Meaning
Voltage sensitivity describes the undesirable shift in an oscillator’s output frequency caused by a change in its applied DC supply voltage. In radio frequency transmitters and phase locked loops, frequency pushing can introduce unwanted phase noise and modulation distortion if the power supply is not perfectly regulated. This sensitivity is usually expressed in megahertz per volt and varies across the tuning range of the oscillator.
It represents a critical design constraint for systems with high dynamic current loads.
Mechanism Origin
Changes in the supply voltage alter the internal capacitance of the oscillator’s active devices. This variable junction capacitance directly changes the resonant frequency of the tank circuit. Active voltage regulation is required to suppress this supply induced drift.
Regulation Strategy
Minimizing the impact of this frequency drift requires the integration of a dedicated low dropout regulator next to the oscillator. This regulator must provide high power supply rejection ratio at the switching frequency of the main system converter. Placing local decoupling capacitors closer to the supply pin further reduces the high frequency voltage ripple.
These design choices isolate the sensitive tuning circuit from transient loads on the main power rail.
Testing Procedure
Measuring this parameter involves stepping the supply voltage by a known increment and recording the resulting frequency change on a spectrum analyzer. The test is performed with the tuning voltage held constant to isolate the power supply effects. Dynamic testing can also be performed by applying a small sinusoidal ripple to the supply line to measure the resulting sidebands.
This characterization helps engineers specify the required noise floor for the power supply, preventing unwanted spurious emissions in the final product.
