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Design Ideas |
The
frequency comparator in
Figure 1 uses two VCOs. C1,
R1+R2, and the voltage at pin 9 determine the frequency
of IC1 (900 Hz). C3, R5+R6, and
the voltage at pin 9 determine the frequency of IC2 (1580 Hz). If fIN is lower
than 900 Hz, then the output P2 of phase-comparator 2 in IC1 is high
and drives the inhibit input of IC2 high via the R4-C2
lowpass filter. Consequently, the VCO in IC2 turns off. fIN
is therefore higher than IC2's frequency (0 Hz), so the output P2
of phase-comparator 2 in IC2 goes low, pulling the output low via
the R8-C4 lowpass filter.
If fIN is higher than 900 Hz and lower than 1580 Hz, then the output P2 of IC1 goes low and enables the VCO in IC2. IC2's frequency is higher than fIN, so the P2 output of IC2 goes high. Simultaneously, Q1 removes the short circuit across R2, and the frequency of the VCO in IC1 decreases to 870 Hz. The ratio of R2 to R1+R2 determines the hysteresis of the first comparator.
If fIN exceeds 1580 Hz, then P2 of IC2 goes low; therefore, the output goes low and Q2 removes the short circuit across R6. The frequency of the VCO in IC2 then decreases to 1520 Hz. The ratio of R6 to R5+R6 determines the hysteresis in the second comparator. So, if fIN increases from 100 to 3000 Hz, for example, the output is high between 900 and 1580 Hz. If fIN decreases from 3000 to 100 Hz, for example, the output is high between 1520 and 870 Hz. (DI #1947)
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