I need to generate a sine signal using a VCO (voltage controlled oscillator) in order to feed a PLL (Phase-locked loop). I need the output to be 12.8MHz and 3.3Vp-p.

I bought at RS a VCO reference 7099357

Despite the datasheet says: "Output Level (CMOS) Output High (Logic “1”) - - 90% VDD" I can only have an output of 1.2Vp-p. Output frequency is 12.8 as expected. The IC is being feed with 3.3V DC.

What feature do I need to look at in order to select a componen that has the desired 3.3Vp-p output? Do you know any specific component I should use?

This is how I'm connecting the IC

enter image description here

VCO (RS-7099357) datasheet

  • \$\begingroup\$ Are you seeing the output with a scope or just measuring the frequency? Maybe you are exceeding the maximum output load capacitance, i.e. 15pF. That enable pin then, are you sure it goes to Vdd? I think it's more a mode select pin and should be connected to ground for normal operation. \$\endgroup\$ Apr 9, 2014 at 11:40
  • \$\begingroup\$ I'm measuring with a scope. I try to ground or open EN (pin 1) but I get the same result output: 1.2 peak to peak volts. The PPL needs 3.3Vp-p to work... \$\endgroup\$
    – Gmae
    Apr 9, 2014 at 14:39

1 Answer 1


Why are you using a VCO (variable) if you only need a fixed frequency?

In any case, according to the datasheet you linked, pin 1 should never be connected to VDD. In the VCTCXO variation, this is the control voltage, which must remain between 0.5 and 2.5 V. In the plain TCXO variation, it should either be left open or grounded.

  • \$\begingroup\$ Ya just beat me to it! This definitely looks like the culprit. \$\endgroup\$
    – gwideman
    Apr 9, 2014 at 11:52
  • \$\begingroup\$ Yeah, what I can't get is why the frequency is correct. And what about the clipped sine wave output? How can one achieve that? I'd find useful a bit of informations more. \$\endgroup\$ Apr 9, 2014 at 11:53
  • \$\begingroup\$ Exactly how are you measuring the output voltage and frequency? \$\endgroup\$
    – Dave Tweed
    Apr 9, 2014 at 11:58
  • \$\begingroup\$ @DaveTweed I'm using a scope to mesure the output. I try to ground or open EN (pin1) and I get the same result 1.2Vp-p at the output. Even if I go from 0.5V to 2.5V in the PIN1 I don't get different frequency... To tell you the truth I'm using VCO because I'm using an evaluation kit from TI ti.com/tool/SERDESUB-16OVT and I'm replacing the component they use to feed the PLL witch I guessed it was a VCO. Should I better use something like Wien bridge if I want a fixed frequency? \$\endgroup\$
    – Gmae
    Apr 9, 2014 at 14:34
  • \$\begingroup\$ @DaveTweed I cannot know if the IC i'm replacing is a VCO since I don't have any reference from the componen, I don't really know if it's a VCO or some kind of oscillator IC. What I know is that the attached picture is the actual schematic and that it outputs 30MHz and 3.3Vp-p. I'm just trying to feed the PLL with a lower frequency \$\endgroup\$
    – Gmae
    Apr 9, 2014 at 14:47

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