Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




Repeater Macro


The repeater macro provides the capability to take a one shot waveform and give it a periodic characteristic. For some waveform sources, such as the User source, creating a periodic waveform requires a lot of data to be generated in order to produce the waveform over the time range that is being simulated. Rather than creating multiple cycles in the User source file, only one cycle needs to be created when the source is used in conjuction with the repeater macro. The repeater macro is shown below.

Repeater Macro Circuit

The repeater macro takes in one parameter: Period. This defines the period of the input waveform that is to be repeated. The repeater macro consists of three resistors, two Sum macros, and a lossless transmission line. The R1 and R2 resistors along with the T1 transmission line produce a delay element whose delay is defined by the parameter Period. The input signal is fed into the two Sum macros. The original waveform will be passed directly to the Sum macro at the output and produce the expected waveform during the first Period seconds. The input is also passed into the delay element through the Sum macro at the input. After Period seconds have passed, the original waveform now starts to appear at the output of the transmission line and is reproduced at the output of the macro. This waveform is also fed back into the input of the delay element so that the cycle will repeat continuously. The R3 resistor in the feedback loop has had its VALUE attribute defined as:


which will produce a 1E12 resistor when the simulation time is zero or negative, and a RMIN (set in the Global Settings) valued resistor when the simulation time is greater than 0. The high resistance at time=0 helps prevent a positive feedback loop from occurring during an operating point calculation.

In order for the repeater macro to work correctly, the input waveform must be at zero once the time of the original period has passed. Any nonzero voltages after the original period has expired will be passed to the output and be summed with the original waveform. If a DC voltage is passed through, a ladder effect will occur where each subsequent period is greater than the previous by the value of the DC voltage. Also, this macro does not work in DC so a DC or Dynamic DC analysis will not converge when this macro is used in a circuit. For transient analysis, the operating point should generally be disabled.

A simple circuit was set up to test the repeater macro. It consisted of a User source connected to the input of the repeater macro. The User source imports in the SAMPLE.USR file that is distributed with Micro-Cap. The SAMPLE.USR file has 1us worth of data stored in the file and simulates a few cycles of a damped sine wave. The repeater macro has its Period parameter set to 1u. The resulting transient analysis for a 5us simulation is displayed below.

Repeater Macro Example Analysis

The top waveform is the waveform directly from the User source. It simulates the 1us of data within the file and then stays at zero volts for the rest of the simulation. The bottom waveform is the output of the repeater macro. Note that it has taken the 1us of data from the User source and has turned it into a periodic waveform with a period of 1us.

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