About VCO output waveform

Discussion in 'Wireless & RF Design' started by JiaZheng, Feb 20, 2013.

  1. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
    Hi everyone, I have done some experiment on VCO and I find a weird phenomenon such that when VCO operate at low frequency range (below 1MHz), it can give a square wave output, as expected. But when it operates at high frequency, the output is becoming a sine wave. I have no idea whether it is common feature among the VCOs
  2. bertus


    Apr 5, 2008

    What is the brand and type of the VCO's?
    It looks like that the slew rate is influencing the output wave shape.

  3. MrChips


    Oct 2, 2009
    A square wave is an infinite summation of sine waves, starting with the fundamental frequency f and adding every next odd integer frequency, 3f, 5f, 7f, ...

    If the system is unable to respond at higher frequencies, you are in effect reducing the high frequencies to the ultimate limit that you are left with the fundamental frequency f, which is a sine wave.

    Hence, a circuit may operate at 1MHz. If it is bandwidth limited to 1MHz you cannot generate any of the required harmonics at 3MHz, 5MHz etc. in order to produce a square wave. This is most noticeable on an oscilloscope that has a limited bandwidth. A perfectly good 5MHz square wave will look like a sine wave on an oscilloscope with 6MHz bandwidth.

    So the next obvious question is: What is the bandwidth of your oscilloscope and scope probe?
    Last edited: Feb 20, 2013
    farman99 likes this.
  4. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
    Sorry for no reply to you. I'm not familiar with this forum. It doesn't remind me there are some replies. I'm using VCO in CD74HC4046. What do you mean about the slew rate? Output slew rate?
  5. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
    What I'm using is a very common scope probe. I don't know its bandwidth but it must be small since it is very cheap. Also the oscilloscope has 50 MHz bandwidth. May be I should buy a good one and test it again. Thanks for your help!
  6. JiaZheng

    Thread Starter New Member

    Jan 28, 2013
    Forget to say. Thanks for your reply!
  7. Papabravo


    Feb 24, 2006
    The "slew rate" is a measure of how fast the output can change. The units are "Volts" divided by "time". You might see this in a datasheet expressed in "Volts/microsecond". It might also be expressed as an upper frequency limit.

    As you know CMOS logic operated above the frequency limit starts to look less like a switch and more like an amplifier biased into "Class A" or linear operation. Thus the sinewave output.