Microwave Engineering Europe - November 2007 - (Page 21)

SOFTWARE DEFINED RADIO 21 ADC sampling at 200 MSPS (mega samples per second), the maximum bandwidth that can be effectively digitized is 100 MHz. In practical implementations, though, the ﬁlter used to band-limit the analog input to Fs/2 has a ﬁnite roll-off, which effectively further reduces the usable bandwidth. Beyond the receiver, the demand for high bandwidth also is key for the transmit section of the radio. Since the cost of the power ampliﬁer is proportional to its output power, a key method of reducing the overall bill-of-materials (BOM) and operational cost is through increasing its efﬁciency. Modern digital pre-distortion algorithms that linearize the power ampliﬁer at the transmitter rely on feeding back to the digital processor a digitized bandwidth that is a multiple of the transmitted signal’s bandwidth. This in turn necessitates the use of an ADC capable of sampling at very high rates. Signal-to-Noise Ratio (SNR) In order to maintain utmost sensitivity, an SDR design has to feature a large SNR so that very low received signals can be discerned, and effectively demodulated. The evolution of wireless standards to higher order modulation schemes (such as 64QAM) imposes more stringent requirements on the SNR performance of the ADC. In situations where the received input power at the antenna is really low, the SNR of the ADC (in conjunction with the phase noise of the local oscillator) becomes the limiting factor and sets the sensitivity for the entire receiver. Until recently, SDR designers had to trade off SNR for sample rate (bandwidth), since the state-of-the-art ADCs at several hundred MSPS were limited to 10 bits, with SNR levels around 50 dBFS. With the introduction of the ADS5463 (12-bit/500 MSPS), the envelope for monolithic 12-bit, ADCs essentially has doubled (previous art was at 250 MSPS). With SNR levels jumping to the mid-60s, implementations previously prohibitive can now become reality. In addition to being able to effectively reconstruct as large an analog signal bandwidth as possible, the sample rate of the ADC offers an added beneﬁt — usually referred to as processing gain. Typically, SNR for an ADC is calculated as the ratio of the power of the fundamental of a sinusoidal tone to the sum of the noise across the entire Nyquist band of the ADC (0 Hz through Fs/2, excluding DC). Typically, total noise is uniformly spread across the Nyquist zone. When the receiver processes a signal of a certain bandwidth within that zone, powerful digital ﬁlters can greatly attenuate the out-of-band noise. When the signal of interest has a bandwidth BWSIG and the ADC samples at a rate of Fs, the effective processing gain (PG) can be calculated as: Now entering the high performance area R&S®SMF 100A – the new microwave signal generator Unsurpassed specifications for telecommunications and radar applications – that‘s what the new R&S®SMF 100A stands for. Take, for example, its output power of typ. +25 dBm (at 20 GHz) – and that‘s without an external amplifier. Or setting times of < 700 μs in the list mode, or the unprecedented SSB figure of -115 dBc (at 10 GHz/ 10 kHz offset). Yet there‘s more: The new R&S®SMF 100A also excels when it comes to operation. The highly self-explanatory block diagram, which is unique in this class, guides you smoothly through all applications. But see for yourself ! Your Rohde & Schwarz sales partner will be glad to give you a personal tour of the fascinating world of microwave high performance. — Equation (1). Figure 1 shows the processing gain that can be achieved by using a very high speed ADC such as the ADS5463, sampling at 500 MSPS. The power of the digital backend of the SDR can fully exploit the beneﬁts of the wideband capabilities of the ADC. Ultimately, the evolution of wireless receivers will entail direct sampling at the RF frequency. Although the ADC technology needed for such a task is not feasible today, it is not unreasonable to expect that eventually technological breakthroughs may enable it. However, jitter needs to be taken into account since, ultimately, it will limit the SNR. The well-documented equation www.rohde-schwarz.com/ad/smf Microwave Engineering ● November 2007 ● www.mwee.com 020_021_022_MWEE.indd 21 24/10/07 14:00:37 http://www.rohde-schwarz.com/ad/smf http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - November 2007

Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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