Digital frequency meter, Electrical Engineering

Q. Explain digital frequency meter.


Explain principle of digital frequency meter and discuss utility of time base selector with suitable diagrams.

Sol. Digital Frequency Meter: The signal whose frequency is to be measured is converted into a train of pulses, one pulse one pulse for each cycle of the signal. Then the number of pulses appearing in a definite interval of time is counted by means of an electronic counter, since the pulses represent the cycles of unknown signal, the number appearing on the counter is a direct indication of frequency of the unknown signal. Since the electronic counters are extremely fast, the frequency of high frequency of signals may be known.

    Basic circuit: The block diagram of the basic circuit of a digital frequency meter is shown. The unknown frequency signal is fed to a Schmitt trigger.

    The signal may be amplified before being applied to Schmitt trigger. In a Schmitt trigger, the signal is converted into a square wave with very fast rise and fall times, then differentiated and clipped. As a result, the output from a Schmitt trigger is a train of pulses, one pulse for each cycle of the signal.

     The output pulses from the Schmitt trigger are fed to start stop gate. When this gate opens (start), the input pulses pass through this gate and are fed to an electronic counter which starts registering the input pulse. When the gate is closed (stop), the input of pulses to counter ceases and it stops counting.

The counter displays the number of pulses that have passed through it in the time interval between start and strop. If this interval is known, the pulse rate and hence the frequency of the input signal can be known. Suppose f is the frequency of unknown signal, N the number of counts displayed by counter and t is the time interval between start and stop of gate. Therefore frequency of unknown signal f = N/t.

Time base : In order to know the value of frequency of input signal, the time interval known as time base can be determined by the circuit given in. The time-base consists of a fixed frequency crystal oscillator. This oscillator, which is known as clock oscillator must be very accurate. In order to insure its accuracy, the crystal is enclosed in a constant temperature oven. Then output of this constant frequency oscillator is fed to a Schmitt trigger which converts the input to an output consisting of a train of pulses then passes through a series of frequency divider decade assemblies connected in cascade. Each decade divider consists of a decade counter divides the frequency by 10. Connections are taken from the output of each decade in the series chain and, by means of a selector switch any output may be selected.

  In the block diagram of the clock oscillator frequency is 1MHz or 10 Hz. Thus the output of Schmitt trigger is pulses per second. At the tap of the switch there are pulses per second and thus the time interval between two consecutive pulses is second or 1 µs.

At tap, the pulses having gone though decade divider 1, are reduced by a factor 10 and now there are 106 pulses per second. Therefore the time interval between them is 10µs. Similarly, there are 104 pulses per second at tap 10-2  and the time interval is 100µs ; 10-3 pulses at tap 10-3 and the time interval is 1 ms; 100 pulses per second at tap  10-4 and the time interval 10 ms, 10 pulses per second at tap10-5  and time interval 100 ms; one pulse per second at tap 10-6 and the time interval is 1 second.

 This time interval between the pulses is the time base and it can be selected by means of the selector switch.

Posted Date: 7/12/2012 6:07:15 AM | Location : United States

Related Discussions:- Digital frequency meter, Assignment Help, Ask Question on Digital frequency meter, Get Answer, Expert's Help, Digital frequency meter Discussions

Write discussion on Digital frequency meter
Your posts are moderated
Related Questions
Q. What are the features of clamping circuits? The clamping circuit does not change the peak to peak or r.m.s value of the waveform .Thus the input waveform and clamped output

Q. What are the uses of MOSFET? The most common use of MOSFET transistors today is the CMOS (complementary metallic oxide semiconductor) integrated circuit which is the basis f

Discuss the following points with regard to the Company selecting Jebel Ali Free Zone as its Middle East base.  (a) How is a company likely to enhance its supply chain by using

For the circuit in figure, find the current and voltage drop at every resistor

A 20kw 220v shunt generator has armature resistance of 0.07 and a shunt field resisitance 900 ohm find field loss?

Advantages of Expert System The system, once set up, requires to be tested employing data with known outcomes to see if it functions correctly. If it's being used to diagnos

Q. Find the voltage gain of the overall circuit? (a) Consider the circuit of the inverting amplifier shown in Figure (a), including an ideal op amp. Show that the voltage gain

Charge Carriers in Semiconductors In a metal, the atoms are imbedded in a "sea" of free electrons, and these electrons can move as a group under the influence of an

Q. A negative impedance converter is used, as shown in Figure. Show that the load current i L is given by v in /R, which is independent of Z L . Note that since the load sees a cu

Design a 4-bit synchronous counter that has the following sequence: 0 ?4? 9?12 ? 14 ?15 and repeat using: i) JK FF ii)D FF