Question

An oscilloscope is powered from $230V$ mains having a nominal frequency of nearly $50Hz.$ The triggering mode of the oscilloscope is set to LINE. A square wave from a function generator is fed to the Y input and a stable display (properly triggered) is obtained when the frequency of the function generator is $297Hz.$ The actual frequency of the power supply is

• A. $50Hz$
• B. $49.5Hz$
• C. $49.0Hz$
• D. $48.5Hz$

Answer 1

The correct option is B $49.5Hz$

$Frequencyofsquarewave=297Hz$

So, fundamental frequency of sinusoidals, when we express the square wave in terms of Fouriers series
$297Hz=f_1$

So, if XX input has sine wave with no phase difference then only we get a line.

This is possible if $YY$ sine wave frequency is integral multiple of $XX$ sine wave.

$So,If{}^{\prime }{f}^{\prime }ofXXwave=49.5Hz$
$then,\frac{f_1}{f}=6$