EDN Access -- 01.20.94 Circuit eliminates hum from pulsed signal

-January 20, 1994

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Design Ideas:January 20, 1994

Circuit eliminates hum from pulsed signal

Stanislav Tomek,
Slovak Technical University, Slovak Republic

The circuit in Fig 1 eliminates hum from a periodic signal by a factor of as much as 100:1. Fig 2a shows an incoming signal that combines a periodic square wave and a much lower frequency hum. Suppose for similicity's sake that the hum is sinusoidal.

The input signal has two envelopes that follow the hum (Fig 2a). The circuit separates these two envelopes and then subtracts them, yielding the waveform in Fig 2b. This signal has an ac and a dc component. A capacitor eliminates the dc component, and the circuit splits the signal into the waveforms in 2c and 2d, inverting negative excursions to get the signal in 2c.

Notice that the sum of Fig 2c and Fig 2d is the inverse of the hum. Therefore, summing Fig 2a with Fig 2c and Fig 2d yields the reconstructed pulse signal in Fig 2e.

The analog multiplexer, IC1, in Fig 1 separates the signal envelopes. The op amp IC2 subtracts them from each other. The capacitor C1 removes the dc level from the signal. The analog multiplexer IC3 divides the signal into positive and negative parts. The inverter IC4 inverts the negative part of the signal. Op amp IC5 sums the signals to derive the output.

The external signal, which has the same frequency and phase as the pulse signal, synchronizes the analog multiplexers. Trimmers R1 and R2 optimize performance.

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