VHF COMMUNICATIONS 3/2009 Gunthard Kraus, DG8GB Possibilities and limits of circuit simulation for radio amateurs Revised version of the lecture to the UKW conference 2009 in Bensheim 1.0 • no program can replace your special- ised knowledge (even if the adver- Sense and purpose of circuit tisement suggests otherwise). simulation The use of circuit simulation always means precious work time is saved. Also alternative ideas can be examined quickly or a prototype can be optimised. Everyone who has already developed circuits knows the questions that torment those who do this work: • How do I find out how the circuit 2.0 functions? The different types of simulator • How do I create the data required? program • Will I find the optimum? • What things that I have not consid- ered could affect the results? The differences are: • Will the circuit work “cleanly” with- out oscillating? • Simulators that work in the time • Do component and manufacturing Domain. These are in principle all tolerances affect the results? SPICE and PSPICE programs. “SPICE” stands for Simulation Pro- • Do I have alternatives if the solution gram with Integrated Circuit Empha- chosen leads to a dead end? sis and “PSPICE” is simply SPICE • etc. for the PC. Circuit simulation can answer most of They provide all the information these questions before a prototype is about stability and starting behaviour. manufactured or a soldering iron is The response to signals with arbitrary touched. However exact knowledge of waveforms is determined by splitting the desired circuit behaviour is absolutely the signal behaviour into very short, necessary. Working with the simulation but practically linear sections. Addi- as well as any changes are ALWAYS the tionally all nonlinearities in the cir- responsibility of the developer cuit are identified. 171 VHF COMMUNICATIONS 3/2009 Fig 1: This is how it looks: a screen without complications, only with the circuit and the necessary text inputs (Properties, Controls and Commands). Other options are the AC-Sweep a linear simulation (a small signal (wobulator to determine the fre- system). Information about distortion quency response), noise simulation with large signals is usually only and the FFT (Fast Fourier transfor- available with cheats that the pro- mation) for the analysis of a signal in gram may offer (e.g. using harmonic the frequency domain (spectral repre- balance) and these are normally se- sentation). Thus they offer the most crets of the software vendors. comprehensive possibilities, but the • EM simulators use the distribution simulation accuracy always depends and the relationships of the electrical on the quality of the SPICE models. and magnetic fields around “struc- These are supplied with the software tures” for analysis. They can be di- or by the component manufacturers. vided in two groups: In reality they specify exactly how the simulation and reality will agree. For things with surfaces (Patch an- tennas, couplers, stripline filters etc.) • Simulators that work in the frequency The complete structures are divided domain, these often use S-Parameters into very small rectangular or trian- (some also convert Y-Parameters to gular “cells”. The individual fields S-Parameters). So vector network for each cell are computed and finally analysers can be used to check the all added together. simulation or to gain the necessary component properties for the simula- For wire antennas (or similar wire tion itself. This achieves a good objects) these are known as “NEC”. agreement between simulation and The structure is also divided into prototype. A disadvantage with this small elements, in this case short in the RF and microwave area (to pieces of wire with linear current and over 100GHz) is that it is principally power distribution. These elements 172 VHF COMMUNICATIONS 3/2009 Fig 2: The result of the simulation. Additionally to the circuit the desired signal is shown when the measuring point is clicked. can be examined more easily to cal- include EMV simulation and also a sim- culate the total behaviour of the ple SPICE simulation (e.g. Target). arrangement by addition of all indi- vidual calculations. The large SPICE software manufacturers always supply free demonstration ver- sions of their products, but these make you grind your teeth in the middle of a 3.0 simulation with a message like: “all components used” or “MEMORY re- SPICE simulation for the radio stricted in this version”. Above all be- amateur cause, unfortunately, the design started can never be transferred to a more com- plex (and more expensive) program vari- ant. The situation is very much relaxed be- cause the restriction “for the amateur” A program for amateur purposes (or usually means reduced price. It is not a training purposes!) should have the fol- problem to spend €50,000 on simulation lowing features: software, these are enormous machines that can be used to design the associated • Little or no cost printed circuit board and examine the EMV