Adding a bit more to this:
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What John Woodgate is talking about, is the special function named "MODULATE" (and its counterpart "MODULATE2") which you can find by typing "MODULATE" into LTspice's component selector menu.? They are in LTspice's [SpecialFunctions] folder.? It models an ideal VCO.? Its output voltage is a sine wave whose frequency is a linear function of the voltage applied to its "FM" input pin.
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Use that to mimic your current-controlled oscillator.? Turn your current into a voltage.? Use either a small resistor, or a B-element, or H-element (CCVS) to get from current to voltage.
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Apply the sum of the two input signals (your "Tune" and "FM" signals) to the MODULATE's "FM" input pin.
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To include a nonlinear transfer function, add a pre-distorter before the MODULATE device's FM pin.? It can be a B-source with a Table() function to make it nonlinear.
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See the example circuit PLL.asc that comes with LTspice, in the ...\examples\Educational folder.? Right-click on the MODULATE device and add these two parameters to any of the Value, Value2, SpiceLine, or SpiceLine2 lines:
- mark=frequency1
- space=frequency0
where frequency0 is the VCO's frequency when the FM voltage = 0, and frequency1 is the frequency when the voltage = 1 V.? Those two set the offset and scale of the VCO's linear transfer function.? Note this refers to the voltage applied to the MODULATE device's "FM" pin, which is NOT the same as your "FM" signal you referred to in your question.
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The MODULATE's "AM" pin should be left floating.? Or connect it to a voltage to control the sine wave's amplitude.
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Andy
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