in order to calculate the unknown frequency of input signal, time interval should be known.
the time interval is between start and stop condition of main control gate.
this time interval is called time base.
the time base consists of a fixed frequency source oscillator; generally crystal oscillator which is very accurate.
this source oscillator is called clock oscillator.
to ensure accuracy of the clock oscilator, the crystal used in the oscillator is enclosed in a tempeatur controlled oven.
the output of the constant frequency oscillator is continuous sine wave.
this is fed to the schmitt trigger circuit.
then schmitt trigger circuit gives square wave output in the frome of train of pulses at rate equal to the frequency of the clock oscillator.
the train of pulses in then passed through the frequency decade divider assembly.
there are many such assemblies connected in cascade fashion.
each decade divider divider assembly consists of a decade counter and divides the frequency by 10.
the outputs are taken from each decade frequency divider by mrans of selector switch, so that by just changing the position of the switch any output can be connected.