Using an RTA Program As an Oscilloscope or Circuit Analyzer

The purpose of this trick is to give viewers and affordable option to view the electrical signals of their circuits and devices using real time analyzer (RTA) programs. The primary benefit to this approach over an oscilloscope is that RTA programs can function as both an oscilloscope for seeing voltage as well as an RTA for seeing frequency response.

An oscilloscope is good for simple tones, but complex signals are difficult to discern. An RTA gives a view of the frequency spectrum of the signal under test. This is good for identifying the harmonic content in a signal, any high-frequency noise content, and also to determine the effects of filters.

Applications include:

• Viewing the actual effect of passive crossovers or filters to see what their exact effect is. This is helpful for custom speaker designs with custom passive crossovers.
• Viewing the output of a circuit before or after noise filters, or just looking for noise itself.
• Viewing and storing oscilloscope outputs or traces.
• Viewing and storing frequency response outputs.
• Viewing the onset of signal clipping (exceeding voltage rails or range) and the harmonics associated with clipping. This also provides a good way to test clipping detectors by tracking the conditions that trigger the circuit.
• Troubleshooting circuits by looking at both voltage and frequency components.
• Measuring the frequency response of audio amplifiers and determining if there are filters in the system - this is useful when determining what the signal looks like in OEM/Factory audio systems (cars, stereos, etc.). If you want to make something sound better that it does from the factory, it is helpful to know what you're working with.

The embedded video offers a narrative explanation of the process. Images include the setup bench and a block diagram of signal routing.

Step 1: Determine the Operating Voltages

In order to use a computer-based real time analyzer (RTA) to measure the electrical behavior of your circuit, you need to determine what voltage range your circuit will produce. The input to most computer sound cards is fairly low, only a volt. DO NOT EXCEED THE INPUT VOLTAGE RANGE! This means that circuits with higher output voltages will need to reduce that voltage down to an acceptable level. This can be done with a voltage divider resistor network or a line output converter circuit or device. If you are looking at the output of an audio amplifier, a line output converter is a perfect device for this purpose. The line output converter takes speaker level signals and reduces them down to line level signals through resistor networks or an audio transformer. You want to take frequency ranges into consideration because some transformer-based line output converters will affect frequency response.

To determine the output voltage of your circuit or device (if you don't know it already) you should measure it with a volt meter to determine both the AC and DC voltage characteristics. If the voltage needs to be reduced, keep track of the ratio (output : input) so you can translate the results. Also be sure to note that your DMM measures average or RMS voltage and your scope easily displays peak voltage, refer to the attached image.

If the output voltage is 10VAC and you apply a resistor network or line output converter that takes it down to 1VAC you have a ratio of 10:1. That means a measurement of 0.5VAC on the program will translate to an actual circuit output of 5VAC (0.5 x 10 = 5).

I've used this method to measure the outputs of high power audio amplifiers. Just keep track of your voltage ranges and pay attention to what load the device sees. Of course, you have other gain stages available so it makes sense to check a measured level with the program and adjust the audio gain on the PC to achieve a usable ratio.

This is a good time to mention that each circuit or device has an output impedance and an input impedance. Your device or circuit should already take this into consideration in the design and most audio inputs have high input impedance (10k ohms or so). If more information is desired on this topic, there are videos online that explain this topic (look for lectures such as "input and output resistance of circuits and the voltage dividers").

Step 2: Gather the Necessary Components

Because this tip and trick requires a real time analyzer (RTA) program, you will need a PC or tablet with an audio input card or feature. You will also need an RTA program to run on the PC or table. There are several programs available (both free and paid) that offer a frequency view and an oscilloscope view.

Depending on the circuit voltage output, you may need a line output convertor circuit or device (see Step 1).

You will need cables to connect everything together, mostly audio cables with terminations that are compatible with the audio input on your PC or tablet.

The device or circuit under test will be needed, as well as whatever means you use to power it up. For some devices this may require the power supply you normally use to test the equipment.

Step 3: Connect the Components

Because you are using the RTA program on the PC or tablet to view the electrical signal of your circuit or device, you need to get the signal from the circuit or device into the PC or tablet. The RTA program needs to be told to look at the audio input for the signal. Refer to the instructions for your RTA program to do this.

Simply put, you connect wires to the output of your circuit or device and connect them to the audio input on the PC or tablet. Refer to Step 1 if you need a line output converter between the circuit and the PC to reduce voltage to an acceptable range.

But, BE CAREFUL to not inject high voltages into your PC or you can damage the audio board!

Step 4: Understanding the Results

The RTA program in this example allows for both an oscilloscope view and a frequency spectrum view. The oscilloscope view behaves similar to a traditional oscilloscope. Because the audio input has adjustable input gain on the PC or tablet, and because you may be changing the signal voltage to an acceptable level, you need to determine the actual ratio to use the oscilloscope view to measure voltage. Do this by using your volt meter on the circuit output and compare it to the display on the screen. Adjust available gain or volume stages so you have a reasonable ratio to make the math easy. If your circuit or device has adjustable output voltages, take measurements at different levels to verify you have a linear gain relationship (meaning the ratio remains constant at different volume ranges). If you aren't interested in actual voltage levels because you already know them, you can skip this step.

The frequency spectrum view is the primary benefit to this method. In this view you will have the ability to chose the resolution of your view and this is observed in octaves (or fractions of octaves). 1/1 octave has the lowest resolution, 1/3 octave view has 3x as much resolution. 1/6 octave has 6x more resolution than 1/1 octave. This program goes down to 1/24 octave resolution which allows for more detail. Which resolution you choose depends on what you are interested in. For most purposes, seeing the highest resolution possible is usually desired.

Another value of interest is the averaging value. This determines how the RTA program will average the results. Use of this variable depends on what you are interested in. If you want to see changes in real-time then keep the average value very low (between 0 - 5). If you want to see a "steady state" representation of the circuit, averaging values greater than 20 are useful. Note that you will have to wait longer for results and to see changes if averages are high.

If you are looking to learn the frequency response of an audio circuit, you will want the circuit to attempt to generate a signal that covers the entire usable frequency range (typically 20Hz to 20,000Hz). This can be done by having the circuit reproduce uncorrelated pink noise or a tone sweep while monitoring the output on the RTA.

The images are outputs from measured circuits including the crossover points of a passive crossover, the factory EQ and corrected response of a 2014 Honda Accord, the factory EQ of a 2017 Malibu LT at 5 volume levels, oscilloscope view of 1kHz clipped tones, and frequency response view of 50Hz tones clipped and not clipped.