Page 224 - Op Amps Design, Applications, and Troubleshooting

206 OSCILLATORS

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We can now select R 2 ^d compute R 3. Both resistors should be in the range of 1 to
680 kilohms unless there is a compelling reason to exceed these suggested
extremes. Let us select JR 2 *° be 56 kilohms. R 3 can now be computed by using the
results of our previous calculation.

^2. = 0.231, or

We will use the nearest standard value of 240 kilohms.

Compute RI and Cj. Once the thresholds have been established on the com-
parator circuit, RI and Q determine the frequency of oscillation. The required R^
time constant can be found by applying Equations (4.36) and (4.37). First, let us
use Equation (4.37) to determine the total period for one cycle.

Since we know the two alternations are equal, we can determine the time for
either alternation by dividing the total time by 2. That is,

We can now apply Equation (4.36) to determine the RjQ time constant.