From (5) it is evident that the cou

From (5) it is evident that the count, N, is linearly proportional to the measured phase angle, φ, independently of the period, Tx, of the input signals. The division coefficient, n, or the coefficient k, can be used to select the measuring ranges.III.Designed Instrument DescriptionA low-frequency digital phase meter based on the method in Figure 1 was designed and constructed. It is controlled by a microcomputer and can communicate with a personal computer (PC) via a serial communication line. Block diagram of the designed instrument is in Figure 2, [8]. The analogue part consists of protection circuits, an amplifier and a comparator in each of the two channels. The protection circuits are simple diode circuits with fast diodes in anti-parallel connection. The amplifier must amplify the input signal to the sufficiently high level not to cause a phase error in the comparator due to possible level shift error. From this point of view the best choice of the comparation level is zero, because the slope of the input signal is the highest just at the zero crossing. The comparator shapes the input signal to the square wave signal with the edges in the instants of zero crossing.The digital part consists of logic circuits, two AND gates, a quartz oscillator, two counters and a microcomputer with a display, a keyboard and communication means. The logic circuits contain three edge-sensitive D flip- flops. Two of them shape the rectangular pulses vo of the duration t1, proportional to the measured phase angle. These pulses make the AND gate 1 open in every period of the input signal and counted pulses from the quartz oscillator can propagate to the input of the Counter 2. The AND gate 2 is kept open during the whole time interval of measurement, Tm, by the control signal generated digitally in the microcomputer by the help of the Counter 1. In the Logic Circuits block this control signal is used in the third D flip-flop which is also controlled by the shaped signal vo to insure the start and the end of the time interval of measurement, Tm, exactly at the rising edges of the shaped signal vo. This is necessary to avoid possible errors due to counting during only a partof the time interval t1 at the beginning and at the end of the time interval of measurement, Tm. The duration of Tmis user-selectable.