Simulation eBook - Probing signals
Probing signals
After spending ages building a schematic the time has finally arrived: the first simulation is run. The WaveForm window pop opens and there are a bewildering number of traces. In fact there is a trace for every node voltage (i.e. the voltage on every net) and for the current through almost every component.
What's gone wrong?
The most likely cause is one of the most overlooked beginners' mistakes: there are no signal probes in the schematic. The simulation has run fine. It just saved and displayed traces for the voltage on every net and for the current through almost every component because it had not been told which particular subset of all the available node voltages and component currents to save and display.
In order to view useful output from a simulation without it being swamped in the WaveForm viewer, the circuit must have at least one voltage probe or one ammeter in the circuit. In basic simulations these will be the Voltage Probe and Ammeter symbols from the EasyEDA Libs. In more advanced simulations, these can be implemented using a .probe or .save command. In either case, at least one type of probe must exist in the simulation schematic. There is also a Diff_V_Probe that can be found in the Search Libraries tool. This probe produces the difference between the voltages on the A and B input pins on its (A-B) output pin to which an ordinary voltage probe can be attached or a net label can be attached and the output voltage displayed using a .probe or .save command referring to the relevant netlabel..
Probing voltages
All voltage measurements in real circuits are actually measurements of voltage differences. In many cases such as when probing a voltage using an oscilloscope probe, it is easy to forget that the voltage being measured is, in reality, the difference between the voltage at the probe tip and wherever the probe ground lead is connected. In the same way it is easy to forget that probing a single ended voltage in a simulation schematic is with respect to wherever the ground node has been placed.
- A common mistake, however, is to attach a voltage probe to ground.
In spice simulations, all voltages are referred to ground so not only is there no need to attach a Voltage Probe to the ground net, in fact doing so will throw an error in the simulation.
In a real circuit, probing a voltage between any two points places a resistive load between them. With a good quality voltmeter that resistance may be very high, in the order of hundreds of MegΩ. With a x10 oscilloscope probe it will be 10MegΩ. There will be some stray capacitance across that resistance. There will also be stray lead inductances. If the voltage being measured is an AC signal then impedances due to these stray and parasitic components will also load the circuit.
Note that in simulations, voltage probes present an infinite resistance and have no stray capacitance or inductance. In effect, voltage probes have an infinite bandwidth.
The following example illustrates some of the probing techniques described above:
The following example shows a number of ways to measure voltages with respect to ground or differentially using;
The
Diff_V_Probewith a Voltage probe to probe the output;An
Esource (a.k.a. Voltage Controlled Voltage Source or VCVS) with a Voltage probe to probe the output;A
B, source (a.k.a. behavioural or dependent source) configured as a VCVS using a Voltage probe to probe the output.
The schematic also demonstrates the importance of:
Giving voltage probes names that are identical to the nets to which they are attached;
Naming all nets in a schematic;
Probing currents
In a real circuit, probing the current in a wire places a resistive load between them. This will cause some voltage drop across the ammeter. With a good quality ammeter that voltage drop may be very low, in the order of millivolts. There will be some stray capacitance across the insertion resistance and from the ammeter connections to ground. There will also be stray lead inductances. If the current being measured is an AC signal then impedances due to these stray and parasitic components will also load the circuit.
Note that in simulations, except for the Ammeter, which has a 1uΩ (1e-6Ω) series resistance, current probes present zero insertion resistance and have no stray capacitance or inductance. In effect, current probes have an infinite bandwidth.
The following example shows a number of ways to measure currents with respect to ground or differentially using;
- As a current using the Ammeter symbol;
- As a linearly scaled voltage using an H Current Controlled Voltage Source (CCVS) (or an F Current Controlled Current Source (CCCS) with a resistor);
- As a linearly scaled current using an F Current Controlled Current Source (CCCS) driving an Ammeter;
- As a voltage that can be an arbitrary function of the current flowing through a 0V Voltage Source using a BV source (or a BI source with a resistor);
- As a current that can be an arbitrary function of the current flowing through a 0V Voltage Source using a BI source driving an Ammeter.
Note however, that although a 0V source can be used to monitor a current, it cannot be used to measure a current so that it can be directly displayed in the Simulation > Show your simulation report... window or plotted in Waveform.