Can u people clarify the below mentioned queries?

1.If we have designed the low pass filter for the cut off frequency (ex. fc=10khz), also the noise has been occurred in the range of 10khz i.e. noise has some phase shift with required signal. How can we filter it?

2.Semiconductor materials, which have the property of PTC AND NTC.

(Positive Temperature Coefficient and Negative Temperature Coefficient)

How the electronics industry made this property?

3.Fourier Series: We know that all waves can be represented in terms of Sine and Cosine waves. Why cant we represent the waves in terms of?

*Square and saw tooth wave (or)

*Square and triangle wave

4.In Time domain, we can analyze Amplitude Vs Time,

In frequency domain, we can analyze Amplitude Vs Frequency.

For which domain we will analyze Amplitude Vs Time Vs Frequency?

Thanks in advance.

Karuna