# How Current Loads Affect Signals

May 15, 2020

What You Can Capture

• Different types of loads will produce different frequency-domain and time-domain behavior.

• There are some simple simulations you can use to extract this important information during circuit design.

• Once you can construct these important signal simulations, you can import simulation models for active components to examine how they affect different types of signals. You don’t need a scope to see how current loads affect signals. Here’s how you can examine resistive and reactive signal behavior in your schematic.

With simple circuits and simple signal sources, it is usually obvious how different circuits and loads affect signal behavior at a circuit’s output. With more complex circuits, and with wideband analog signals, it is not always obvious how signals are affected by the circuit itself, or how current loads affect signal behavior. While you could work out these aspects of signal behavior by hand, not everyone is a mathematician, and you’ll need some tools to speed up analysis of complex circuits.

When you have access to a powerful simulation engine in your schematic design software, you can quickly examine how load impedance affects signal behavior in the frequency domain vs. time domain. Some simple simulations will show you how a load and its upstream circuit modifies a signal. Here’s how to execute these tasks directly from your schematic using PSpice and which simulation tools you’ll need for these tasks.

## Frequency Domain or Time Domain?

When analyzing signal behavior, the choice of working in the frequency domain or the time domain depends on what information needs to be extracted from your design. If you are driving the circuit with a digital signal, you generally don’t need to worry about much more than what happens in the frequency domain as the signal’s power spectrum is well-defined. Instead, you should be looking at the transient behavior of the signal in the circuit, in order to ensure the rise time of your signal is within spec.

When working with broadband signals or harmonic signals, whether you need to work in the frequency domain or time domain depends on your particular application. Most likely, you will need to determine signal behavior in both regimes. The goal in these types of simulations is to examine whether the input signal is not excessively distorted in the time domain and whether the frequency content of the signal is preserved once the signal reaches the load.

Here are some important points to examine when working with different types of signals. You’ll want to take a trace of the signal at the load component (either the voltage across the load or the current entering the load).

 Signal Type What to Examine Harmonic Examine the transfer function for the circuit before the signal reaches the load, and determine whether the transient response delays the rise to full strength. FM Source Verify that the frequency sidebands are not distorted at the load. AM Source Verify that the amplitude envelope is not distorted at the load. Impulse Examine the impulse response at the load as the reactive behavior can distort the impulse response. Also examine the transient behavior in the time domain.

 When you add a load to an existing circuit, the load couples back into the circuit and creates a new equivalent circuit. This means you can’t just simulate a load component in isolation, you need to add it into your circuit and simulate the entire system. Here’s how you can easily do this with resistive and reactive loads in your circuit simulation tools directly from your schematic.

Because the effects of individual components on input waveforms can’t be examined by taking the components individually, we have to look at these components in larger circuits. The schematic below shows a simple circuit with an FM source (1 A, 1 kHz, modulation index of 4) connected to a Pi filter and a resistive load (R2). This type of source has strong modulation that can be easily seen in the time domain, but the temporal behavior at R2 is not always obvious. The Pi filter below functions as a low-pass filter, but it could also be changed to a high-pass filter by interchanging the capacitors and inductor. FM source with 50 Ohm output impedance connected to a Pi filter and a downstream load (R2).

With the load being resistive, we should expect simple low-pass filtering and a phase shift in the waveform at the load compared to the source waveform. The cutoff frequency for this filter is at ~2.3 MHz, so we should not expect serious attenuation at the source’s carrier frequency. However, the resistive output on the source (R1 = 50 Ohms) will affect the transient response and will modify the transfer function. By looking at the waveform at the load in the time domain and the frequency domain, you can see how the load and any upstream circuitry modifies signal behavior.

The image below shows a simple transient analysis that compares the source current waveform (green) and the current at the load (red). We can see that the transient response dies off within the first ~1 ms, and the waveform at the load appears consistent at all later times. There is clearly some attenuation in the signal level, but this is largely due to the combined impedance of the Pi network and the load, rather than just the load itself. Current waveform at the source (green) and the load (red) in the time domain.

