*Susanne Nell*

*EDN*

The main purpose for building the circuit in Figure 1 is to study the idiosyncrasies of X5R, Z5U, and Y5V multilayer ceramic capacitors. The circuit is also an inexpensive VCO (voltage-controlled oscillator) with only five components.

Figure 1. |
This simple oscillator shows the effect of a dc bias on amultilayer capacitor, C _{2}. |

Many types of ceramic capacitors for surface-mount placement are on the market. The parts become continually smaller because of space problems on the board, and the capacitance values continually increase to compete with more expensive tantalum-electrolytic units. Unfortunately, capacitors with X5R, Z5U, or Y5V dielectrics have some undesirable properties. They exhibit voltage-dependent capacitance values. The idea behind the circuit in Figure 1 is to check the influence of a dc bias voltage on the frequency of a simple oscillator. The net result is a low-frequency VCO with a relatively large voltage-gain figure, which depends largely on the type of capacitor you use.

Figure 2. |
The frequency of the oscillator in Figure 1exhibits almost a 4-to-1 shift for a 4.7-µF Z5U multilayer capacitor. |

The circuit is a simple oscillator using a Schmitt-trigger inverter. The frequency is a function of R_{1}, C_{1}, and C_{2}. C_{2} is the ceramic capacitor with voltage-dependent capacitance. Using the value of C_{1}, you can shift the frequency independently of C_{2}. This design uses a stable-foil-type capacitor for C_{1} to avoid bias-voltage-dependent effects in the measured results. If necessary, you can compensate the temperature coefficient of the capacitor with a combination of NTC, PTC, and metal-film resistors for R_{1}. For measurements, this design uses a simple metal-film resistor. The capacitance change with temperature is normally less than 10% from 10 to 35 °C for Z5U and Y5V and much lower for X5R. Figure 2 shows the measured voltage-versus-frequency graphs with different values and types for C_{2}. For Figure 2, C_{1} = 10 µF; the orange curve represents a 4.7-µF, 10 V, Z5U multilayer capacitor, and the purple curve represents a 10-µF, 10V, Z5U multilayer capacitor. Figure 3 shows similar plots for values of C_{1} (orange: 1 µF; purple: 10 µF).

Figure 3. |
The value of C_{1} has little effect on the frequencycurves for the circuit in Figure 1. |

The moral of the story is: Be wary when using high-capacitance ceramic capacitors with high or variable dc bias; the varying capacitance can greatly influence circuit performance.

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