Extending ARTA/STEPS frequency beyond 22KHz using cheap USB audio device - Page 1
Forum section: electronics / testgear

author: Insatman
author_rank: Frequent Contributor
timestamp: February 26, 2022, 03:54:27 am
content: Getting extended frequency range from ARTA STEPS, for Amplitude vs Frequency Measurements.I’ve been using ARTA STEPS program to characterize various filters and amplifiers for frequencies too low for my spectrum analyzer w/ tracking gen. Up until recently I’ve used a USB external sound dongle I bought for ~$15. This unit is spec’d at 24bit/44Khz. So that implies a Nyquist limit of 22Khz for both ARTA and STEPS programs. This is demonstrated easily if you run STEPS past 22KHz and get crazy results above 22KHz.I wanted more range in STEPS program. My spectrum analyzer is spec’d at the low end down to 9Khz but the tracking generator performance sucks there. It’s almost unusable below 50Khz, so I was wanted to get STEPS to work up to 50+ KHz. I know there are USB DACs up to 192KHz advertised, but I wanted a 192KHz ADC as well. Eventually I found a USB sound device made for musicians for around $50 that advertised 24bit/192KHz performance for both input and output.Getting it to work with STEPS for amplitude vs frequency measurements was a journey however due to several factors. I hope this writeup will shorten the journey for anyone wishing to repeat my efforts to get extended frequency range from ARTA/STEPS. I was able to get this setup working after several hours of testing and at one point I disassembled the unit to get access to the PCB and make measurements with scope probes to better understand how it works and if I needed to modify it. Turns out no mods are necessary, just setting up the pots, switches and connections correctly does the trick.The Device under Test (DUT) is connected between the Right (ch2) output and the Left (ch1) input. This odd arrangement is required due to the particular way this unit is internally configured. The pots and switches must also be setup correctly, which is detailed below. What follows is an account of the journey where this setup was derived.First I was unable to get the Microsoft driver to work correctly. You could set the bitrate to 192KHz in both the driver and the ARTA software, but the 22KHz Nyquist limit was still showing it’s ugly head. This told me that the bitrate was still 44KHz. I switched to the ASIO driver (https://www.asio4all.org/) which is free. This solved the bitrate problem.Second issue is more complicated to explain. The unit I bought has two mono microphone inputs (left/right), two main mono outputs (left/right) and a stereo headphone jack output. The microphone inputs have potentiometers for input level and there are pots for main and headphone jack amplitude. The main outputs share their source with the headphone jack.On the rear of the unit there are five switches. One is to select external or USB power input. The second turn on and off a +48V bias supply for the microphone inputs, The third is a mono/stereo select, the fourth selects the source for the headphone jack, either “Mic” or “Mic + PC”. The fifth switch selects the source for the “PC REC” or ADC to USB as either “Mic” or “Mic + PC”.To get the unit to work correctly with ARTA/STEPS, the switches must be set as follows.USB power: either source works+48V Bias: OFFMono/Stereo Select: StereoHeadphone Source: “Mic + PC”PC REC: “Mic”The four pots on the front of the unit need to be set as follows.Left (ch1): About 2 dots past midpoint. Increasing will boost signal but risks clipping resulting in harmonic generation.Right (ch3): Off or completely to the left. Do not attach signal to the right input.Headphone: Off or completely to the left.Main: Midpoint. Increasing will boost signal but risks clipping resulting in harmonic generation.ARTA/STEPS Setup.Audio Device: Single channel left mode, 192K samplesMeasurement: Set frequency range desired, I use 10Hz to 60Khz. Note above 60Khz the frequency response of the sound card falls off quite a bit.Using these settings and a cable directly between the Right output and Left input you will get the system response. A graph is shown below. This data can then be used to generate a Frequency Response (FR) file which when implemented is analogous to using the normalize function on a spectrum analyzer with tracking gen. Once the FR sweep is complete in STEPS, use the Export function to save the data as a “Mag + Phase” .CSV file. (this works on the free version of ARTA/STEPS). Open the file in EXCEL or your spreadsheet editor of choice. Delete the phase column. Add a row at the top of the file. Content is not important, just type some ascii. I put the file name here. Add one more row directly above the first frequency data row. Put a value lower than the first frequency in the list as frequency and copy the first Magnitude number in the Mag column. Save the file as a .CSV type but use a .MIC file extension. An example file is shown below.In STEPS program open the FR window. Load the file you just created. Push the “Use FR compensation” button. Close the window . Now repeat the STEPS measurement. You should get a flat line at 0dB. See example photos below. Now you can insert the DUT in the line between Right (ch2) output and Left (ch1) Input and measure your amplitude vs frequency.Please excuse the poor photography below.