Bluetooth Audio receiver - high noise level - Page 1
Forum section: electronics / beginners

author: den
author_rank: Contributor
timestamp: May 12, 2018, 08:17:12 am
content: Hi forum,long story short - I want to upgrade my tape Car radio with a bluetooth receiver. For that I have bought a small bluetooth loudspeaker. The plan is to connect its output to a tape amplifier in car radio.Now the problem - there is a ton of noise in the output of the BT receiver. I feed it with a sine wave, but on top of the audio wave there is another high frequency sine wave. I've tried to power it also with a battery - the problem persists, so it's not a power supply ripple. Has anyone had this experience? Maybe the BT microchip is shitty, or can the problem be somewhere else? Some pictures attached.Added: the noise is also in the input of the amplifier stage, so it's not the amplifier fault.Thanks in advance,Den

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: May 12, 2018, 11:08:11 am
content: Can't say much if we don't really have a schematic. You say it isn't a power supply ripple issue, but there is a significant amount of noise on the power supply rail, additional decoupling may be warranted (recommended, that may mitigate the issue).If the decoupling doesn't solve the noise on the power rail, then it may be a poor board layout issue.Edit: Im blind.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 12, 2018, 11:08:49 pm
content: I cannot tell from the PCB what product that is wrong, or how it is supposed to be used - I'm assuming its battery powered, and just outputs to a 3.5mm plug?Generally bluetooth receivers are noisy for one of two reasons - noise on the power supply due to inadequate decoupling, and radiated noise from the bluetooth SOC itself which has a lot "going on".There is also the issue of ground loops if you were powering this from the cigarette lighter, or USB output from the dash (you mentioned battery, so it isn't this).Often to mitigate the above the audio output is differential, since the noise from the SOC (MCU, radio, amplifiers, probably SMPS controllers too on one chip) so easily couples to outputs, making the audio differential allows it to be cancelled out - and often such SOC's have bult-in audio amps for directly driving 32-ohm headphones. If that bluetooth unit has a dual opamp on board to convert to single ended, and is also powered by the same power supply, noise from the power rail (and the layout) can couple to the output.As for power supply, are those scope grabs from the power rail? or the output? It would help to scope the power rail as well just to check if that noise is there. Just because you use a battery, doesn't mean there isn't noise on the line.Ultimately though, its just a question of measuring things, in different situations to narrow down the problem. Have you tried placing the bluetooth unit in a different location? Different power supplies?

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 13, 2018, 10:39:25 am
content: The amplitude of your signal appears to only be a couple of millivolts, this does not seem right. Are you taking the signal from the speaker output? Are you sure the audio level are adjusted correctly at the signal source, the transmitter and the receiver?

author: den
author_rank: Contributor
timestamp: May 16, 2018, 07:45:29 pm
content: You are correct. Here I am showing minimal volume. I can increase it to a volt or so, but then simultaneously seeing noise is not possible.To answer Buriedcode question - these are signals taken at the output to the loudspeaker.Unfortunately didn't find time yet to test it further.