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PLL filter blocks undesired frequencies

Stephen Kamichik, Ile Bizard, PQ, Canada


You often need to block signals of specific frequencies; of these frequencies, 50- or 60-Hz line frequency is the most common. You can use the PLL notch filter in Figure 1 to block unwanted frequencies. IC1, an LM567C, is a tone decoder. Components C1, R1A, and R1B determine the frequency, F, that IC1 detects:


PLL filter blocks undesired frequencies
Figure 1. A tone decoder and a switch block frequencies that external components determine.

When you feed frequency F to Pin 3 of IC1, the output, Pin 8, goes low because the output transistor in IC1 is saturated.

The LM567 decoder comprises an inphase and quadrature detector, which a VCO (voltage-controlled oscillator) drives. The VCO determines the decoder’s center frequency. The bandwidth of the decoder is

where V is the rms (root-mean-square) input voltage and C2 is capacitance in microfarads. The bandwidth is a percentage of the frequency.

The tone decoder’s output runs to the control pin, Pin 13, of IC2, a CD4066 quad bilateral switch. The input voltage connects to the CD4066’s input pin, Pin 1. That signal controls the switch. The CD4066 switch is closed, or on, when the control pin is high at logic one and open, or off, when the control pin is low at logic zero. When IC1 detects the unwanted frequency – in this case, 60 Hz – IC1’s Pin 8 and, thus, IC2’s Pin 13 go low. That action opens the switch, which blocks the signal with the unwanted frequency (Figure 2).

Figure 2. The components in Figure 1 block frequencies of approximately 60 Hz.

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