Microwave Engineering Europe - October 2007 - (Page 36)

36 DIRECT SYNTHESIS Table 2: Instrument speciﬁcations. (1)I and Q baseband signal bandwidth is 264 MHz for a combined channel bandwidth of 528 MHz. (2)Band #1 only. (3)Band Group #1 only. (4)Bandwidth of a single sideband. (5)BWBB – bandwidth for a single UWB band (528 MHz). ﬁlter covering the target band. The amplitude and group delay distortions introduced by the band-pass ﬁlter may be compensated as part of the calibration procedure. 3:– Direct RF synthesis In this arrangement a single channel AWG generates the UWB signal directly at the ﬁnal frequency. The speed and the analog bandwidth requirements for the AWG depend mainly on the speciﬁc Band Groups to be covered and not on the hopping nature of the ﬁnal signal. For Band Group #1 (maximum frequency 4,752 MHz) a minimum of 10 GS/s sampling rate and 5 GHz analog bandwidth are necessary. Band Group #2 requires 15 GS/s sampling speed and 7 GHz analog bandwidth. The new Tektronix state-of-the-art AWG7102 is capable of generating 5.8 GHz bandwidth waveforms at 20 GS/s, so it is possible to generate hopping signals in the Band Group #1 with enough performance margin. Direct RF Synthesis requirements for calibration are low. Controlled thermal behavior, low drift in time, and the lack of external equipment allow for factory calibration while keeping an acceptable signal quality over a long period of time. Refer to Table 2 for more details on the AWG requirement for different setups. Instrument setup Experimental data has been gathered using a test setup comprising a Tektronix AWG7102 with 20 GS/s when interleaved and a Tektronix TDS6154C with WiMedia analysis software (see Figure 1). All the WiMedia signals have been generated using Tektronix Arbitrary Waveform generators. The AWG7000 Series Arbitrary Waveform Generator delivers a combination of unrivaled sample rate, bandwidth and signal ﬁdelity. With sample rates from 5 GS/s to 20 GS/s (10-bits), together with 1 to 2 output channels, the toughest measurement challenges in the communications, digital consumer and semiconductor design/test industries can be easily solved. The open Windows (Windows XP)-based instruments deliver ease of use and allow connectivity with peripherals and compatibility with third-party software. All the analysis has been performed using a high-bandwidth oscilloscope with a sampling rate of 40 GS/s, 15 GHz bandwidth, 64 MSample record-length, and UWB analysis capability. Amplitude and phase distortions introduced by the analysis oscilloscope are extremely low thanks to the built-in real-time, DSP-based compensation techniques and the time-domain calibration procedures performed architectures used in wireless communication testing. The necessary modulation and baseband bandwidths, the frequency hopping nature of the UWB-WiMedia signals and the inﬂuence of any amplitude, timing, and frequency response misalignments of the I and Q components of the signal makes necessary additional efforts to reach the required quality level for the resulting signals. Below are the three potential architectures for UWB signal generation. 1:– IQ baseband generation & quadrature modulation This is the traditional vector signal generation architecture. Frequency hopping may be implemented in two ways: by synthesizing a baseband IQ pair with the required frequency shift for each symbol or by changing the LO frequency at the IQ modulator. Practical implementations for the base band generation of the hopping signal require dual channel AWGs with sample rates around 2 GS/s and analog bandwidths in the 1 GHz Range. The implementation of frequency hopping by controlling the carrier frequency at the IQ modulator requires the capability of hopping more than 1 GHz in less than 70 ns. Current implementations, given their limitations in sample rate and hopping speed, are limited to the generation of non-hopping signals. As two independent signal paths are used for I and Q base band components, their alignment is extremely critical to obtain satisfactory results. Careful and long calibration procedures requiring additional high-performance analysis equipment are necessary and, due to the thermal and time drifts associated, may have to be carried out frequently. 2:– IF generation and up-converter In this method a single channel AWG is used to generate an UWB signal to feed an upconverter covering the required frequency range. Practical requirements for the AWG depend on the implementation of the hopping frequency operation. 1.5 GS/s sampling speed is the minimum requirement to generate non hopping signal. Generating a hopping signal would require twice as much (> 3.2 GS/s). Up-converters used in such a system would require a minimum of 750 MHz or 2 GHz up-conversion bandwidth for a non-hopping and a hopping signal respectively. Although this method also requires careful magnitude and phase calibration procedures to reach the highest levels of modulation and spectral accuracy, its requirements are much less demanding as I and Q components are by deﬁnition aligned and they share the same signal path. The main limitation of this strategy is handling the signal images that will show up in the spectrum. This effect may be minimized by using an analog band-pass Figure 1: Test setup. Microwave Engineering Europe ● October 2007 ● www.mwee.com 034-036-038-040-042-044_MWEE.ind36 36 20/09/07 17:34:10 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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