Basic difference between PLL Frequency Pushing vs PLL Frequency Pulling
This page describes basic difference between PLL Frequency Pushing and PLL Frequency Pulling. It mentions how to measure VCO FREQUENCY PUSHING and PULLING. Both of these parameters are very useful in the design of RF frequency synthesizer using VCO, frequency divider and loop filter.
VCO is the short form of Voltage Controlled Oscillator. It takes control voltage and supply voltage as inputs and delivers sine wave of certain frequency as output. By keeping the control voltage and supply voltage to certain constant values, drift in the output can be avoided.
Here term frequency pushing refers to a change in VCO frequency due to a change in the power supply voltage. Typical value for one of the minicircuit VCO device is 0.30 MHz/V.
Here term frequency pulling refers to a change in VCO frequency due to a change in the load on the VCO output. Typical value for one of the minicircuit VCO device is 0.80 (MHz) peak-to-peak at 12dBr.
How to measure VCO FREQUENCY PUSHING
As mentioned VCO pushing is result of sensivity of VCO with respect to voltage supply.
The figure-1 depicts measurement set up for the VCO frequency pushing.
Following are the steps for the measurement:
• Initially supply voltage is set to normal value and VCO frequency is noted.
• Supply is varied and VCO frequency is recorded for these different voltages.
• Supply voltage is incremented by 1 volt and VCO frequency is recorded/measured for different tuning values.
• Supply voltage is decremented by 1 volt and frequency is recorded for different voltages one more time.
• At one tuning voltage the change in frequency caused due to 1 volt change will give frequency pushing figure of VCO.
How to measure VCO FREQUENCY PULLING
As mentioned VCO pulling is result of non ideal load connected with VCO. The figure-2 depicts measurement set up for the VCO frequency pulling.
As pulling is different and vary with frequency, it is measured at various points over operational bandwidth. Following are the steps for the measurement of VCO pulling.
First make the set up as shown and use bias tee which separates RF and DC. Hence
RF outout is measured using spectrum analyzer as shown.
• Tune the phase shifter device to get the maximum ( OR minimum) frequency
with short at the end of RFoutput line.
• Now obtain reflected phase change of 180 degree by replacing short with open.
• Again Fine tune phase shifter to obtain minimum (or maximum) frequency.
• Record the VCO frequency pulling using peak search in the spectrum analyzer.
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