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Designing for Wideband RF

Rob Reeder, Analog Devices -January 28, 2014

As the use of technology continues to increase and improve, designing for wideband radio frequency (RF) designs will be a key issue in 2014. High-frequency, high-speed converter designs exist in many applications today, with radar, wireless infrastructure and instrumentation pushing the boundaries and demanding the use of high-speed gigasample-per-second (Gsps) converters with resolutions of 8 to 14 bits.

 

Utilization of system-simplifying wideband RF sampling architectures over the DC – 2 GHz band is currently limited in many performance-sensitive applications due to the insufficient dynamic range, noise-performance, and circuit-design and applications complications, associated with the existing ultra-high-speed data converters. Higher input bandwidth with associated dynamic performance is valued by engineers because it means: fewer frequency conversion stages in the RF signal path; easier to manage frequency plans; better resolution in RADAR systems; and enhanced system performance and functionality.

 

Keep in mind that many parameters must be met to satisfy the “match” for your particular application. Key elements to incorporate into your design include:

  • Laying the proper foundation, which means choosing the right Gsps converter for your design
  • Remembering that a converter’s full-power bandwidth is different from converter “useable or sample” bandwidth
  • Choosing the front-end topology: amplifier (active) or transformer (passive), once the application bandwidth and high-speed ADC are known
  • Specifying a balun where phase imbalance will become important in what the high-speed ADC understands as optimal second-order linearity, for example
  • Optimization yielding the best results given the front-end design. It is almost impossible to “match” a front end at every frequency today with 100-Msample/s converters, let alone over a band that’s greater than 1000 MHz.

 


Figure 1: Generic Front end Passive Network and Gsps A/D Converter

Remember that these are just a few of the many parameters that need to be achieved when designing your next wideband RF application.  Even when a balun is chosen, don’t throw away its performance by using poor layout techniques, and be wary about matching the network properly.

 

Because each application is unique, feel free to ask me questions here or connect with me on EngineerZone at RReeder.

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