Increase the frequency range of the RC voltage-controlled oscillator

　　A typical voltage-to-frequency converter is also called a VCO (voltage-controlled oscillator), where the input voltage of the IC has a simple regulation characteristic for the output frequency. Its general form is F=kV/RC, where RC is the time constant of the associated timing resistor and capacitor. These devices have a wide range of output frequencies, but few can be tuned over the entire range of a set of RC time constants. However, if the timing ratio is changed with changes in input voltage, an implementation method can be used to expand the tuning range to almost the entire frequency range.

　　One way to achieve this is to replace the timing capacitor with a variable capacitor whose value changes inversely with its bias voltage, known as a varactor diode. For this design, the Analog Devices AD654 voltage-to-frequency converter was considered because it is simple and has a bandwidth of at least 1MHz.

　　Figure 1 shows a typical implementation using a fixed resistor and capacitor. For the values ​​in the figure, when the input changes from 0V to 10V, the frequency range is approximately 10Hz to 30kHz. After replacing the timing capacitor with an NTE618 hypermutation varactor (as shown in Figure 2), the same 0V to 10V input range can obtain a tuning range of approximately 10Hz to 1MHz or more.

　　Figure 3 compares the tuning curves for the two converter topologies. Note that the range is increased considerably, but at the expense of linearity, which also affects temperature stability. In summary, accuracy is traded for tuning range, which should be acceptable in basic applications where no exceptional accuracy is required.

　　Hyperabrupt varactors can achieve large frequency changes with small bias changes because they have a large capacitance ratio. For some hyperabrupt varactors, the ratio can be as high as 15, for example the NTE618 is a hyperabrupt varactor used in AM receivers. Since the converter frequency increases at higher voltages, the capacitance decreases, thereby increasing the frequency. This combination of responses produces a wide tuning range. The 0.01μF coupling capacitor isolates the bias voltage of the varactor from the operating voltage of the converter core. Lightly biasing the varactor with a large 1MΩ resistor avoids adding load to the oscillator.

　　This behavior is somewhat calculable and predictable, and can even be done from a data sheet. The tuning curve for a varactor diode can be generated in Microsoft Excel. This information is then used in the voltage-to-frequency conversion equation for the converter. For the NTE618, the approximate relationship between capacitance and voltage is:

　　Figure 4 shows the similarity between the calculated and measured values. The difference is greater at higher frequencies because the capacitance of the varactor diode is reduced to a level comparable to the circuit and device stray capacitance. Careful layout can minimize this problem and increase the range.

　　Note that at low input voltages, the varactor response and the fixed capacitance converter response are almost identical, since the varactor has an inverse exponential relationship to voltage. Achieving this range has the useful consequence of extending the tuning range without the need for switches between converters. Other useful and interesting applications can be explored using this approach in conjunction with a phase-locked loop, modulator, or function generator.