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CHAPTER 9 RF Design Tools > Modeling - Pg. 214

214 RF CIRCUIT DESIGN With Circuit Envelope, only one large signal tone, for the LO, is required. Circuit Envelope simulation is a hybrid time- and frequency-domain simulation technique. Signals that are within the envelope bandwidth that is centered on each large-signal analysis tone are generated without requiring any additional large-signal analysis tones. The envelope bandwidth is equal to 1/(simulation time step). Tones that are within 0.5/(simulation time step) above 0 Hz are also generated. The large-signal analysis frequencies in this Circuit Envelope simulation are the LO frequency and its harmonics. The LO Frf2-Frf1 Flo-Frf2 Flo-Frf1 Baseband Envelope Baseband centered on Flo FrF1 FrF2 Flo signal can be generated by an oscillator and/or a frequency divider, in which case its frequency is determined automati- cally by the simulator while solving the oscillator and frequency divider. Fig. 9-13 displays the Circuit Envelope simulation results. The equations show how a particular spectral tone can be plotted as a function of the input signal amplitude. The design variables Flo, Frf1, and Frf2 specify the spectral tone. The input signal amplitude is VRFamp_dB. Note that, in this example, the LO and its harmonics had to be simulated along with baseband signals near DC due to the large difference in signal frequencies. While a time-domain simula- tor cannot simulate such circuits efficiently, frequency-domain simulators like Harmonic Balance and Circuit Envelope are well suited for circuits of this type. 0.5/(time step) 1/(time step) MODELING As opposed to system-level behavioral modeling, which describes what the system does, modeling of active and passive on-chip elements during circuit design is done to improve the FIG. 9-12. Spectrum for a direct downconversion.