Bode plotters
Bode plots were originated and named for H. W. Bode, who used the plots for studying amplifier feedback. The original plots required semilog graph paper. Two graphs were required: one for gain in decibels plotted against frequency on the log scale and the other for phase shift in degrees plotted against frequency on the log scale. The arrival of computer simulations such as National Instrument’s Multisim has resulted in greater utilization of Bode plotters.
The Bode plotter in Multisim allows the user to produce a graph of a circuit’s frequency response and is quite useful in analyzing filter circuits. The The Bode plotter generates a range of frequencies over a specified spectrum. An AC source must be present somewhere in the circuit for it to work. The frequency of the AC source does not affect the plotter.
The Bode plotter measures the ratio of magnitudes (voltage gain in decibels) between two points (V1 and V2). It also measures the phase shift (in degrees) between two points. A logarithmic base is used when the compared values have a large range, which is typical when analyzing frequency response. The horizontal axis, or x–axis, always shows frequency in hertz.
When measuring the voltage gain, the vertical axis, or y–axis, shows the ratio of the circuit’s output voltage to its input voltage. The units are in decibels. When measuring phase, the vertical axis always shows the phase angle in degrees.
Questions
1. What is the function of a Bode plotter?
2. What are the two graphs that a Bode plotter generates?
3. What is required to use the Bode plotter?
4. What units must the input magnitude be in?
5. With both graphs, what does the horizontal axis represent?