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By All About Circuits

Teardown: Bluetooth FM Transmitter

Courtesy of All About Circuits

Most cars will come with a radio that has FM capabilities but not every car has a Bluetooth music system. To get around this, some clever people designed a Bluetooth device that can connect to devices such as phones and MP3 players and then broadcast the music over FM so your cars music system can play the music. Of course, these devices sit in a legal grey area due to transmission laws but in this teardown we will open one up to see how they do it!

The Bluetooth Transmitter

The device has a rather numerousness amount of IO which include a display, various buttons, a volume control knob, audio line input, a card reader, and two USB ports. One of the USB ports is for charging (indicated by the electric shock sign followed by 2.1A) while the other is for loading files from USB. The back of the unit has the model indication T11 and a DC supply of 12V – 24V. This implies that there are internal regulators to produce the specific voltages needed as car battery voltages can vary a lot.

The Bluetooth FM Transmitter

The Bluetooth FM Transmitter  

The card slot and audio line

The card slot and audio line

The two USB ports (charging and reading)

The two USB ports (charging and reading) 

Information found on the back of the transmitter

Information found on the back of the transmitter 

Getting into the transmitter was rather easy with the face plate coming off without effort and the use of only two screws keeping the front on. However, the display was incredibly sensitive and as soon as I applied pressure to the transmitter (to get the face plate off), the screen cracked.

The two screw holes and the cracked screen. Thank goodness this is a teardown!

The two screw holes and the cracked screen. 

The Innards Of The Transmitter

Removing the front cover reveals the internal PCB and other various components. The top side of the PCB, however, reveals mostly passive components and IO related parts such as switches and a single potentiometer. This means that the magic behind the transmitter must be on the underside of the PCB!

The front cover removed revealing the PCB

The front cover removed revealing the PCB 

Interestingly, this device also connects with phones to act as a hands free set and when the front cover was pulled off so was the microphone. It turned out that the microphone was glue to the front cover and used very small wires (hence being prone to damage). Removing the display revealed many stitching via and a portion of missing copper which suggests that there is some kind of radio IC behind (as the copper on the PCB would interfere with the radio transmission).

The various IO including the wires going to the microphone

 The various IO and the wires going to the microphone

The display removed showing stitching via

 The display removed showing stitching via

The potentiometer knob removed

The potentiometer knob removed 

The Underside Of The PCB

The underside of the PCB revealed a lot of ICs, modules, connectors, and a strange fact on the PCB. As it turns out, this PCB is only a two layer board instead of a more standard 4 layer (when considering the use of radio modules and the need to conform to EMC rules). At first glance, it appears that the transmitter is powered by an ARM microcontroller that connects to an external radio module and possibly serial memory as well.

The bottom side of the PCB (reminds me of a Raspberry Pi)

 The bottom side of the PCB

The first IC of interest is the ARM AP8048A (MySilicon) which is a highly integrated audio SoC with an ARM Cortex-M3 core, SD/MMC card controller, SARADC, audio DAC, RTC, and IR decoder. It can operate up to 98MHz, has 128KB SRAM, embedded 3.3V regulator, UART, PWM, and even FAT16/FAT32 file system.

The main controller – the ARM AP8048A

The main controller – the ARM AP8048A 

The second IC just above the ARM controller is the BergeMicro 25Q80ASTIG which is an 8Mbit dual and quad SPI flash. This IC will most likely hold configuration information include default connections, settings, and various other pieces of information that would be undesirable to loose. The datasheet for the IC states that the BergeMicro 25Q80ASTIG can operate up to 120MHz with data rates as fast as 360MBits/s possible, a minimum of 100,000 read/write cycles, and software/hardware protection.

The SPI serial flash IC

 The SPI serial flash IC

The bottom left of the PCB has various passive components, a crystal (12MHz), and an IC with the identification 802747445TCC. Searches on Google return no results and so the true identity of this IC will remain a mystery. However, using some basic deduction we can deduce what this IC may do. This IC is surrounded by inductors, many capacitors, and resistors. Considering that it sits next to large thick copper planes and is very close to the V_DET and power input suggests that this IC is a step down DC-DC regulator for powering the other ICs on the PCB.

Unidentifiable IC – Most likely a power IC

 Unidentifiable IC – Most likely a power IC

The module that is attached to the top of the PCB is what handles the Bluetooth communication. The IC that controls the module has the identification MEDIATEK MT6622N which is a monolithic single chip that contains a 32-bit RISC microprocessor, integrated LDO, high speed UART, RAM and ROM, and low power features.

The Bluetooth Module

The Bluetooth Module

 

Functional diagram of the IC (from the datasheet)

 Functional diagram of the IC (from the datasheet)

Summary

This transmitter shows how modular design has transformed the engineering world. Instead of needing to create entire circuits, modules can be almost glued together and then packaged into a product which reduces the designing and production time. Such modules help to offload complex tasks from the main processor that may need to focus on more important tasks such as audio streaming and display updates. Such modular design also results in simpler PCBs which helps to reduce the number of layers and therefore the cost.