4.2: The Program

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First, notice the use of #define for pi and function prototypes. The functions, though small, make the main section much more readable.

#include <stdio.h>
#include <math.h>

#define M_PI 3.141592653

void give_directions( void );
double find_fc( double res, double cap );
double find_xc( double freq, double cap );
double find_dB( double gain );
int main( void )
{
       double r, c, xc, gain, dB, steps;
       double fc, f, fstart, fstop, ffactor;
      
       give_directions();

       printf("Enter the resistance in ohms:");
       scanf("%lf", &r);
       printf("Enter the capacitance in farads:");
       scanf("%lf", &c);

       fc = find_fc( r, c );

       printf("\nThe critical frequency is %lf hertz.\n\n", fc);

       printf("Enter the start frequency in hertz:");
       scanf("%lf", &fstart);
       printf("Enter the stop frequency in hertz:");
       scanf("%lf", &fstop);
       printf("Enter the number of steps per decade to display:");
       scanf("%lf", &steps);

       printf("Frequency (Hz)\t\t\tGain (dB)\n"); /* \t is a tab */

       ffactor = pow( 10.0, 1.0/steps );

       f = fstart;

       while( f <= fstop )
       {
             xc = find_xc( f, c );
             gain = xc/sqrt(r*r + xc*xc); /* could use pow() for square here,
                                             but mult by self executes faster */

             dB = find_dB( gain );
             printf("%10.1lf\t\t%10.1lf\n", f, dB ); /* %10.1lf is 10 spaces
                                                   with 1 digit after decimal */

             f *= ffactor; /* shortcut for f=f*ffactor; */
       }
}

void give_directions( void )
{
       printf("Bode Table Generator\n\n");
       printf("This program will display dB gains for a simple RC circuit\n");
}

double find_fc( double res, double cap )
{
       return( 1.0/(2.0*M_PI*res*cap) );
}

double find_xc( double freq, double cap )
{
       return( 1.0/(2.0*M_PI*freq*cap) );
}

double find_dB( double gain )
{
       return( 20.0 * log10( gain ) );
}

Enter this program and test it using R = 1 k\(\Omega\), C = 100 nF, start frequency = 100 Hz, stop frequency = 20 kHz, points per decade=8. Consider what might go wrong with this program and how you might circumvent those problems. For example, consider what might happen if the user entered 0 for the resistor value, or a stop frequency that was less than the start frequency.