Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
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.