How does DSP Designer Work?
Problem
Coding for real-time signal processing is tedious and error prone. Even when you seemingly get it "right," errors typically creep in. Clearly there is a need for computer automation to free the engineer from detailed micro-code book keeping required by DSP so that better algorithms can be implemented. Simulation using SPICE partially solves the problem. Substituting a SPICE delay line for the z transform, z-1, is the usual approach. But as complexity increases the substitution breaks down, with instabilities above the sampling frequency that are clearly impossible. Even more troublesome is the increased complexity when algebraic "feedback" occurs. Take, for example, a simple Buck regulator plant model. The output voltage feeds back to the input, making the current through the inductor proportional to the difference in output and input voltage. But the output voltage is proportionate to this difference. The simplest approximation is to selectively use backward Euler integration to break this nasty feedback, though a better solution solves the algebraic equations. At that, the detailed implementation is so error prone that the designer accepts loss in performance, just to get the job done. Moreover, most DSP variables are hidden in the code, so the simulation predictions cannot be validated.

Solution
DSP Designer is a software/firmware suite based on Intusoft's ICAP/4 SPICE toolset. Simulation models have been added that make z-transform based modeling fast and easy. This starts with a z-delay to get a more perfect delay model, then adds in a good Analog-to-Digital converter model, which replicates quantizing errors in both frequency and time domain. The continuous conduction mode PWM model is also employed for the fastest simulations ever. Building on the SpiceNet configurable schematic, multiple control loops are broken and simulated in seconds to measure the stability parameters, plus gain and phase margins.

Example circuits are given for target designs that include a modified proportional- integral-differential MPID, design. A novel concept for current feedback the Virtual Current Controller is also developed. Both of these controllers are implemented in hardware using Microchip and Texas Instruments test beds.

Simulations are normally performed for AC, DC and TRAN cases. In order to provide the same capability in the DSP, Real Time (RT) code is provided. This links with a serial interface to the DSP to provide this capability. The most challenging part is the incorporation of a transfer function analyzer, TFA, inside the DSP. The TFA uses the single injection modeling technique inside the ICAP/4 software. IntuScope scripts are used to control the DSP serial interface, and using the RT code, the automated scripts make and display the measurement results.

A new data acquisition plugin has been added to IntuScope so that lab oscilloscope measurements can be transferred by a serial link into an IntuScope waveform for viewing.