Sunday, May 11, 2014

R Board Overview

I wanted a simple way to digitally control attenuation, different amplifiers and filters, and the ability to try different analog processing quickly.  This led to another BREC board with simple RF processing (R Board).  It can also be viewed as a QFN practice board since there are several small, simple and cheap QFN parts that can be used to refine assembly techniques.

The R board (or RF input) is designed to produce better RF isolation and level control along with band specific processing.  Four channels are available using an RF mux.  Each channel has a different attenuator or amplifier present.  Channels are selectable via an SPI interface.  The board can act as a stackable A-board daughter board.

The module block diagram is:
A channel is selected via the SPI interface.  The SPI interface is implemented in two chained serial shift registers to reduce IO pin count to the BBB.  Each channel has a separate regulated 3.3V low noise supply.  When a channel is selected, the mux path is enabled and the regulator enabled.

The RF switch used is an SKY13384 (non-reflective).  It provides 4 channels that are usable from 20M-4GHz. When a channel is not selected it is terminated in 50 ohms.  A mux is used on the input and output side of a channel.  Care must be taken in user defined channels to note that when not selected a channel’s input and output has a DC path to ground through 50 ohms.

Channel 0 is a small band specific daughter board connected via two 6X1 headers.  These headers include the RF input and output, +5V (BBB supply), a digital supply enable, and two digital signals from the SPI output register (All digital signals are 3.3V logic).  This channel is designed to be small and readily pluggable.  A daughter board is roughly the width of a BBB and carriers (~2.1 inches) and can be an inch or less wide (slightly wider to cover forward mounting bolts if desired).

Channel 1 is a digital step attenuator using the SKY12347.  This is a 6 bit attenuator providing 0.5 to 31.5dB of attenuation from DC to 3GHz.  It is used in a parallel configuration.  It is only available in a 24 pin QFN package (like most attenuators at this point).

Channel 2 is a BJT amplifier - either common base or common collector configuration.  It includes 0805 L and C components configured to provide impedance matching as well as tuned or resistively terminated ports.  Final user values and components determine the configuration.  The BJT layout is targeted at a BFR93A SOT23 package (or any similar footprint).

Channel 3 is a broadband amplifier using the AD8354 providing approximately 20dB of gain from 1MHz to 2700MHz with 50 ohm termination and a noise figure of about 4dB.  It is operated in the 3V mode.

As built schematic :

As built unit #1:

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