enter image description hereI'm looking at the datasheet for the analog devices hmc431lp4 voltage controlled oscillator, and I can't tell if its output waveform is a sine wave or square wave. The datasheet doesn't label the oscillator as being harmonic or relaxation. I'm wondering because I am trying to understand a homemade radar system I found online. Do FMCW systems like this project require a sinusoidal RF signal to operate?

Link to datasheet.

Link to project I'm looking at.

  • \$\begingroup\$ Just added a screenshot from the front page of the datasheet \$\endgroup\$
    – Saunders
    Aug 24, 2019 at 20:27
  • \$\begingroup\$ I don't know the answer but the harmonics figures should give a clue. I notice that it is rated at > 500 W which, I suspect, should be mW. \$\endgroup\$
    – Transistor
    Aug 24, 2019 at 20:36
  • \$\begingroup\$ So because the datasheet includes information on output harmonics the oscillator most likely produces a sinewave? \$\endgroup\$
    – Saunders
    Aug 24, 2019 at 20:41
  • \$\begingroup\$ I would think so. A squarewave wouldn't have any even harmonics, only odd. \$\endgroup\$
    – Transistor
    Aug 24, 2019 at 20:45
  • \$\begingroup\$ @Saunders an ideal square wave would have no 2nd harmonic and around -5dB third harmonic. The fact that the third harmonic is below the 2nd, and 30dB below the fundamental, indicates that they're not going for anything remotely square-ish. Everything else, put together, implies "pretty much sine". \$\endgroup\$
    – hobbs
    Aug 25, 2019 at 18:02

2 Answers 2


This VCO outputs a frequency between 5.5 and 6.1 GHz.

It is sort of "implied" that the output signal will be a sinewave because:

  • a "decent" square wave at that frequency would need a signal bandwidth of several hundreds of GHz. That's because a square wave relies on harmonic frequencies (multiples of the base frequency) to become "square". Read up on Fourier analysis to understand this

  • this chip is designed to be used in RF transceivers, usually as a local oscillator for mixing up/down RF signals. This works best using sinewaves as RF designers want one frequency to deal with, not one frequency and all its harmonics (like a square wave would have).


Neither. The second and third harmonics are specified in the part of the datasheet page which you cut off. Writing it like this clearly indicates that the harmonics are considered to be spurious to a sine wave. So even if you would actually design a circuit layout which let's say is able to transport up to e.g. 20GHz BW, you would not see a "rectangle", since the second harmonic is already significantly suppressed.


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