USB audio interface output has lot of high frequency noise. - Page 1
Forum section: electronics / general

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 12, 2023, 10:24:20 am
content: I have a TASCAM US-4x4 audio interface and while I can't hear the noise but watching the waveform on a scope I see a lot of noise. I found that the noise is above the audio frequency. If I set the a low pass filter of 22kHz on the scope then the waveform looks clean.I wonder if this noise is a defect of the unit or it's intentional?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 12, 2023, 12:32:41 pm
content: I would first exclude any grounding issue - have you?Other than that, even pro audio, like yours, is a little light on DAC reconstruction filters. I saw that on my own Behringer.

author: bw2341
author_rank: Regular Contributor
timestamp: July 12, 2023, 05:56:54 pm
content: How are you connecting the oscilloscope?It looks like this has a balanced (differential) output. Are you measuring with two probes and differential mode on the oscilloscope? If you are only using one probe and leaving the other connection floating, it is not a proper test of the output.If it still has noise with two probes and differential mode, check the input impedance of the oscilloscope. A high input impedance (1 megaohm) is not representative of a normal audio input. Try the 50 ohm input impedance setting on the oscilloscope.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 12, 2023, 07:31:44 pm
content: Yes.- Check that you're not picking up EMI noise via the scope probes.- Explain what you mean by "a lot of noise". What's the amplitude?

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 13, 2023, 03:21:29 pm
content: I connect the balanced output with the + to the probe. The - and Ground are shorted together and connected to the ground of the scope. I don't have a differential probe.The amplitude of the noise is about 50mV RMS. The noise level doesn't change with the volume controls.

author: bw2341
author_rank: Regular Contributor
timestamp: July 13, 2023, 06:12:38 pm
content: A differential probe is not needed for differential measurements. You need a two-channel scope and two probes.First, set your oscilloscope to differential mode. The method varies with different scopes and is not usually labeled with "differential".On an analog scope, there may be an "ADD" button in the vertical section that changes the display to the sum of Ch1 and Ch2. If so, there is usually an "INVERT" button on Ch2. If you activate "ADD" and "INVERT" at the same time, you get a differential measurement of Ch1-Ch2. Make sure the vertical scale settings are the same for both channels and the zero position are set to the same reticle grid lines before you switch to differential mode.On a digital scope, you use a math function to display Ch1-Ch2.Connect Ch1 to the positive terminal. Connect Ch2 to the negative terminal. Do NOT short the negative to the ground. Connect the ground clips to the ground terminal. Play the same audio as you did before. Do you still see the same noise?

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 13, 2023, 08:37:02 pm
content: I did that and it still has the same amount of noise. All the noise are above the audio frequency range.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 13, 2023, 08:55:57 pm
content: What's the frequency?

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 13, 2023, 09:13:53 pm
content: Mostly around 100kHz but I found that to totally get rid of the noise I can use a low pass filter with the cut off of about 22kHz.