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| This is a plot of the previous filter's passband insertion loss in dB and group delay, also tested by Harold, and plotted by a VNA group delay test program. This plot, at 2 dB per vertical division and 0.5 kHz per horizontal divison, shows that the passband ripple is under 1 dB and the 6 dB bandwidth is 2.9 kHz. The group delay below, being the derivative of phase with respect to frequency, also quantifies the degree of linearity of the filter's phase vs. frequency characteristic in passband, as seen in the above plot. The sharp increases in group delay at the ends of the passband, about 2.7 milliseconds in this case, are characteristic of filters that have been optimized for skirt selectivity. These group delay 'ears' represent a loss in fidelity that can be particularly troublesome in narrow CW and data filters. |
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Paul Kiciak, N2PK