MPhil thesis of Lo Chi Wa - Department of Electronic & Computer ...

on the process variation. Therefore, a gain adjustment is required to make each LSB ofthe channel selection word to correspond to 200kHz frequency change.With the correct offset and gain adjustments, the 125 channel selection words willrepresent the exact 125 channel frequencies in the receiver band. The criterion tocalibrate the offset and gain adjustments is that the phase-locked loop works correctlywithin these 125 frequencies. The offset adjustment coefficient can be adjusted until theaverage value of the tuning voltage is the nominal value (0.55). Then the gainadjustment coefficient can be calibrated until the average tuning voltages of the first andsecond half of the frequency range are both around 0.55V. Fig. 133 shows the tuningvoltage against channel number with too large, too small and optimal gain adjustments.Only one calibration at the most beginning is required and the coefficients for the gainand offset adjustments can be always stored and reused.Tuning voltage0.55Vtoo large gaintoo small gainoptimal gainchannel 0channel 124Fig. 133 Tuning voltage vs. channel number with different gain adjustmentsThe measured tuning voltage of the varactors as a function of the channel number isshown in Fig. 134. The average value is 0.55V. The maximal variation of the tuningvoltage is 0.24V for total 1.2-MHz frequency error. It is due to a) the finite frequency125