5575 output is a noiselike signal, created from the interfering WCDMA carrier. If this signal is large enough, it can add to the Figure 2: Effects of 2nd order distortion. Figure 3: 2nd order distortion due to WCDMA carrier. Figure 4: Transmitter leakage effects. Microwave Engineering Europe ● June 2008 ● www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - June 2008 Contents Comment News Cover Feature Designing and Simulating a Wireless LAN Antenna 60GHz: Achieving the Ultimate Wireless Dream New Radar Developments Include HFETs to Challenge DMOS/LDMOS and a 77-GHz CMOS PA for Automotive Applications Testing Raises Concerns Over 802.11-Based High-Speed Bluetooth IP2 & IP3 Design Considerations with Direct Conversion I/Q Demodulator Receiver Products Calendar

Microwave Engineering Europe - June 2008

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