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In such questions , we have to understand the significance of mod value of a counter and difference between the operations of various types of counters , the nature of waveforms being produced by them etc.So I mention the steps one by one :

a) A ring counter is a type of counter composed of a type of circular shift register. The output of the last shift register is fed to the input of the first register.So in this periodicity comes out to be simply n cycles for n bit counter means value is going to be repeated every n cycles.So the frequency of the clock which is given as input is going to be divided by the same mod value which is the period of repetition when it is fed to a counter.

So in this case we have 25 bit Ring Counter.So mod value = 25

Therefore , frequency resulted due to ring counter  =  Input frequency / mod value

                                                                         =   2 * 106 Hz / 25

                                                                         =   80000 Hz

b) Now this output signal is fed to ripple counter .Now the characteristic of ripple counter is it is purely asynchronous counter and hence if we have say n flip flops , then the clock frequency of ith flip flop is provided by (i-1)th flip flop.So we have mod value of n bit ripple counter = 2n .Here we have 4 bit ripple counter .So mod value = 24 = 16

Therefore , frequency resulted due to ripple counter = Frequency provided to it / Mod value

                                                                           = 80000 / 16

                                                                           = 5000 Hz

Now this is fed to Mod 25 counter.

c) As now we have Mod 25 counter , so ,

                 frequency resulted due to this counter = Frequency provided to it / Mod value

                                                                         =  5000 / 25

                                                                         =  200 Hz

d) Now we have 4 bit Johnson's Counter .It's mod value is 2n for n bit counter as its operation is slightly different from ring counter but it is also prepared using shift register.So for 4 bit Johnson's counter , mod value = 2 * 4 = 8.Hence ,

               

                 frequency resulted due to this counter = Frequency provided to it / Mod value

                                                                         =  200 / 8

                                                                         =  25 Hz

Hence the final output frequency  is  25 Hz

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See for any counter of Mod-M if input frequency is Fi  then output freq Fo = F/M.

Here first counter is 25 bit Ring counter of which Mod no is 25.So output freq at W is 2MHz/25=80000 Hz

Then This would be input freq to next counter which is 4 bit ripple counter.Its Mod no is 24=16

So output freq at X is (80000/16)Hz=5000Hz

5000 Hz would be input freq to next mod 25 counter whose mod no =25

So output feq at Y is (5000/25)=200Hz

Now last this 200 Hz applied on 4 bit Jhonson counter...Hence its mod no(m)=8

So output freq at end at Z is =(200/8)=25 Hz

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