### 2. The Multiple Frequency Problem

Several strategies exist to deal with the multiple frequency problem:

1. Divide and Conquer - Use a DC Input
   - Circuit is now a DC/DC converter
   - Can run POP/AC
2. Choose your frequencies to be harmonic multiples (limited to fixed frequency converters)
   - e.g. 100kHz switching frequency and 50Hz line frequency
   - You can POP on 50Hz, and the 100kHz appears as a 2000th harmonic!
3. Brute force it - Multi-Tone AC Analysis
   - Inject multiple time-domain sinusoidal frequencies into the loop
   - Extract the frequency response with Fourier techniques