Jefferay Lawton, Microchip Technology
Varactor (or “varicap”) diodes are used primarily in radio-frequency (RF) circuits to provide a capacitance that can be varied by changing the applied voltage. These types of diodes often are used for tuning circuits, such as RF oscillators and filters found in wireless applications like wireless microphones and radios. Designers, then, should know about the benefits of using a nonvolatile digital-to-analog converter (DAC) to provide the biasing voltage of a varactor diode used as a voltage-controlled capacitor.
The varactor diode is operated under reverse bias, which creates a depletion zone around the P-N junction. Changing the level of the reverse bias changes the thickness of the depletion region and, thus, the effective capacitance of the diode. Increasing voltage causes a decrease in capacitance.
Varactor diodes are specified with a nominal capacitance value and the range of capacitance that can be achieved with a maximum and minimum voltage level. Increasing the bias voltage range increases the capacitance range available, but designers can also look for varactors with a larger capacitance- to-voltage ratio.
A convenient solution for creating a varying bias voltage is to use a DAC. Most DACs have an output voltage range of 0 V to +5.5 V. If a higher voltage bias is required, though, then a high-voltage DAC can be used. However, it may be more cost-effective to use a low-cost, high-voltage operational amplifier in a non-inverting configuration to provide level shifting of the output voltage from a common 5.5-V DAC.
The LC-tank circuit portion of a voltage-controlled oscillator allows for FM modulation in wireless microphones and radios. Its back-to-back varactor configuration minimizes the effects of RF modulation.
Using a DAC does introduce sources of potential error. The varactor is affected by any form of amplitude variation of the bias voltage, resulting in an undesired shift in capacitance. Deterministic errors can be accounted for when using the microcontroller to program the DAC output voltage. The primary sources of error that should be considered include varactor nonlinearity, offset errors, and DAC integral nonlinearity (INL). RF modulation may also be caused by voltage induced from a noise source – perhaps from an antenna in the system. The figure shows an LC-tank circuit portion of a voltage-controlled oscillator. This circuit allows for FM modulation in the aforementioned wireless microphone or radio.
Here, a back-to-back varactor configuration minimizes the effects of RF modulation. If a varying signal is injected, the bias across one diode increases as the other decreases, keeping overall capacitance unchanged. Note that the two diodes are in series with each other, so capacitance is half of a single varactor setup.
To also prevent RF signals from affecting the circuitry outside the tuning circuit, the bias voltage is fed through an isolation resistor or an RF choke. There are other benefits to using a DAC to bias a varactor diode. For example, multiple-output-channel DAC devices can be used in a multistage application. Additionally, in a four-channel DAC, three channels could potentially be used for separate band-pass filters for low-, mid-, and high-frequency filtering. The fourth output could be used for offset voltage calibration elsewhere in the circuit, or it could be turned off when it isn’t in use. Space and design time can be saved by avoiding having to set up separate biasing schemes.
Some DACs, such as the MCP4728, also offer on-board nonvolatile memory, which can store configuration data such as output-voltage levels and channel status (on/off). This enables the device to be reset or powered up into a known set state, which could allow a pre-programmed tune to be stored. The tune could be recalled when a desired event or input occurs or when power is lost and restored.