Cadence PSPICE SCHEMATIC Bedienungsanleitung PDF herunterladen (Seite 22)

Date/Time run: 09/17/08 10:58:17

** Profile: "SCHEMATIC1-simplesweep" [ C:\Documents and Settings\John Davies\My Documents\OrCAD intro\RC freque...

Temperature: 27.0

Date: September 17, 2008 Page 1 Time: 11:02:04

(A) simplesweep (active)

Frequency

1.0Hz 10Hz 100Hz 1.0KHz 10KHz 100KHz

P(V(OUTPUT))

30d

60d

90d

DB(V(OUTPUT))

-60

-40

-20

-0

SEL>>

Figure 29: Amplitude in dB and phase (Bode plot) of the output from a high-pass ﬁlter as a

function of frequency.

At the half-power frequency, the ﬁlter should have a gain of −3dB and a phase of 45°. (You

should be able to derive these numbers.) Check this on the plot.

Add a parametric sweep to the plot for capacitors of 1, 10 and 100 nF. Explain how the

behaviour changes.

How does the circuit behave if the capacitor and resistor are interchanged?

5 Transient response of an RC circuit

The frequency response of a circuit is helpful to understand its behaviour when it is driven with

simple sine waves, or signals that can be constructed from them. In other cases it is clearer

to look at the behaviour in time. For example, how does the circuit respond to a sudden step

or pulse on the input? Figure 30 on the following page shows the same RC circuit but with a

different source. The idea is that we suddenly connect the circuit to the battery at t = 0, so its

input voltage jumps from zero to V1, and observe the result at V

Output

(t).

For how long should the simulation be run? Typically there is a transient (short-lived)

variation, after which the voltages reach a steady state. The duration of the transient behaviour

is set by the time-constant of the circuit, τ = RC. This is 0.1 ms for the values shown. It is

closely related to the half-power frequency.

This time we use the ﬁnal type of simulation, called Time Domain (Transient). The settings

are shown in ﬁgure 31 on the next page. I’ve chosen to run for ten time-constants so that

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Cadence PSPICE SCHEMATIC Bedienungsanleitung Seite 22