behaviour etc. For the private • No restrictions or limitations, thus person money is replaced by time. In fully functional practice that means a reduction of com- • An easy import of new component fort as well as a separating the simulation models from the Internet from the PCB CAD. There are already affordable solutions for PCB design that • A good user group removing the 173 VHF COMMUNICATIONS 3/2009 Fig 3: The simulation of S-parameters requires a different spice directive and a new simulation command. dependency on expensive support for questions or problems solving The use of LTSpice will be demonstrated using the example of an 110MHz Tsche- Fortunately the company Linear Tech- byschef low pass filter for a characteristic nologies has made a free version of their impedance of 50Ω driven with a 25MHz PSPICE software LTSpice IV available square wave signal. The screen shown in to engineers on their homepage [1]. Fig 1 is produced. There is no problem to download the software, even the registration before Beside the circuit there is the facility to download can be refused. Importing new include additional text lines this requires models from the Internet is problem free some explanation: and there is an easy to use symbol editor. The line “Pulse…” below the circuit defines a square wave signal with Vmin The only drawback in using the software = 0V and Vmax = 1V. The rise and fall is that it has a very simple user interface times are 0.01ns with a pulse length of and it takes some time to become famil- 20ns and a period of 40ns. The signal iar with the mixture of keyboard and begins immediately at the time t = 0s. mouse commands required. The line “.tran… “ defines the time Note: there is a Tutorial (in German or period, in the time domain, that will be English) for this program on my simulated. The time period is 0 to 150ns homepage [2] that can be downloaded that will be divided into maximum 0.2ns free of charge. It contains an extensive time steps. collection at practical projects for train- ing. After the simulation is started the screen is divided and an empty result diagram 174 VHF COMMUNICATIONS 3/2009 appears for the simulation (Fig 2). Click- The principle of this technique is perfect ing the mouse on the point of interest in power matching (Ri = Ra) everywhere in the circuit immediately displays a dia- the system. gram of the signal at that point (from the initially starting point of the circuit). Using suitable measuring instruments Using different mouse clicks and sub- (directional couplers) the deviations of menus the result can be adjusted until the the ideal resistance are measured at the desired results are achieved (e.g. several input and output (technical term: “Ports”) diagrams, different values or different to calculate the “reflection factors” (S- colours for the curves…). Parameters S11 and S22). For an S-Parameter simulation the inter- The power output can be calculated from nal resistance of the source (50Ω) must the input power multiplied by the “for- be set in the “Property” menu and an AC ward transmission factor” (S-Parameter Sweep programmed with a SPICE simu- S21). lation COMMAND (Wobulator from 1Hz to 200MHz with 201 points per The effect of a component from the decade, decade sweep). The Spice direc- output back to the input is the “reverse tive “.net I (R1) V1” specifies an S- transmission factor” (S-Parameter S12). Parameters display. The final result is shown in Fig 3. A complete system has many compo- nents connected in line. Using suitable software the total behaviour can be easily determined. 4.0 4.2. Using PUFF S-Parameter simulation for the This well-known DOS program available radio amateur from [9] is probably the standard pro- gram used by radio amateurs for nearly 20 years and has not retired yet. It is low priced, has minimum memory require- 4.1. Some basic information about S- ments, high stability and simulation accu- Parameters racy. It is simple to operate using key System descriptions and computation commands with the most important basic start from approximately 1MHz upwards models available. Clear screen layout with the highest frequencies (today more makes it the RF pocket calculator for the than 100GHz) easily measured. These developer making it easy to quickly test require a completely different model: new ideas. In addition it has been up- graded again and again and even made Everywhere in the system the same sys- compatible with Windows XP. tem resistance is used (usually 75Ω for entertainment electronics or video tech- Note: my homepage [2] contains all nology and 50Ω for most other areas). documents required and help (including a This defines the internal resistance of the detailed list of the control commands signal sources, the output resistance of with explanations and a guidance plus a the modules used, the characteristic im- “PUFF_XP-Servicepack” for trouble free pedance of all cable connections and all operation under Windows-XP - sorry but terminating resistors.