An important point to consider in this simulation is the resolution in the time domain. Here, the resolution was set to 10 μs, which is 1% of the carrier frequency’s oscillation period. This gives a total of 1000 data points per 1 ms period in the above traces. In PSpice, these simulation parameters can be easily set when you create a simulation profile.

## Continuing Frequency Domain Simulation Results

When you want to compare the source and load signals in the frequency domain, you can use a Fourier transform of the above results. To really get a sense for how different frequency components in the signal are affected, you should normalize the current waveform at the load so that the peak load current is equal to the peak source current in the time domain. The graph below shows how the circuit affects an FM signal in the frequency domain. In the red trace, there is a slight roll-off, but this is not due to the Pi filter’s transfer function as we are dealing with lower frequency signals. Frequency content in the source waveform (green) and load waveform (red). Note that the load current has been normalized by its maximum value to provide a better comparison.

Note that the above simulations were performed with a resistive load; the only attenuation in the frequency domain occurs because of the Pi network, while the larger reduction in the time-domain signal is due to the load resistance. To examine what happens with a reactive load, the schematic below has replaced the resistive load R2 with an inductive load L2. The circuit shown above has been modified to have an inductive load (L2).

The new load has high inductance and will present an impedance of 62.83i Ohms at the carrier frequency, which is much lower than the original 1000 Ohm resistor. When we run the simulation again, we see very different results. The time-domain source and load results for this new circuit are shown in the graph below. Current waveform at the source (green) and the load (red) in the time domain.

Here, the transient response at the load component dies off very quickly (before ~0.1 ms) due to the smaller overall resistance in the circuit. In terms of the time-domain current waveform, we see that there is some resonance at different frequencies. This can be seen in the amplification in the red trace (load) compared to the green trace (source).

It may not be obvious where the circuit is amplifying the input modulated signal. This is where another Fourier transform can be used to extract the frequency content in each signal and compare them on a spectrogram. The Fourier transforms of each signal are compared in the graph below. Frequency content in the source waveform (green) and load waveform (red).

From here, we can see that there is wideband amplification in peaks 2-4, with amplification being most prominent in peak 3. If we consider the layout in this modified schematic, the load L2 and output capacitor C1 form an LC tank circuit with resonance frequency of 2.32 kHz. The other components in the Pi filter will modify this resonance to a lower value, and we can see that this resonance sits at approximately 1.6 kHz.

There is also a slightly larger roll-off beyond ~2.5 kHz. One would expect this is due to the fact that the inductive load causes the Pi filter to become a more complex 4th order filter, thus the rolloff should be larger. This can be investigated by measuring the transfer function with a frequency sweep. Note that this would be done with and without L2 connected in the load position, which would allow the two transfer functions to be compared.

This should show how the frequency-dependent behavior of a load component will affect the upstream components in a circuit and the resulting signal seen by the load. In the case of a resistive load, the resistance in the circuit provides some attenuation, while the Pi filter creates some phase shift and distorts the input signal. When we have an inductive load, the inductance couples back to the Pi filter to create a more complex resonance spectrum.

## Working With Active Load Components

Active load components will produce more interesting behavior and need to be simulated using a verified model. Fortunately, many component manufacturers have taken time to develop verified simulation models for use in PSpice and other simulation programs. This saves you a significant amount of time and guesswork when examining the electrical behavior of different components and how they affect an input waveform.

When you need to simulate signal behavior in a complex circuit, you need a set of simulation tools that can show you how current loads affect signals in the frequency domain and time domain. The best simulator will be accessible within your PCB design and analysis software as pre-layout simulation features. The front-end design features from Cadence integrate with the powerful PSpice Simulator, creating a complete system for designing schematics and examining how current loads affect signals.

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