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narrow impulse has all harmonics, square wave has no even harmonics, thus 2nd harmonic is due to asymmetry of square wave not being 50% duty cycle.
THere is direct correlation between null of 2nd harmonic and perfect symmetry. Usually -20dB is ok, -30dB is better, and -40dB is hard to get unless controlled impedances with R terminators or using PECL.
In Bipolar BJT's it is commonly caused by faster fall time than rise time., thus pullup R helps balance rise/fall times at some compromise in power dissipation etc.
In bipolar FET's N type is a bit lower RdsON so may be faster than P type of same family with same load capacitance, but it all depends.
Newer 3V CMOS it has good symmetry and impedance <50Ω, rather than 74HC types which range from 300~200Ω for 12 to 5V. .. and higher for older technology CMOS at lower voltage.
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