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Wave form in mixer input ports

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STOIKOV

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In a double-balanced Gilbert mixer, there are two input ports RF and LO.
For local oscillator (LO) input, the wave form has to be sinusoidal or square ?
Depends is mixer is working as down-converter or up-converter ?
 

flatulent

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For best gain the LO level should be high enough to turn the transistors full on to full off. This can be done with a high level sine wave or a lower level square wave. Since square waves are difficult to generate at microwave frequencies, sine waves are used.

One thing about the nomenclature is that this mixer topology goes back to 1950 when Gilbert was in grammar scchool.
 

    STOIKOV

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STOIKOV

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thanks for your reply,
One more doubt about input signals, if I simulate or characterize a Gilbert cell, RF signal must have a DC component in order to bias the MOS. What about LO signal ? Just levels 0 to 1 if sqare wave or sinusoidal without DC component; or must have DC component ?
 

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You need a DC level also on the LO signal. All transistors should be in the active ( decent Vce or Vds) when both the LO and RF AC components are zero and the DC components are present.
 

    STOIKOV

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STOIKOV

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I see, so if I am designing an integrated ring VCO as local oscillator to apply its signal to the LO port, how can I adjust the DC value of the output signal ? Aditional blocks like opamps ?
Will you have an schematic that shows wave forms and biasing ? Most of papers literature just show circuits without biasing.

Be well.
 

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