AC Analysis
Understanding AC Analysis (RF Simulation)
Before an RF engineer builds a $5,000 amplifier circuit board, they build it on the computer using simulation software. The most important tool in that software is the AC Analysis engine.
The AC Sweep
An RF circuit behaves completely differently depending on the frequency. A capacitor might act like a solid brick wall to a 10 MHz wave, but act like a perfectly open door to a 5 GHz wave. AC Analysis mathematically proves this.
- The engineer commands the software to inject a theoretical 1-Volt sine wave into the input of the circuit.
- The software then Sweeps the frequency. It tests 10 MHz, then 11 MHz, then 12 MHz, all the way up to 5 GHz.
- For every single frequency step, the software's mathematical matrix solver calculates exactly how much the sine wave is delayed (Phase Shift) and how much it is weakened or strengthened (Gain/Attenuation).
Small-Signal vs. Large-Signal
The most critical limitation of AC Analysis is that it is strictly a Small-Signal linear simulation.
The software assumes the input signal is incredibly tiny (microscopic voltage). Because the signal is so tiny, the complex, non-linear transistors in the circuit are mathematically 'linearized' (treated as simple, flat amplifiers). This makes the calculus extremely fast, allowing the computer to solve a massive circuit in 2 seconds.
However, if the engineer intends to blast 100 Watts of raw power into the amplifier, the small-signal AC Analysis is completely useless and highly inaccurate. The extreme power will violently distort the transistors, requiring the engineer to use a much slower, vastly more complex Harmonic Balance (Large-Signal) simulation to see the true physics.
Key Equations
AC Analysis (Alternating Current Analysis), commonly known as AC Sweep, is an absolutely foundational mathematical simulation technique utilized within advanced EDA (Electronic Design Automation) software...
Key specifications:
000 a | 10 MHz | 5 GHz | 11 MHz | 12 MHz
Power: P(dBm) = 10log(PmW), 0dBm = 1mW
Comparison
| Aspect | AC Analysis Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | While DC Analysis calculates the static,... | Application-dep. | Critical | Verify in sim |
| Operating range | Understanding AC Analysis (RF Simulation... | Application-dep. | Critical | Verify in sim |
| Performance | The most important tool in that software... | Application-dep. | Critical | Verify in sim |
| Integration | The AC Sweep An RF circuit behaves compl... | Application-dep. | Critical | Verify in sim |
| Trade-off | A capacitor might act like a solid brick... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
What does an AC Analysis output look like?
A Bode Plot. The software generates a massive graph with two distinct lines. The top graph shows the Amplitude (measured in decibels), proving exactly how much the circuit amplifies the signal at every frequency. The bottom graph shows the Phase (measured in degrees), proving exactly how much the circuit delays the wave at every frequency.
Why run a DC Analysis first?
Before the software can run an AC Analysis, it must mathematically calculate the DC Operating Point (Biasing). The computer must determine the static DC voltage flowing through every transistor before it can calculate how the tiny AC radio wave will "ride" on top of that massive DC voltage.
Is AC Analysis the same as Transient Analysis?
No. Transient Analysis simulates Time (like an oscilloscope). It shows exactly how the voltage spikes and crashes millisecond by millisecond. AC Analysis simulates Frequency (like a Spectrum Analyzer). It ignores the concept of time and assumes the radio wave has been running forever (steady-state).