Configuring Moving Average Filter Lengths for Decimated Signal Processing
Matching moving average filter length to the decimation factor eliminates alias leakage by placing transfer function zeros directly on downsampling fold points.
Passband Droop
Amplitude Attenuation
Non-uniform frequency response within the passband of a filter or amplifier reduces the signal strength at the band edges relative to the center. Passband droop measures this loss of gain as the signal frequency approaches the cutoff frequency. This variation introduces amplitude distortion in wideband signals, which degrades the performance of digital communication receivers.
It is typically expressed in decibels.
Electrical Cause
Finite quality factors of inductors and capacitors in the filter circuit prevent a perfectly flat amplitude response. Resistive losses and parasitic capacitances cause the transmission coefficient to decrease gradually before the nominal cutoff frequency is reached. Digital filters also experience this behavior due to the sync response of digital-to-analog converters.
This effect is mitigated by using equalizer circuits or digital pre-distortion algorithms that boost the signal amplitude at the outer edges of the band. These techniques restore a flat frequency response across the entire operating bandwidth of the system.
Measurement Protocol
Sweep generators and network analyzers measure the transmission loss across the frequency range of interest. The test system records the output amplitude at multiple points to construct the frequency response curve of the device. This curve allows metrologists to calculate the maximum passband droop of the filter under test.
Temperature changes can shift the component values, which increases the droop and alters the filter characteristics.
Signal Impact
High amplitude variation within the signal bandwidth causes intersymbol interference and reduces the noise margin of the system. This degradation makes the receiver more susceptible to thermal noise and signal crosstalk. These performance losses are minimized by enforcing tight design tolerances on the analog components.
Integrating Moving Average Filter Windows with Analog Converter Downsampling Rates
Aligning moving average filter lengths as integer multiples of ADC decimation rates eliminates aliasing foldover and maximizes noise bandwidth reduction.