Abstract
Nonlinear frequency responses are a common problem in radio frequency
(RF) components. Specifically, in wireless communication systems,
signals are often unequally amplified or attenuated across a specified
frequency band. Common communications components such as filters,
amplifiers, and mixers can lead to nonlinear frequency responses, which
can cause errors in transmitting and receiving. This article outlines
the design and demonstration of a static and dynamic finite impulse
response (FIR) digital equalizer circuit. Using predistortion topology
with a coupled feedback loop, the adaptive LMS algorithm was
implemented. The FIR filter was simulated in MATLAB and Vivado and then
implemented onto an Eclypse Z7 FPGA evaluation board with no timing
errors. Simulations showed that the custom RTL module gave the same
frequency response that was produced in MATLAB calculations. The filter
was able to dynamically equalize the frequency responses of different
nonlinear boards that were used as the devices under test (DUT).
Measurements showed that the equalizer was able to compensate for system
distortion from 0.2 to 0.8 Nyquist frequency. The phase response
remained relatively linear across the band of interest, with a group
delay flatness less than 10ns.