What is Frequency Pushing?

Frequency Pushing in a VCO or VCXO is the change in the output frequency due to a change in supply voltage while the control voltage remains constant. A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency can be controlled by an input voltage (control voltage). A VCO usually requires a supply voltage which is used to power/bias the device and then a control voltage that is used to vary the output frequency of the oscillator. The output frequency of the VCO is directly proportional to the control voltage.

In an ideal world, the supply voltage to a VCO remains constant. However, in the real world there can be slight fluctuations. The change in output frequency due to a change in the supply voltage of the VCO is called Frequency pushing.

Frequency pulling is usually represented in Hz/mV. Let us take an example here - If the supply voltage of a VCO increase by 50 mV, and the frequency pulling of the VCO is 200 Hz/mV, then the output of the VCO would change by 10,000 Hz or 10 KHz.

Therefore, it is important to minimize any fluctuations in supply voltage to minimize output frequency pulling in VCO.

How to measure VCO Frequency Pushing?

As already discussed, frequency pushing in a VCO is an outcome of the sensitivity of a VCO to fluctuations in voltage supply. Therefore, to measure the frequency pushing, we need to initially set the supply voltage to the recommended value and note the VCO frequency for a particular tuning voltage. Next, the supply is varied and VCO frequency is recorded for the different voltage levels. This process is then repeated for different tuning voltages as frequency pushing can be different for tuning voltages.