Microwave Engineering Europe - June 2008 - (Page 36)

36 RECEIVER ARCHITECTURE Figure 5: Effects of 3rd order distortion. • Isolation, Tx to Rx 2110 MHz: 55 dB • Diplexer insertion loss, Tx path: 1.2 dB In the case of a Wide Area basestation, the transmit power may be as high as +46 dBm. At the transmit port of the diplexer the power will then be at least +47 dBm. This high level modulated signal will leak to the receiver input, and some portion of it will drive the I/Q demodulator: • Receiver input power: -8 dBm • Rx BPF rejection at 2110 MHz: 40 dB • RF gain preceding LT5575: 20 dB • Signal entering LT5575: -28 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: -114.7 dBm Refer this signal back to the receiver input: • RF gain preceding LT5575: 20 dB • Equivalent interference level at Rx input: -134.7 dBm • Thermal noise at receiver input: -101.2 dBm This equivalent interference is 33.5 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. Third Order Distortion (IP3) The third order intercept point (IP3) will have an effect upon the baseband signal when two properly spaced channels or signals enter the nonlinear element. Refer back to the transfer function: y(t) = x(t) + a2x2 (t) + a3x3(t) + …, where x(t) is the input signal consisting of both desired and undesired signals. Consider now the third order distortion term. The coefﬁcient a3 is equal to 2/(3ZoIP3) where IP3 is the single tone intercept point in W. Note that the twotone IP3 is 4.78 dB below the single-tone IP3. Two signals entering the nonlinear element will generate a signal centered at zero frequency, if the spacing between the two signals is equal to the distance to zero frequency. Let x(t) = A(t)cosωt + B(t)cosωut, where the ﬁrst term is the desired signal and the second term is an unwanted signal. The unwanted signal may be a tone or a modulated signal. If it is a tone, then B(t) is simply a constant. If it is a modulated signal, then B(t) represents the signal envelope. The output signal is then equal to y(t): y(t) = A(t)cosωt + … + a3(A(t)cosωt + B(t)cosωut)3 + … higher order terms = A(t)cosωt + … + 3a3A2(t)B(t)cos2ωtcosωut + 3a3A(t)B2(t)cosωtcos2ωut + … = A(t)cosωt + … + ¾a3A(t)B2(t)cos(2ωuω)t + … The third order distortion term of interest here is ¾a3A(t)B2(t)cos(2ωu-ω)t. In order for this distortion to appear at baseband, set ω = 2ωu. The power of the distortion is 1/Zo * E{(¾a3A(t)B2(t))2}, which can be expanded to: Pbb = 9a32/(16Zo) * E{A2(t)} * E{B4(t)} (equation 7) Consider the case of a modulated desired signal and a tone interferer; B(t) may be replaced by B (see Figure 5). The value of E{B4} can be expressed as (2ZoPu)2, where Pu is the power of the tone interferer. We can use equation 2 to express E{A2(t)} in terms of the desired signal power thermal receiver and A/D converter noise to degrade sensitivity. Compute the equivalent thermal noise at the receiver input with no added distortion: • Sensitivity: -121 dBm • Processing + coding gain: 25 dB • Signal to noise ratio at sensitivity: 5.2 dB • Thermal noise at receiver input: -101.2 dBm Now refer the distortion signal back to the receiver input: • RF gain preceding LT5575: 20 dB • Equivalent interference level at Rx input: -118.7 dBm The baseband second order product in this case is 17.5 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 -115 dBm. This is illustrated in Figure 3. Single WCDMA carriers can also appear out of band, as speciﬁed in section 7.5.1. These can be directly adjacent to the receive band at levels as high as -40 dBm. Here again, the second order effect of such carriers upon sensitivity is negligible, as the preceding analysis shows. Another threat to sensitivity comes from transmitter leakage in FDD systems, as shown in Figure 4. In an FDD system, the transmitter and receiver are operating at the same time. For the WCDMA Band I case, the transmit band is 130 MHz above the receive band. A single antenna is commonly used, with the transmitter and receiver joined by a diplexer. Here are some typical basestation coupled resonator-type diplexer speciﬁcations: Figure 6: 3rd order distortion due to WCDMA carrier plus tone interferer. 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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