LMF60 datasheet(17/20 Pages) NSC

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20 Designing with the LMF60 (Continued)

24 CHANGING CLOCK FREQUENCY

INSTANTANEOUSLY

The LMF60 will respond well to a sudden change in clock

frequency Distortion in the output signal occurs at the tran-

sition of the clock frequency and lasts approximately three

cutoff frequency (fC) cycles As shown in Figure 12 ifthe

control signal is low the LMF60-50 has a 100 kHz clock

making fC e 2 kHz when this signal goes high the clock

frequency changes to 50 kHz yielding 1 kHz fC

The transient response of the LMF60 seen in

Figure 13 is

also dependent on the fc and thus the fCLK applied to the

filter The LMF60 responds as a classical sixth order Butter-

worth lowpass filter

TLH9294 – 27

fIN e 15 kHz (Scope Time Base e 2 msDiv)

FIGURE 12 LMF60-50 Abrupt Clock Frequency Change

25 ALIASING CONSIDERATIONS

Aliasing effects have to be taken into consideration when

input signal frequencies exceed half the sampling rate For

the LMF60 this equals half the clock frequency (fCLK)

When the input signal contains a component at a frequency

higher than half the clock frequency as in

Figure 14a that

component will be ‘‘reflected’’ about fCLK 2 into the fre-

quency range

below fCLK 2 as in Figure 14b If this compo-

nent is within the passband of the filter and of large enough

amplitude it can cause problems Therefore if frequency

components in the input signal exceed fCLK 2 they must be

attenuated before being applied to the LMF60 input The

necessary amount of attenuation will vary depending on

system requirements In critical applications the signal com-

ponents above fCLK 2 will have to be attenuated at least to

the filter’s residual noise level An example circuit is shown

Figure 15 using one of the uncommitted Op-Amps avail-

able in the LMF60

TLH9294 – 28

FIGURE 13 LMF60-50 Step Input Response

Vertical e 2VDiv Horizontal e

1 msDiv fCLK e 100 kHz

TLH9294 – 29

(a) Input Signal Spectrum

TLH9294 – 30

(b) Output Signal Spectrum Note that the input signal at

fs 2 a f causes an output signal to appear at fs 2 b f

FIGURE 14 The phenomenon of aliasing in sampled-data systems An input signal whose frequency

is greater than one-half the sampling frequency will cause an output to appear

at a frequency lower than one-half the sampling frequency In the LMF60 fs e fCLK

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