250 Chapter 8 24 dB/octave below crossover as frequency decreases. The output of the first integrator A2 is the same but with a 6 dB/octave slope decreasing with frequency applied across the whole range, so the part of the response that was flat now slopes downward at 6 dB/octave slope with frequency, while the 24 dB/octave section has its slope reduced by 6 dB/octave to give 18 dB/octave. The second integrator A3 performs the same process again, so its output is a fourth-order bandpass response with skirt slopes of 12 dB/octave. The third integrator A4 does the same thing, its output having a 6 dB/octave slope in its low-frequency section, and a 18 dB/octave slope in its high-frequency section. The fourth integrator completes the process, and its output is the lowpass signal, with a flat low-frequency section and a 24 dB/ octave roll-off above the crossover frequency. Note that all the plots in Figure 8.46 pass through the -6 dB point at the crossover frequency. This configuration could of course be adapted for fixed-frequency operation by removing the variable elements. 8.44 Variable-Frequency Filters: Other Orders You may at this point be wondering what happened to third-order state variable filters; there appears to be no reason why these should not be constructed in exactly the same