Load Capacitance for an Oscillator

Discussion in 'Circuit Help' started by Smooth, Jan 7, 2017.

  1. Smooth

    Smooth

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    Hi,
    Recently I've purchased a crystal oscillator from IQD frequency products, (IQXT-210, I attached the documentation).
    While setting up the circuit to check the oscillator's performance with an oscillometer, I bumped into something, hope you guys could help me.

    IQXT-210 is a 10MHz TCXO with a clipped sine output and the device expects to see a load of 15pF / 10kOhms // 10pF. I don't understand this load description. I searched the net, but that didn't help.
    So my questions are: How do you interpret their load description? How do you set up this type of load?

    Thanks in advance.
     

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    Last edited: Jan 7, 2017
    Smooth, Jan 7, 2017
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  2. Smooth

    davenn Moderator

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    can you please post the info here or link to it where people DONT have to sign up to a site to view it
     
    davenn, Jan 7, 2017
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  3. Smooth

    Smooth

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    yeah, sure, my bad. I edited the original post.
     
    Smooth, Jan 7, 2017
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  4. Smooth

    davenn Moderator

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    not sure if that means it has an internal cap across the output or that it needs a cap across the output in parallel with the actual load or, finally, the actual load that the Osc. drives needs to represent a 10/15pF load.

    I will ask around as well, if I get an answer I will post :)


    Dave
     
    davenn, Jan 7, 2017
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  5. Smooth

    hevans1944 Hop - AC8NS

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    It is designed to drive HCMOS, so the spec is probably the maximum capacitance allowable for the load to guarantee the rise time spec. The oscillator output would normally be buffered by a gate or HCMOS driver to distribute the clock. Don't use long PCB traces over a ground plane to connect the oscillator to whatever it is intended to drive.

    Notice there are four versions, with and without "pulling" control input, two versions for HCMOS loads and two versions for clipped sine waves (probably intended for a phase-lock loop control input). The signal swing for clipped sine waves is only 0.8 V with a 10 kΩ load in parallel with 10 pF.
     
    Last edited: Jan 7, 2017
    hevans1944, Jan 7, 2017
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  6. Smooth

    Smooth

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    I see, thank you! Question: why do they mention the 10kOhms reactance then?
     
    Smooth, Jan 7, 2017
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  7. Smooth

    hevans1944 Hop - AC8NS

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    Didn't see any reference to reactance. The maximum load with the clipped sine version is 10 kΩ resistive in parallel with 10 pF capacitance. More resistance, less capacitance in the load in preferable. The HCMOS version is expected to drive the very high impedance of an HCMOS gate with a maximum of 15 pF distributed capacitance to meet 8 ns rise/fall time spec.

    What are you going to DO with this little gem? If you have the "pulling" version you could phase-lock it to a rubidium or cesium atomic clock for an ultra-stable oscillator signal. Are you in over your head on this one?
     
    hevans1944, Jan 7, 2017
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