Microwave Engineering Europe - June 2008 - (Page 37)

RECEIVER ARCHITECTURE 37 as 2ZoPs, where Ps is the power of the desired signal. The power level of the distortion at baseband is then: Pbb = 9/2PsPu2Zo2a32 (equation 8) If the undesired signal is modulated, use equations 2 and 5 to express E{B4(t)} as 3(2ZoPu)2, where Pu is the power of the tone interferer: Pbb = 27/2a32Zo2PsPu2 (equation 9) as high as -15 dBm entering the receiver. To minimize dynamic DC offset, the I/Q demodulator must present a 2nd order intercept point on the order of +40 dBm at the receiver input. Also, there are modulated interferers up to -40 dBm that can degrade the effective noise ﬂoor of the receiver if the 2nd order intercept point is not high enough. Leakage from the transmitter, which operates simultaneously with the receiver, can have the same effect. The 3rd order linearity is less important, because interfering signals must be properly positioned to pose a threat to sensitivity. The WCDMA speciﬁcation does require minimal degradation in sensitivity in the presence of a pair of -48 dBm interfering signals. In this case, if the 3rd order intercept point of the receiver is less than 0 dBm, there will be an appreciable loss of sensitivity due to these signals. In the direct conversion receiver example, Section 7.6.1 of the WCDMA speciﬁcation calls for two interfering signals as shown in Figure 6. One of these is a -48 dBm CW tone, and the other is a -48 dBm WCDMA carrier. These are offset in frequency so that the resulting 3rd order product appears centered about DC. Compute the intermodulation product generated in the I/Q demodulator: • RF gain preceding LT5575: 20 dB • Signals entering LT5575: -28 dBm • LT5575 IIP3, two tone: +22.6 dBm • LT5575 a3: 0.0244 A MATLAB simulation performed using a pseudo-random channel predicts the following: • Distortion at LT5575 output: -135.8 dBm This result agrees well with the equation 8, which predicts a distortion power of -135.7 dBm. Refer this signal back to the receiver input: • RF gain preceding LT5575: 20 dB • Equivalent interference level at Rx input: -155.8 dBm • Thermal noise at receiver input: 101.2 dBm The equivalent interference in this case is 54.6 dB below the thermal noise at the receiver input. The resulting degradation in sensitivity is <0.1 dB, so the receiver easily meets the speciﬁcation of -121 dBm. Conclusion These calculations highlight the importance of 2nd and 3rd order linearity to a successful direct conversion receiver design. For a WCDMA application, 2nd order performance is critical for two reasons. First, there are CW tone interferers Microwave Engineering ● June 2008 ● www.mwee.com http://www.lairdtech.com/wirelesssystems http://www.lairdtech.com/wirelesssystems 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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