Frequency Modulation Simulation in Altium Designer

December 11, 2019

This microwave mixer can be examined in a frequency domain simulation in Altium Designer

When working with analog signals, you need to ensure your device is operating linearly in order to prevent problems like harmonic distortion during operation. Nonlinear interactions in analog devices lead to distortion that corrupts a clean analog signal. It may not be obvious when an analog circuit clips just from looking at your schematic or datasheets. Instead of tracing through your signal chain manually, you can use simulation tools to get insight into the behavior of your device. Some important simulations with sinusoidal signals, such as a frequency modulation simulation, can be easily performed with the pre-layout simulation features in Altium Designer®.

In this post, I’ll continue from a previous simulation and bring an FM source into a circuit with a transistor. Here, the idea is to see what range of input values I can use with my analog source in order to ensure that the device operates in the linear range, i.e., when my nonlinear stops behaving linearly. This is quite important in amplifier design, as you need to know the compression point, which determines the input power level where intermodulation products become prominent and degrade your signals.

Getting Started With a Frequency Modulation Simulation

In a previous post, we looked at load line analysis for a circuit with an NPN transistor. Based on the DC sweep results, we can see when the collector current starts to saturate as the collector-emitter voltage is ramped to higher levels. This allowed us to extract the load line for this circuit, and to see how the threshold voltage changes.

In this simulation, I’ll show you how to bring a sinusoidal FM source into your simulations and examine when clipping occurs. In this frequency modulation simulation, we can then examine the Fourier components and determine when new harmonics are generated. We can then modify the simulation by changing the DC bias to see how the FM signal clips and identify the range of input values that lead to linear behavior throughout the relevant frequency bands. This is an important aspect of RF signal chain design with GaAs or GaN FET drivers.

I’ve reused the simulation schematic from my previous post, with the exception that I’ve replaced the DC source seen by the base with a frequency modulated source. You can access this simulation source (named VSFFM) from the Simulation Sources.IntLib library in the Components panel. My schematic is shown in the image below. In this schematic, the basic idea is to use the transistor as a switch allow the FM wave to the load as the base voltage increases. However, you could also use a common collector configuration (V_FM at the base) and measure the output across R_E.

Frequency modulation simulation schematic

Here, I’ve set the carrier frequency to 1 MHz, the modulation index to 5, and the baseband frequency to 100 kHz. The AC range has initially been set +/- 0.25 V with some DC offset. In your circuit, you can base your appropriate range of AC values on your load line results. If you look at the load line results, you’ll be able to find the range of collector-emitter voltage values that produce a linear output; we would like to quantify whether this input range is appropriate for this circuit.

Here we want to run a transient analysis as this will show the behavior of the system in the time domain. I’ll be measuring the collector current, collector-emitter voltage, and the power seen by the load resistor (R_LOAD). Simply go to the “Simulate” menu and click Edit Simulation Setup to locate the parameter sweep and transient analysis settings. In the transient analysis setup (shown below), I’ve set “Default Cycles Displayed” to 10. This was set because the ratio of the carrier frequency to the baseband frequency is 10, so an entire modulation cycle will be seen in the output. If you set this number lower, you will not be able to see results for a whole modulation cycle.

Frequency modulation simulation setup

In the parameter sweep window, I’ve set the primary sweep parameter to the base voltage. I’ve chosen to vary the base voltage from 1 to 7 V in increments of 2 V so that you can see how this will affect the output. This will allow me to see the load current and power clips and to know when we can see a clean pulse. Another option is to w

Frequency Modulation Simulation Results

To get my results, simply go to the Simulate menu and run the simulation, or press F9 on your keyboard. As long as you’ve defined models for all of your components in your schematic and there are no errors in the generated netlist, you’ll see a set of graphs appear on the screen. My simulation produces a set of six plots, but I want to focus on the three shown in the image below.

Transient analysis results. Frequency modulation is clearly visible in these waveforms.

The top graph shows the collector current at a base voltage of 7 V. The middle set of waveforms shows the collector current as the base voltage is swept from 1 to 7 V. It should be obvious that the collector current clips heavily at low base voltage values. This is also seen in the bottom waveform, which shows the power at the load resistor.

Note that, if you set the bias point in your FM source to 0 V, you will have severe clipping as you will be attempting to drive the transistor in reverse, thus the DC bias point is required when working with this transistor.

Creating an FFT

To create a fast Fourier transform (FFT) chart, simply select a waveform in the transient analysis results, go to the Chart menu, and click on Create FFT Chart. The Fourier spectra below show the frequency components in the load current (top graph) and power in the resistor (bottom graph). These graphs were plotted from the parameter sweep results, although you could also create plots with the base voltage set to specific values (you can set this in the schematic directly). We can see higher order frequency content in the spectra (up to 7th order), although there is some harmonic distortion due to the clipping in the transient analysis results.

Frequency spectrum for the load resistor current and power

If you like, you can add a wave to these charts in a new graph for the FM source, and perform an FFT for this source. From our results, we see that using a base voltage of 7 V is nearly ideal for the signal we are working with, where the FM source has a DC bias of 0.25 V and amplitude of 0.25 V about this bias point. To clean up the signal, the amplitude of the FM signal should be decreased, or the base voltage should be increased.

You could also export the simulation/FFT data to an Excel file, which will allow you to calculate the level of distortion seen at the load. Since we are dealing with sweep results, you could apply these harmonic distortion calculations for all the FFT spectra shown above, giving you a curve showing harmonic distortion as a function of base voltage.

The unified environment in Altium Designer allows you to take your schematic data and perform a frequency modulation simulation or any other analysis you like. This is much better than working in a separate program to run these important analyses. Altium Designer also gives you access to a complete set of post-layout simulation tools for signal integrity analysis.

Previous Article
SMPS Circuit Design: Which Switching Frequency to Use?

The issue of switching losses, noise and component size are important considerations in an SMPS circuit.

Next Article
PCB Trace Width vs. Current Table for High Voltage Design

Take a look at our PCB trace width vs. current table for high voltage/high power PCB designers.