Electronic Design

# PLL Compares Capacitors

With this simple test circuit (see the figure) , a given capacitor's (Cx's) value can be instantaneously revealed as being higher, lower, or within the percentage limits compared with a reference capacitor (CREF). As a result, the tester, which is built around the commonly used and inexpensive CMOS PLL CD4046, can be used to select capacitors for use in such circuits as active filters, PLL circuits, tone decoders, and so forth.

Capacitors Cx and CREF decide the ON and OFF periods, respectively, of the square wave at the VCO's output. Integrator R4 C1 develops a dc voltage proportional to the square wave's duty cycle. Comparators a and b compare the dc voltage against the two adjustable reference levels VREF-HI and VREF-LO. The three LEDs (two red and one green) indicate the test results.

If capacitor Cx = 1.01 CREF , then the ON period (tON), OFF period (tOFF), duty cycle (n), and integrator output (VINT) will have the following relationships:

tON = 1.01tOFF , n=1.01/2.01=0.502, VINT = nVDD = 6.024 V

Similarly, for Cx =0.99CREF: tON =0.99tOFF, n=0.99/1.99=0.497 and VINT =5.964 V

Hence, for a comparison check that is \[plus-minus sign\]1%, the two reference voltages are: VREF-HI = 6.024 V and VREF =5.964 V.

When tested, it was found that the accuracy for lower capacitor values is limited by the stray capacitor values is limited by the stray capacitance inherent to the circuit. For higher values (lower VCO frequencies), accuracy is limited by the ripple voltage overriding the dc voltage at the integrator's output. The ripple voltage makes two LEDs turn on simultaneously.

During testing, it was found that the device compared capacitors in the range of 100 pF to 10,000 pF for a comparison limit of ±1%.