SOL. Ramp Type VM: The operating principle of the ramp type DVM is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage or to decrease from the level of the voltage to zero. This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicating tubes.
Conversion from a voltage to a time interval is illustrated by the waveform diagram.
At the start of the measurement cycle, a ramp voltage is initiated, this voltage can be3 positive going or negative going. The negative going ramp is continuously compared with the unknown input voltage. At instant that the ramp voltage equals the unknown voltage, a coincidence circuit a comparator, generates a pulse which open a gate. This gate is shown in the block diagram of.
The voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second comparator generates an output pulse which closes the gate.
An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units (DCU's) which totalize the number of pulses passed through the gate. The decimal number, displayed by the indicator tubes associated with the DCU's is a measure of the magnitude of the input voltage. The sample rate multivibrator determines the rate at of which the measurement cycles are initiated. The oscillation of this multivibrator can usually be adjusted by a front panel control, marked rate, from a few cycles per second to as high as 1000or more. The sample rate circuit procides an intiating pulose for the ramp generator to start its next ramp voltage. At the same time a reset pulse is generated which returns all the D.C. Voltages to their 0 state. Removing the display momentarily from the indicator tubes.