BC127 noise in audio amplifier

A very small signal from a sensor is amplified through a series of analog amplifiers. The final signal is an audio signal which is sampled by a microcontroller. This signal is then sent digitially over i) a USB line audio connection to a PC, ii) to the BC127 module, which transmits is wirelessly to a PC.

When operating with the BC127 Bluetooth module, there is alot of noise in the audio (the system work fine in the ‘USB’ mode). The module has its own 3.3V regulator and sufficient decoupling capacitors. The entire system operates off a 3.7V li-ion battery. The noise fluctuates when parts of the circuit are touched or moved around.

Is there a way to get rid of such noise and what could be the root cause of it?

Hi @cb6,

Can you sharethe topology are you using for the testing? Can you clarify

Please correct me if I am not talking about yours
1- USB line
PC <—> Audio amplifier
2. Wireless
PC <—(Wireless)—> BC127 <—> Audio amplifier
Is the PC audio amplifier??
Which FW are you using?

Hi, thank you for your response.
The system is configured as follows:
1 - USB:
Sensor → Audio amplifier → microcontroller → (USB cable) → PC
2 - Wireless:
Sensor → Audio amplifier → microcontroller → BC127 → (Wireless) → PC

The BC127 module is using Melody 7.1, and is placed on a small PCB above the main PCB which contains the rest of the electronics. Each stage of the circuit has its own voltage regulators.

The noise is only present in the Bluetooth mode.
The noise fluctuates when parts of the circuit are touched or moved around.

Hi @cb6

You can update the device to the latest FW Melody 7.3 https://source.sierrawireless.com/resources/airprime/software/bc127-firmware/#sthash.cXciTIqD.OsRDsLzw.dpbs.
Check it again and share ​your result.


Updated to the latest Melody firmware but the problem still persists.
I have found that shielding the other circuitry reduces the noise. However, it is sporadic and changes when the wires or shielding moves around even a little.