S
Member
Our VCO frequency shifts 2MHz with supply ripple. Characterizing pushing properly is painful. What's your standard setup for measuring VCO pushing/pulling?
VCO pushing/pulling - how do you characterize it?
L
Admin
Pushing and pulling are different mechanisms, so treat the setups as two separate experiments.
Pushing (supply → f₀): modulate or step Vcc with a low-impedance source, keep the load and tuning voltage fixed, and plot Δf / ΔV (often in MHz/V). Use a quiet supply and a spectrum analyzer or frequency counter with enough averaging; ripple on the supply line itself will fake pushing. Quote the frequency offset or modulation rate if you use AC ripple, because pushing can be frequency-dependent.
Pulling (load → f₀): keep supply and Vtune fixed, and vary the load reflection (sliding short, tuner, or a set of known |Γ| loads) while watching f₀. Pulling is usually quoted vs VSWR or |Γ|. Buffer stages and isolators exist mainly to kill this term.
A practical check: if "2 MHz with supply ripple" collapses when you harden decoupling at the VCO pin but not when you pad the RF output, you are mostly seeing pushing, not pulling.
Open question: at what offset from the carrier do you care about pushing most — close-in PN, or far-out spurs from switching regulators?
— rfforum 编辑部 / Editorial Team
Pushing (supply → f₀): modulate or step Vcc with a low-impedance source, keep the load and tuning voltage fixed, and plot Δf / ΔV (often in MHz/V). Use a quiet supply and a spectrum analyzer or frequency counter with enough averaging; ripple on the supply line itself will fake pushing. Quote the frequency offset or modulation rate if you use AC ripple, because pushing can be frequency-dependent.
Pulling (load → f₀): keep supply and Vtune fixed, and vary the load reflection (sliding short, tuner, or a set of known |Γ| loads) while watching f₀. Pulling is usually quoted vs VSWR or |Γ|. Buffer stages and isolators exist mainly to kill this term.
A practical check: if "2 MHz with supply ripple" collapses when you harden decoupling at the VCO pin but not when you pad the RF output, you are mostly seeing pushing, not pulling.
Open question: at what offset from the carrier do you care about pushing most — close-in PN, or far-out spurs from switching regulators?
— rfforum 编辑部 / Editorial Team
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