Microwave Engineering Europe - June 2008 - (Page 33)

RECEIVER ARCHITECTURE 33 If the desired signal is modulated, then the 2nd order result is a modulated baseband signal. The power of this term is 1/Zo * E{(½a2A2(t))2}. This can be expanded to: Pbb = a22/(4Zo) * E{A4(t)} (equation 4) In order to express this result in terms of the desired signal power, we must relate E{A4(t)} to E{A2(t)}. For a Gaussian random variable, the following relation is true: E{A4(t)} = 3 * [E{A2(t)}]2 (equation 5) The distortion power can then be expressed as 3a22/(4Zo) * [E{A2(t)}]2. Now express the expected value in terms of the desired signal power: Pbb = 3a22ZoPs2 (equation 6) It is the conversion of any given tone to DC, and any modulated signal into a baseband signal that makes second order performance critical to direct conversion receiver performance. Unlike other nonlinear mechanisms, the signal frequency does not determine where the distortion product falls. Any two signals entering the nonlinear element will give rise to a beat note/term. 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. y(t) = A(t)cosωt + a2(A(t)cosωt + B(t)cosωut)2 + … higher order terms = A(t)cosωt + ½a2A2(t) + ½a2A2(t) cos2ωt + 2a2A(t)B(t)cosωtcosωut + … = A(t)cosωt + … + a2A(t)B(t)cos(ω-ωu)t + … RF testing out of control? Only Keithley gives you the RF test tools you need to rein in today’s devices and tame tomorrow’s challenges. The second order distortion term of interest is a2A(t)B(t)cos(ω-ωu)t. This term describes the distortion product centered about the difference frequency between the two input signals. In the case of two unwanted tones entering the element, the result will include a tone at the difference frequency. If the two unwanted signals are modulated, then the resultant includes a modulated signal centered about their difference frequency. We can apply these principles to a direct conversion receiver example. The block diagram of a typical WCDMA basestation receiver appears in Figure 1. Here are some key characteristics of this example: • Base Station Type: FDD, Band I • Base Station Class: Wide Area • Number of carriers: 1 • Receiver band: 1920 to 1980 MHz • Transmit band: 2110 to 2170 MHz The RF section of this receiver includes a diplexer, a bandpass ﬁlter, and at least one Low Noise Ampliﬁer (LNA). The frequency selective elements are used to attenuate out-of-band signals and noise. The LNA(s) establishes the noise ﬁgure of the receiver. The I/Q demodulator converts the receive signal to baseband. In the examples illustrated later in this paper, we will use the characteristics of the LT5575 I/Q demodulator as representative of a basestation class device of this type. Lowpass ﬁlters and baseband ampliﬁers band-limit and increase the signal level before it is passed to the A/D converters. The diplexer and RF bandpass ﬁlter serve as band ﬁlters only; they do not offer any carrier selectivity. The second order linearity of the LNA is much less important than that of the demodulator. This is because any LNA distortion due to a MODEL 2920 RF SIGNAL GENERATOR MODEL 2820 RF SIGNAL ANALYZER ■ Test the most complex signal structures, including 802.11n WLAN MIMO and 802.16e Wave 2 WiMAX. Conﬁgure a 4x4 MIMO test system costeffectively. Generate and analyze signals up to 6 GHz repeatybly and accurately with our instruments’ software-deﬁned radio architecture. Reduce your time to market and cost of test with MIMO systems optimized for R&D and production test. ■ ■ ■ Go to www.keithley.com/tame and try a demo. www.keithley.com/contact info@keithley.de Microwave Engineering ● June 2008 ● www.mwee.com http://www.keithley.com/tame http://www.keithley.com/contact http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - June 2008

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.