MF4 datasheet(9/14 Pages) NSC

9 / 14 page

2.0 Designing With The MF4

(Continued)

2.3 CHANGING CLOCK FREQUENCY

INSTANTANEOUSLY

The MF4 will respond favorably to an instantaneous change

in clock frequency. If the control signal in

Figure 12 is low the

MF4-50 has a 100 kHz clock making f

c = 2 kHz; when this

signal goes high the clock frequency changes to 50 kHz

yielding f

c = 1 kHz. As the Figure illustrates, the output signal

changes quickly and smoothly in response to a sudden

change in clock frequency.

The step response of the MF4 in

Figure 13 is dependent on

c. The MF4 responds as a classical fourth-order Butterworth

low-pass filter.

2.4 ALIASING CONSIDERATIONS

Aliasing effects have to be considered when input signal fre-

quencies exceed half the sampling rate. For the MF4 this

equals half the clock frequency (f

CLK). When the input signal

contains a component at a frequency higher than half the

clock frequency f

CLK/2, as in Figure 14a, that component will

be “reflected” about f

CLK/2 into the frequency range below

CLK/2, as in Figure 14b. If this component is within the pass-

band of the filter and of large enough amplitude it can cause

problems. Therefore, if frequency components in the input

signal exceed f

CLK2 they must be attenuated before being

applied to the MF4 input. The necessary amount of attenua-

tion will vary depending on system requirements. In critical

applications the signal components above f

CLK/2 will have to

be attenuated at least to the filter’s residual noise level.

DS005064-11

FIGURE 1. Schmitt Trigger R/C Oscillator

DS005064-12

VIH ≥ 0.8 Vcc

VIL ≤ 0.2 Vcc

Vcc = V

+ −V−

FIGURE 2. Split Supply Operation with CMOS Level Clock

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