Sunday, December 22, 2013

B/C Boards Overview


1                   B/C Board

The B and C boards share a common PCB and differ only in population and configuration.  They mate with an A board which provides BBB connection (the additional space taken by the BBB dual row headers is needed).  The board provides a wide band synthesizer, mixer, gain block, and input filter.

1.1         Architecture

The module architecture is as follows.

1.1.1        Power and Misc

The board uses the BBB system 5V from the A-board.  Two low noise 3.3V regulators are used, one for the synthesizer (due to its high power consumption), and one for everything else including output drive.  An I2C eeprom is include to store the device tree for the cape and allow automatic probing of the board.  Due to space, fixed SMT resistors are used for board identification rather than a DIP switch.

1.1.2        Filters

There are filters on the PCB.  The first is a bandpass GPS SAW filter (1575.42MHz center, 2MHz bandwidth). By default this filter is populated in the C-Board configuration.  The second filter is a 3rd order low pass LC filter which can be configured by the user. By default, it is configured as a 1500MHz LPF in the B board configuration. Both are terminated in 50 ohm inputs and outputs.

1.1.3        Amplifiers

A single stage fixed wideband +20dB amplifier is used. It has a noise figure of 3.8dB and is specified from DC to 2.2GHz.

1.1.4        Active Mixer

An active wideband mixer is used.  It is a low power and low cost device used in an unbalanced input and output mode to avoid the cost and space of wideband transmission line transformers. The package is a DFN8.  Unfortunately, most high performance RF parts are only available in lead-less packages.  Care has been taken in layout to provide ample space surrounding the part for alignment and re-seating if necessary.  Based on previous experience with hand construction involving QFN/DFN parts, the pads have been extended to provide solder paste channels and unaided visual alignment guides.  While this may reduce the overall performance, the alternative seems to be to not use the parts or suffer with high fall out rates of hand built units. 

1.1.5        Wideband Synthesizer

A dual output wideband synthesizer (37MHz to 4400MHz) is included.  One output is used to differentially drive the active mixer while the other is made available via a SMA output.  The auxiliary output is resistively terminated and single ended output, again to provide a wideband response while avoiding the need for wide band transformers.  The serial control interface is implemented via GPIOs from the BBB, through the A-board.  A common SDAT, SCLK is used for all boards.  The serial chip select (CS) and lock detect indicator (LD) are assigned to different GPIOs based on B/C board type to allow both to co-exist with an A-board.

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