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Design Notes


This example illustrates the various methods available in both Microwave Office (MWO) and Visual Systems Simulator (VSS) to perform a two-tone analysis on cascaded RF blocks.  In this case, two amplifiers are included in a chain with a filter and two lossy elements.  The graphs compare the simulation results from harmonic balance (MWO), complex envelope (VSS), and the new RF Inspector (VSS).


The behavioral NL_AMP, BPFC (Chebyshev Bandpass Filter), and ATTEN (modeling a cable and an attenuator) elements are used.  The filter is in the subcircuit "Filter" so it can be shared with VSS. In harmonic balance, PORT2 is used to specify a two-tone source.  Tone 1 is taken from the project setting and Tone 2 is given as a parameter, "Fdelt", or the difference in frequency between two fundamental tones.


The behavioral AMP_B and RFATTEN elements are used.  The filter is imported from the MWO subcircuit "Filter" using the LIN_S element.  In VSS, the TONE element can be used to specify both input frequencies with a single element.  By enclosing the two frequencies in curly braces, {}, a vector is indicated and both tones will propagate.


The "Chain Budget" graph shows the cascaded gain budget for the system.  This analysis is available only with VSS.  The gain following each element in the chain is shown, so it follows that we have 28dB gain at the output end of the chain.  The X-axis labels, the schematic symbol for each element, are automatically created.

The "Chain Gain" graph compares the broadband frequency responses from VSS (using VSS Frequency Analysis) and MWO (regular harmonic balance frequency sweep).  Virtually identical results are produced from both analysis engines.

The "Chain IP3" graph shows the IP3 calculated by both simulators.  Note that the VSS analysis uses a cascaded budget IP3 measurement, which performs an idealized IP3 estimate. This estimate uses the linear gain of the amplifiers and does not take into account any compression effects. The time domain IPn measurement can be used for a more comprehensive IP3 measurement.

The "Chain Spectrum" graph compares MWO, VSS, and RFI results.  Since RFI, is the new capability being illustrated in this example, a detailed discussion follows:

RFI measurements are found on the "RF Inspector" node under "System" measurements.  Here, we are displaying a power spectrum calculation on the red trace.  Note that if you click/hold the mouse on the MWO (blue) or VSS (magenta) trace, you get a display of frequency and power.  On the RFI (red) trace, however, a click/hold produces a display of frequency, power, and also information on where the frequency product came from.  Further, if you double-click on the RFI (red) trace, you will get an interactive display that allows you to obtain detailed information on exactly where each tone comes from.

System Diagram - Chain VSS

Schematic - Chain_MWO

Graph - Chain Gain

Graph - Chain Budget

Graph - Chain IP3

Graph - Chain Spectrum