Question

In a 4-stage ripple counter, the propagation delay of a flip-flop is 30 ns. If the pulse width of the strobe is 30 ns, the maximum frequency at which the counter operates reliably is nearly
1. 9.7 MHz
2. 8.4 MHz
3. 6.7 MHz
4. 4.4 MHz

Answer 1

Correct Answer - Option 3 : 6.7 MHz

Concept:

In case of ripple or asynchronous counter total propagation delay T_pd = n t_pd

Where n = no. of flip flops

t_pd = propagation delay of each flip flop

If the strobe signal is given, the pulse width of the strobe is also added, i.e.

T_pd = n t_pd + T_s

Calculation:

Given:

n = 4, t_pd = 30 nsec, T_s = 30 nsec

T_pd = 4 × 30 + 30

T_pd = 120 + 30

T_pd = 150 ns

Maximum frequency at which counter operates will be:

\({f_{max}} = \frac{1}{{{T_{pd}}}}\) 

Putting on the value, we get:

\(= \frac{1}{{180 \times {{10}^{ - 9}}}}\) 

\(= \frac{{{{10}^3}}}{{180}}MHz = 6.67\;MHz.\) 

≈ 6.7 MHz