Compared to VCO calibration, Kvco cal is much more straightforward. First, what is Kvco? Kvco is VCO’s frequency/voltage gain in unit of Hz/V. It measures how much VCO output frequency changes relative to VCO’s input control voltage (Vt or Vtune) change. As shown in below diagram, Kvco is the slope of the curve of Frequency vs Vt.
Next why do we need to calibrate Kvco? Simple, Kvco can’t be too small otherwise slow VCO response and narrow output frequency adjust range. Kvco can’t be too large either otherwise VCO output phase noise will worsen.
How to calibration Kvco? Below is the circuit. PLL works in closed loop.
- set PLL frequency divider so that fvco = ftarget + Delta. Wait until PLL settles and then use ADC to measure Vtune as Vt1.
- set PLL frequency divider so that fvco = ftarget – Delta. Wait until PLL settles and then use ADC to measure Vtune as Vt2.
- firmware calculates Kvco = (Vt1-Vt2)/2/Delta and then programs some PLL control register with this result to adjust Kvco.
In above, Delta can be 1Mhz, 10Mhz, etc. whichever fits your design.