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How have you measured this frequency? With an oscilloscope? Which
bandwidth does it have? What probe do you use?
Normally frequencies in this range have to be measured with special
equipment. If you use an oscilloscope with a standard probe, the
capacitance is that large that higher frequencies are damped. So your
signal appears to be a sinus (50MHz), because all overtones (that make
a square 150MHz, 250MHz, ...) are damped.