Radio Frequency Multiplexer
The customer wanted a 16-way RF multiplexer to add to an existing product line. He specified a 4-way, gallium-arsenide absorptive switch IC with integrated silicon control logic, but provided no other details. ECROS Technology wrote the functional description, which the customer reviewed, and produced three fully-functional pre-production units, all in three weeks.
Microstrip PCB Design
A significant challenge was the 3 GHz bandwidth of the switches, which was not to be significantly degraded by the overall design. This made very careful PCB design necessary, using microstrip techniques. To speed up PCB fabrication, the standard specification 4-layer process of Advanced Circuits was used for the pre-production units with plans for a custom layer stack-up for production. The separation of the top layer from the next layer down, which was necessarily chosen for the RF signal return path, was 0.009 inches. For a 50 ohm characteristic trace impedance, this required a 0.015 inch trace width. However, the production stack-up was to have a 0.020 inch layer separation, allowing the RF traces to be increased to 0.036 inch width, lowering series resistance and decreasing loss.
To accommodate this known change, ECROS Technology used advanced design rule settings of the PCB layout tools to force top-layer ground away from the RF traces, leaving room for the increase in width. Vias were placed at intervals along the edge of top layer ground, allowing RF return current from fringing fields to flow into the RF return plane below. Logic traces were routed around these vias. Because the increase in RF trace width was planned for, no re-routing will be needed for production PCBs, saving time and money for the customer.
Corners are known to be problematic with impedance-matched microstrip. There are methods of chamfering corners to maintain the characteristic impedance, but the method used in the RF multiplexer was to avoid corners entirely by routing RF traces along smooth curves.
The customer required the multiplexer to be controlled by the host system using either four parallel logic inputs, a SPI interface or a Two-Wire interface compatible with the Philips I2C standard. This made an embedded microcontroller essential. The Atmel® ATtiny48 was selected as it has both SPI and Two-Wire interfaces. The four pins used for the SPI interface were shared with the parallel interface, so one more pin was used to allow the host to select between them. The RF switches have four logic inputs to individually select each of the four multiplexer paths, so with the five devices needed to implement a 16-way multiplexer, a total of 20 control signals were needed. With the Two-Wire interface, two powers, two grounds and reset, all 32 pins of the microcontroller were used!
With at least three layers in the PCB necessary to provide RF routing on the top layer, a continuous RF return plane on the next layer and somewhere to route control signals, a 4-layer design was used. Non-RF routing was therefore very easy. Nevertheless, to completely separate RF and logic return signals, non-RF components were not allowed to directly connect to the RF ground plane and instead used a separate ground plane on the fourth layer. This plane was divided into bands running to each switch IC, crossing the RF paths at right-angles. The planes were connected only at the switch ICs. Thus, return currents from RF signals and control signals had no complete path in the other plane.
The control firmware for the microcontroller was written in Atmel's AVR Studio using the GCC compiler in the WinAVR package. The ATtiny48 has on-chip debugging features and the firmware was loaded and debugged using an Atmel AVR Dragon. Careful planning ensured that the customer would be able to easily load the firmware into the multiplexers in production, as the SPI signals and the reset line, used for in-circuit programming, are all exposed at the host system connector.
The host system can directly supply +5V DC power to the multiplexer or can apply between 6 and 14 V DC and use the on-board regulator. The GaAs switch devices require a negative supply, and this is generated by a charge pump device and a few capacitors. To make absolutely sure that the switching of the charge pump does not interfere with the RF signals, it is double-filtered with a small inductor and a ferrite bead before local decoupling right at the RF switch supply pin.