Electronics design for multimedia device

Early prototype

To enable software development to progress before product hardware was in place we assembled and tested two identical prototypes using off the shelf development kits and hand-built breadboard prototypes for the main functionality.

These prototypes weren’t pretty nor compact, but they enabled early test and development of interfacing products and of system software. These units were shipped to the client and have proved to be invaluable for the project.

Printed circuit board form factor

The plastic enclosure development and electronics for the project ran in parallel. A key step was to ensure that the electronics would fit within the available enclosure space envelope and that the electronics would properly function.

From the outset it was clear that all of the required electronics functionality wouldn’t fit onto a single printed circuit board. The design was split into two boards with specific functionality to minimise the number of interconnections between the boards. This also allowed a useful design partition so the board with the more complex functionality could be tested in isolation from the second board.

Schematics

We drafted schematics for both printed circuit boards that included the main functional elements of the product. Critical signal nets which were key for the products correct operation were connected up. These included high speed digital nets and impedance-controlled nets.

The result of this was two draft printed circuit boards generated with 3D cad models within Altium designer. These were then collaboratively and iteratively adjusted in collaboration with the clients product designer to achieve the right balance between form and functionality. There were many Skype screenshare sessions, sharing of CAD models and updates until the positions of connectors, size and shape of the pcbs and mechanical securing’s were fixed.

An added complication was that the plastics could only accommodate a 5mm gap between the top and bottom printed circuit boards. A lot of care was needed to check that the components and leads didn’t clash. Extensive use of multi-board design was used in the project to ensure the boards would properly fit together and not short out when connected together.

Electronics development

Following the form factor design freeze the detailed electronics design began. Designs for the top and bottom rigid printed circuit boards together with a flexible interconnection pcb were completed following robust internal design processes to ensure designs are properly reviewed and signed off before issuing.

While the product boards were being manufactured, we designed and produced a small break-out pcb to allow us to simulate and debug the high speed and audio aspects of the product without reliance upon a connection to the controller that was being developed in parallel. Initially this was intended solely for internal development and test use within Ignys but later in the project both the contract electronics manufacturer and the client also found these useful for their own tests.

Prototypes for all three boards were manufactured and populated and delivered to us for testing where a number of potentially show-stopping issues were quickly found.

Changes

Networking connectivity didn’t work at all on the board, even at low data rates. We could connect to the Gigabit Ethernet Switch via its management interface but that was all. Referring back to the datasheet and requesting the device manufacturers support confirmed that the device was correctly connected. We knew that it should work as the device had already been tested with a development kit.

The designed pcb differed from the tested development kit as we had taken advantage of the devices ability to automatically detect and swap its differential p and n inputs over and to re-order the four sets of differential pairs to optimise the printed circuit board layout.

After checking and rechecking the devices configuration and connectivity we swapped over the connections to the device with an adapter cable and found that it sprang into life. The datasheet was wrong!

The USB3.0 connectivity from between the controller and the base worked at its maximum speed of 5GBps when connected via our break out test board. When connected via the flexi-pcb it had significant speed limitations. Further testing with off the shelf cables and copper tape revealed that although the flexi-pcb was the correct characteristic impedance the track lengths required for the product form factor resulted in significant attenuation over the cable.
The track dimensions to meet the required differential impedance are governed by the spacing between the signal and ground printed circuit board layers as well as the track widths and differential pair separation. In the first design we had worked with the flexi-pcb company to determine the correct layer stack and separation for the flexi-pcb. With the results from our testing we determined that we needed to thicken the tracks to reduce the signal attenuation but still maintain the correct differential impedance so had to increase the distance between the tracks and ground.

Solutions

We contacted several flexi-pcb manufacturers and found one that could meet the requirements, at the same time the client’s main supplier was also able to meet the requirements. We re-spun the pcb artwork to double the track widths and to have the updated layer stack. The updated flexi-pcbs allowed the serial connection to run at its full 5Gbps speed.

Modification information for the required changes to the printed circuit boards was documented, reviewed and issued to allow working units to be produced from the A model electronics to keep the project progressing.

This was quickly followed up with B model electronics which worked well.

The third party interfacing equipment had some errors and omissions that reduced the audio quality and needed a final adjustment of the bottom board printed circuit board. It was vital that these adjustments were completed and checked quickly so after a couple of late nights and long days the finalised artwork for the design was completed.

Compliance

In addition, Ignys provided support for product CE compliance by setting up the Technical Construction File folder structure and providing support with the risk assessment, documentation and EMC pre-compliance for the product.