Microwave Engineering Europe - November 2007 - (Page 22)

22 SOFTWARE DEFINED RADIO Figure 2: ADS5463 SNR and SFDR performance over analog Input Frequency at 500 MSPS. enabled the inclusion of an analog input buffer, capable of delivering high levels of SFDR across many hundreds of MHz. The analog input buffer of the ADS5463 allows the user to easily achieve datasheet performance because it isolates the sensitive analog input from the switching within the ADC. Additionally, it provides constant impedance across input frequency. As can be seen in Figure 2, the ADS5463 enables SFDR performance of over 70 dBc for IFs at least as high as 500 MHz. This dramatic improvement in performance substantially simpliﬁes the design of the radio, especially since it is coupled with very high levels of SNR and processing gain. Utilizing a very high input frequency can further reduce the cost of the radio since it removes an extra downconversion step and its associated BOM. Conclusion The promise of true Software-Deﬁned Radio depends heavily, among other technologies, on the evolution of high speed A/D Conversion. At the heart of both the receiver and the transmitter, the ADC sets the performance for the entire radio. Recent breakthroughs in mixed signal technology have enabled performance at unprecedented sample rates and analog input frequencies, simplifying the radio design and allowing for broader operating bandwidths and higher levels of sensitivity. As ADC technology keeps pushing the envelope, it will continue to pave the way for the advent of truly re-conﬁgurable, multi-standard radio. About the author Yiannis Papantonopoulos is Systems and Applications manager for High Speed ADCs at Texas Instruments. He can be reached at yiannis@ti.com. that relates SNR to jitter for a sampled system is given in Equation 2: — Equation 2 — where fin represents the analog input frequency, and tjitter the RMS value of the system’s jitter. The internal jitter of the ADC sampling circuitry is added (in a root of the sum of squares fashion) to the externally provided sampling clock to the ADC. It is interesting to note that the limitation of SNR is independent of the actual sampling frequency, but directly related to the analog input frequency. This fundamental limitation is a major design consideration when deciding the placement of the intermediate frequency (IF) in receiver design. The beneﬁt of simpliﬁed Rx architecture and ﬁltering (and, hence, reduced cost) is countered by the limitations imposed by jitter and clocking the ADC as the IF is increased. Spurious-Free Dynamic Range (SFDR) The linearity of an ADC, most often characterized by its SFDR, becomes critical in situations where the incident power at the receiver’s antenna is of substantial power levels. This can happen when the wanted signal is strong (a desirable situation), or when an in-band interferer is strong (an undesirable situation). In the latter case, the linearity of the ADC dictates whether the wanted signal can be effectively demodulated. This is particularly true when the desired signal’s power is low. The presence of a large interferer effectively limits the application of any AGC function, since the total signal (wanted plus interferer) may already be approaching the full-scale range of the analog input. Thus, the ADC’s inherent linearity performance becomes the bottleneck. Just as jitter limits how high an SDR designer can place the IF, SFDR also weighs into that decision quite heavily. Many ADCs available in the market today exhibit high levels of linearity that are, however, limited to input frequencies below 200 MHz. Hence, the beneﬁts of high IF placement cannot be realized due to the roll-off in SFDR performance. New analog structures utilizing cutting edge BiCMOS process technologies have Company Information Texas Instruments Incorporated www.ti.com MICROWAVE ENGINEERING EUROPE Free subscription at: www.mwee.com/subscribe Microwave Engineering Europe ● November 2007 ● www.mwee.com 020_021_022_MWEE.indd 22 24/10/07 14:01:01 http://www.ti.com http://www.mwee.com/subscribe 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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