# CM2 Practical

Posted on Feb 5, 2014

The square wave is generated by a 7555 timer operating with a supply of 10V and thus generating exactly this voltage as output voltage. With the single quadrant multiplier (RC4200, Fairchild, obsolete) the two quadrant square root circuit is formed. 10V of input voltage is converted to 10V output exactly (2. 5V to5V, 1V to 3. 16V, 0. 1V to 1V etc. ).

The signal is limited to a lower limit of 0V and a switchable limit of 2. 5, 5, 0. 75 or 10V. Two comparators and associated LEDs indicate when the limits are reached. It turned out to be helpful to adjust the input signal to a maximum amplitude (lin or sqrt) by just observing these LEDs. This amplifier has a gain of 20 so that the nominal input voltage of 10V becomes 200V. The full power bandwidth with an output load of 300pF is 400kHz approx. and thus sufficient for all important harmonics when the square root of a 20kHz AF-signal has to be amplified to 200Vpp. The output is current limited to a few mA and short circuit proof. The oscillator is a sine wave push-pull oscillator. This kind of circuit has a low output impedance and generates an output amplitude which is quite exactly depending on the supply voltage only. This makes the circuit nicely "predictable" and this is why it operates with its own regulated power supply of 8V. Though its low output impedance it is still buffered because the measuring voltage not only has to have a low impedance, but a very low impedance so that high currents through the measured capacitor do not alter the voltage unacceptably. In order to allow the measured capacitance (capsule) to be connected to ground the measuring voltage is galvanically decoupled by a transformer which allows not only the current measuring circuit to be ground referenced too but also the introduction of a...

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