Question

Lab# 7 assignment Introduction A ring oscillator is a device composed of an odd number of NOT gates whose output oscillates between two voltage levels, representing true and false. A schematic diagram of a simple three inverter ring oscillator is shown in Fig. 1. Fig.1: A 3-inverter ring oscillator. The NOT gates, or inverters, are attached in a chain; the output of the last inverter is fed back into the first. Because a single inverter computes the logical NOT of its input, it can be shown that the last output of a chain of an odd number of inverters is the logical NOT of the first input. This final output is asserted a finite amount of time after the first input is asserted; the feedback of this last output to the input causes oscillation. A real ring oscillator only requires power to operate; above a certain threshold voltage, oscillations begin spontaneously. If 't' represents the time-delay for a single Inverter and 'n' represents the number of Inverters in the Inverter chain, then the frequency of oscillation is given by Vint Lab Assignment Question (1) For a ring oscillator with that uses inverters that exhibit the time delay of 10ms, calculate the frequency of oscillations for- 1. A ring oscillator with 4 inverters. 2. A ring oscillator with 8 inverters.

Answer 1

Answer to question no 1

(1) A ring oscillator with 4 inverter means only 3 inverters are used for oscillation and one is used to invert the oscillatory signal. Hence for frequency calculation ,only 3 inverters will be considered .

$f_{0}=\frac{1}{2nt_{d}}=\frac{1}{2*3*10ms}$

$=\frac{1000}{2*3*10}=16.67Hz\; (Approx)$

(2) Similarly for 8 inverters oscillator frequency will be

$f_{0}=\frac{1}{2nt_{d}}=\frac{1}{2*7*10ms}$

  $=\frac{1000}{2*7*10}= 7.14 Hz \; (Approx)$

Answer to Question no 2

(a)

Total time delay = 2*n*td = 2*3*5ns = 30ns.

(b) Total time delay for 5 inverters ring oscillator

= 2*5*5ns = 50ns