Qmf

1*10 RC

1*10 RC

^M35

4 V? " printed circuit card. The power supply and regulator components are mounted on a "cut to fit the chassis" perfboard, seen just above the transformer. The power-on switch, meter, function-selector switch (frequency or capacity), input BNC connector, and "unknown capacitor" (Cx) terminals are all on the front panel. The PC card is plugged into a 22-pin edge connector that is mounted on two metal brackets. Everything is enclosed in an aluminum 3" X 12" X 7" chassis with a bottom plate. Incidentally, a PC card that I find very useful is the Vero DIP board no. 06-0147B. It handles integrated circuits as well as discrete compo-ents and can be clipped to fit tight quarters with a hacksaw or, better yet, a jeweler's saw.

Operation

Switch the instrument on and set the function selector switch to 100 on the frequency side of the switch. Connect a 50-to-100-picofarad capacitor (ceramic or mica) to the hot side of a 115 V ac line, and connect the other side of the capacitor to the center lead of the BNC connector. Connect a ,1-microfarad capacitor (paper or ceramic) from the ground side of the 115 V ac line to the chassis or ground side of the BNC connector. Adjust the 10-turn pot in series with the meter so that the meter reads 60 Hz. These capacitors are necessary to reduce the 115 volts to a level that will not burn out the input circuit. (They are not necessary if you use the output of a 6-volt fila ment transformer.)

The instrument is now calibrated. You can use any frequency that you are sure is accurate on any of the ranges from 10 Hz to 100 kHz to do this calibration. Notice that the frequency side of the function-selector switch is connected to the "capacitor calibrate" side of this same switch. This puts the proper capacitor in series with the measuring circuit. In order to be sure that this accuracy is maintained in the frequency measurement, these capacitors should have a tolerance of not more than ±1.0%. Since these capacitors are available, we can use them to calibrate the instrument to measure capacity.

For example, suppose we want to measure capacity between 0 and 100 pF. Set the function selector switch to 100 pF on the capacitor-calibrate side of the switch. Connect the input BNC connector to an audio generator, or to your Heathkit function generator, and set the generator to 100 kHz (the frequency value on the frequency side of the switch). Increase or decrease this frequency so that the meter reads full scale. Now flip the function selector switch to the Cx position. The capacity meter is now ready to measure values of capacitance between 0 and 100 picofarads. The same procedure is used on the other ranges. ■

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