1. Field of the Invention
This invention relates generally to a method of trimming printed circuit board components, and, more particularly, to a method of changing the capacitance of a tuning capacitor used to tune the resonant output frequency of an oscillator circuit by removing material from the capacitor, where removing the material includes drilling or milling into a printed circuit board.
2. Discussion of the Related Art
Certain circuits must be accurately tuned to a particular output frequency during manufacture. For example, electronic tolling transponders must output a signal at an exact center frequency, such as 915 MHz, so that they perform their intended function. Other types of tuned circuits for other applications also must be tuned to a desirable output frequency for proper operation.
FIG. 1 is a schematic diagram of a representative oscillator circuit 10 that provides an output signal at a certain frequency, and is the type that can be used in a transponder. The circuit 10 includes a co-axial resonator 12 that primarily determines the oscillating frequency. The resonator 12 can be any suitable resonating device know in the art for the purposes described herein, such as a TEM or co-axial resonator. The oscillating signal appears at the collector terminal of an amplifying transistor 14 and is coupled to the resonator 12 through a coupling capacitor C.sub.5. The transistor 14 amplifies and sustains the oscillator signal and provides the buffered output frequency through the emitter terminal of the transistor 14 and an output capacitor C.sub.4. The output signal is fed back to the base terminal of the transistor 14 through the collector-to-base internal capacitance. Capacitor C.sub.3 swamps out the effects of the collector-to-emitter capacitance of the transistor 14.
A DC voltage potential is applied to a voltage divider network made up of resistors R.sub.1 and R.sub.2. The voltage divided DC signal is applied to the base terminal of the transistor 14 to bias it into the active region. Capacitors C.sub.1 and C.sub.2 provide AC filtering and noise reduction, and inductor L.sub.1 provides DC blocking to the power supply. Each of the components that are discussed above are typically discrete components that are surface mounted on a printed circuit board (PCB), such as a double-clad FR-4 circuit board, known in the art. Circuit boards of this type generally start out as opposing copper planes deposited on opposite sides of a fiberglass substrate, and all of the copper patterns are made by etching away all of the unused copper.
Because the natural oscillation frequency of the inexpensive resonators used in these types of devices can be affected by the circuit configuration and loading by the transistor and other circuit elements, the circuit 10 must be tuned to the desirable frequency during manufacture of the transponder. For this purpose, a tuning capacitor 16 is connected to the oscillation output of the resonator 12 to tune the output to the desired frequency. The capacitance of the capacitor 16 acts to reduce the output frequency of the resonator 12. When the capacitance of the capacitor 16 goes up, the frequency of the output signal goes down, and vice versa. Therefore, by setting the capacitance of the capacitor 16 to a certain value, the frequency output can be accurately tuned.
To provide capacitance tuning of this type, the resonator 12 is selected to have an oscillation frequency greater than the desired output frequency. Charts, look-up tables, and experimental data can be provided to identify the necessary tuning capacitance to reduce the frequency of the measured output of the untrimmed circuit 10 to the desired output when it is being manufactured. Various techniques are known in the art for setting the capacitance of the tuning capacitor 16 to provide the necessary capacitance. One technique includes adding additional capacitors in parallel as discrete components to reduce the frequency. However, readily available capacitors generally only come in minimum sizes of 0.5 pf, 1.0 pf, 1.2 pf, etc. Tuning tolerances sometimes require the added capacitance to be in increments of 0.1 pf or even 0.05 pf to attain the desired output frequency.
To overcome the problem of the limited sizes of available capacitors, a laser trimming technique has been developed in the art the reduce the actual plate size of a tuning capacitor so that it provides the desired capacitance for the proper resonant frequency. Specialized laser trimming capacitors are available as discrete components that can be trimmed by a laser beam to reduce the capacitor plate size, and thus, its capacitance. Once again, charts or look-up tables can be provided to identify how much material needs to be removed, to provide a certain capacitance. In one specific laser trimming example, the laser beam reduces the size of only a top plate of the capacitor to achieve the desired tuning capacitance.
Laser trimming is an effective way of altering the capacitance of a tuning capacitor for the purposes described above. However, laser trimming machines are expensive (approximately $250 K and up) and take up valuable space in the manufacturing facility. Improvements can thus be made in the art to provide capacitor trimming at a reduced expense. It is an object of the present invention to provide such improvements.