Microwave Engineering Europe - October 2007 - (Page 42)

42 DIRECT SYNTHESIS Figure 6: User deﬁned TFC in RFXpress custom mode. only PLCP preamble and PSDU. PPDU with the same characteristics can be grouped together to form packet groups. The number of packets within the packet group can be deﬁned. Spacing between the packets can be speciﬁed in symbols, pMIFS or in pSIFS. Deﬁne your own TFC It is not uncommon that one may want to test the receiver with a non-standard TFC pattern. As shown in Figure 6, RFXpress permits conﬁguring a custom hopping pattern and saving it as a user-deﬁned TFC pattern to be recalled later. Figure 7 shows the RFXpress signal ﬂow chart. Signal under your control All kinds of IQ impairments including carrier leakage, quadrature error, and IQ imbalance can be added to the IQ baseband signal. If using an IQ modulator for up-converting to higher bands, the signals can be thoroughly tweaked to test the performance of the receiver. Also it is of utmost importance that the receiver is tested in all real world scenarios. For example, a wireless LAN (802.11 a/b/g) signal can be added that was created using a tool like MCIQ Complex Modulation Software for AWGs. Information on MCIQ is availble at www. tek.com/products/signal_sources/mciq.html as an interferer in RFXpress. Figure 8 shows a 2.4GHz CW tone being added as an interferer. Connect, capture, replay Most of the modern communication devices are Tran receivers and therefore it is very important that the receiver is tested with real life signals. In today’s world where research and development needs to be coordinated between multiple locations, it is important that the same signals are reproduced in all locations in the same way to characterize or to test the particular behavior of the DUT. The signals captured in one location from an oscilloscope can be sent to another location for replay. The other location just needs to recall the AWG setup ﬁles to get the same kind of signal behavior as observed in the ﬁrst location. This enables an unprecedented way of reproducing signals at any site with the same level of accuracy and precision, making it easy for designers to overcome challenges in the design cycle. Tone nulling The most common technique for creating a notch in the frequency domain is to zero out tones that overlap with the radio astronomy band. The advantage of this technique is that there is no increase in complexity of the transmitter. The transmitted OFDM signal is constructed using an Inverse Discrete Fourier Transform (IDFT). As a rectangular window is applied to the data, each tone has a wider than expected spectrum, where the spectrum has the shape of a sinc function. Although the sinc function has zero crossings at each of the tone locations, zeroing out only a few tones results in a shallow notch. For example, to obtain a notch with a depth of 23 dB for the radio astronomy band, a total of 29 tones needs to be zeroed out. This corresponds to a total loss of 120 MHz of bandwidth. Figure 7: RFXpress signal ﬂow chart. Figure 8: Out-of-band interferer. Figure 9: Connect, capture and replay a waveform from an oscilloscope. Microwave Engineering Europe ● October 2007 ● www.mwee.com 034-036-038-040-042-044_MWEE.ind42 42 20/09/07 17:50:45 http://www.tek.com/products/signal_sources/mciq.html http://www.tek.com/products/signal_sources/mciq.html http://www.mwee.com

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Microwave Engineering Europe - October 2007 Contents Comment News CMOS RF: Si-On-Sapphire Goes Mainstream Cover Feature: New Data Protection Concept for UHF RFID Tags CMOS RF: RF Design Team Touts CMOS Spin for 3G PAs Wireless HID – Are You Following the Standard to Another “Average” Product Development? Phase Optimisation of the RF Front-End Direct Synthesis of UWB-WiMedia Signal Generation 4G Chips to Target 700 MHz Applications Femtocells Mobilize to Fight Wi-Fi in the Home Products Product Feature: AXIEM Pioneers the Future of EM Technology Calendar

Microwave Engineering Europe - October 2007

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