Question

When ____ of N zero crossings count becomes small, a signal is considered to be present.

(a) mean

(b) correlation

(c) standard deviation

(d) variance

I have been asked this question by my school teacher while I was bunking the class.

The doubt is from RF and IF Processing Tradeoffs in chapter Technology Tradeoffs in Software Defined Radio of Cognitive Radio

Answer 1

Right answer is (d) variance

Easiest explanation: When zero crossings are simply counted, a small variance of N zero crossings indicate a signal to be present. A disadvantage of this technique is the presence of high speed logic and lack of equalization.