March 27, 1997

Drive high-voltage varactor diodes from just 5V

Steve Hageman, Hewlett-Packard Co, Santa Rosa, CA

You can build nearly any analog circuit from 5 or 3V-only parts; that is, until you try to design a 5V-only, RF VCO using varactor diodes. Because these diodes frequently need drive voltages of 20V or more to get the most out of their tuning range, they always pose problems for designers. A varactor diode is one of those elements that you just can't tweak to run at lower voltages--device physics firmly sets the limits.

The usual approach is to add another low-current output to whatever system power supply you have or to add a dedicated dc/dc converter to your circuit. However, an alternative is to use one of the available photovoltaic-isolator chips. These chips use optoisolator/solar-cell technology to provide an isolated, floating voltage supply of as much as 10V/C. Compared with a modular dc/dc converter, this approach has lower current-output capability and higher component count. However, the isolator circuit doesn't pollute your system with another clock source that needs filtering.

Figure 1a shows how to connect two International Rectifier (El Segundo, CA) PVI1050s in series to provide a 20V tuning drive for the varactor diode. Each PVI1050 has two 5V outputs, which, in series, form a 10V output. This circuit connects two PVI1050s to achieve a 20V output. Rise and fall times of the circuit to a 0 to 5V input step are less than 30 msec each.

The varactor diode in this case controls the frequency of a 20- to 32-MHz VCO circuit. Figure 1b shows the tuning linearity of the driver/VCO circuit in this configuration. Tuning the diode from a 5V-only system voltage reduces the tuning range to less than 20 to 26 MHz.

For this circuit to work, you must filter any RF currents from getting back to the photovoltaic stack. Because of the high impedance at the PVI1050 outputs, any RF current that leaks back can get rectified and cause abnormal circuit behavior. You can use resistors, ferrite beads, and feedthrough and bypass capacitors to limit any RF currents from leaving the VCO circuit. Limiting the current is good practice in any RF circuit because the total circuit is less sensitive to emitting and receiving unintended radiation.

Many RF systems, such as CATV tuner modules, use a negative varactor drive. You can adapt the circuit in Figure 1 for these systems by switching the PVI1050 output leads to generate a negative voltage. You can stack another PVI1050 on the circuit to generate 30V for CATV modules that require a 26V tuning voltage. (DI #2004)

| EDN Access | Feedback | Table of Contents |