Question

A 10 kHz even-symmetric square wave is passed through a bandpass filter the centre frequency at 30 kHz and 3 dB passband of 6 kHz. The filter output is
1. a highly attenuated square wave at 10 kHz
2. nearly zero
3. a nearly perfect cosine wave at 30 kHz
4. a nearly perfect sine wave at 30 kHz

Answer 1

Correct Answer - Option 3 : a nearly perfect cosine wave at 30 kHz

The given signal is 10 kHz even symmetrical square wave.

Since the bandpass filter is centered at 30 kHz it follows half-wave symmetry. Hence only odd harmonics exist.

The frequency components present in this wave: 10 kHz, 30 kHz, 50 kHz, 70 kHz.

Now, 30 kHz component will pass through filter output is nearly perfect cosine wave at 10 kHz cosine as the signal is even signal.