Examples of Circuit Envelope Simulation

The following figure illustrates an example setup for using the Envelope simulator to find mixer intermodulation distortion (IMD).

Note You must have the Circuit Envelope simulator license to simulate examples. You may build the Circuit Envelope example without this license, but will be unable to run the simulations.

Note This design, IMDRFSwpEnv.dsn, is in the Examples directory under RFIC/Mixers_prj. The results are in IMDRFSwpEnv .dds.

Example setup for using the Envelope simulator to find mixer IMD

In this example:

• An RF center frequency of 2000 MHz and an LO frequency of 1750 MHz have been established by a VarEqn component. The spacing between tones has been established by the equation fspacing=100 kHz.
• An I_nTone source establishes two intermodulating RF frequencies by means of the following equations:
  Freq[1]=RFfreq- fspacing/2 and Freq[2]=RFfreq+fspacing/2
• An I_1Tone source establishes the LO frequency by means of Freq=LOfreq.

  Hint Using current sources instead of voltage sources leads to faster simulations, because one fewer equation per source is generated. The function dbmtoa converts power to current at a default reference impedance of 50 ohms. P_1Tone and P_nTone components can also be used.

• A ParamSweep component establishes RF_power as the parameter to be swept. This component also establishes the Start, Stop, and Step values for the power sweep.
• In the Envelope Simulation component, LOfreq and RFfreq have been assigned to Freq[1] and Freq[2], respectively.
• Stop time has been determined by tstop, which in turn is defined by an equation in the VarEqn component. Similarly, Time step has been determined by tstep.

  Note Because this example will later use the fs() function, the number of time points (determined by numpts=20 in the VarEqn component) must be even. numpts is the number of timepoints that are simulated per period of the modulation frequency. Modulation frequency is determined by fspacing/2, and fspacing has been established as 100 kHz.

• Transient responses are discarded by extrapts, the number of extra points to simulate at the start. This is the same as the Sweep offset parameter (under the Env Params tab).

The following figure shows the results of the simulation.

IF spectral power is plotted against frequency in kHz, by means of the equation

IFspectrum=dBm(fs(Vif[1]))

Note The function fs performs a time-to-frequency transform, transforming the IF (Vif[1]) into the frequency domain.

The value zero on the x-axis represents the IF, with values to the left and right representing the mixing products that are offset from the IF. The marker M2, at +150 kHz, indicates one of the third-order IMD products. The equation TOIoutput uses simple geometry.

More Examples

For more Circuit Envelope simulations, refer to these example projects:

• For ways of generating sources for use in envelope simulations (such as Π/4-DQPSK, FSK, QAM, and CDMA), see Tutorial/ModSources_prj.
• To simulate amplifier spectral regrowth and adjacent channel power leakage with digitally modulated RF signals at the input, see RF_Board/NADC_PA_prj.
• To simulate PLL transient responses, see RF_Board/PLL_5th_Order_prj and DECT_LO_Synth_prj.