It’s been a busy several weeks of prototyping, research and development. I’m happy to announce an initial wireless J-Box design is now available for peer design review and comments.
You will find the schematic and a brief explanation and theory of operation outlined below.


Download Schematic PDF

Starting from upper left, we see where the RF flows through Switched Antenna Segment connectors and through the RF Switch. The RF Switch is what controls whether this J-Box allows RF to flow onward to the next antenna segment or not. Disabling a segment(s) retunes the overall antenna to resonance for a given band.  The RF relay is a 1-coil latching relay, which is controlled by U2, a Darlignton transistor array.  There are two inputs to this array, one to open and another to close the latching RF relay.  This is all controlled by GPIO outputs from the J-Box Controller, U1 that is shown on the upper right. This is a CC2640R2F wireless microcontroller by Texas Instruments (more on that later).

On the lower left side, two connections to the RF Harvesting wire are attached. This RF Harvest wire is used to gather RF energy from the antenna wire segments attached to the J-Box using a combination of inductance and capacitance. As the harvested RF energy enters the PCB, a Lightning Surge Protector protects against static discharges caused by nearby lightning from time to time, protecting the components which follow. Two 12V zener diodes are placed back to protect against over-voltage from large RF fields; e.g., full legal limit power. These diodes clamp the incoming signal to plus and minus 12 volts peak-to-peak, creating a square wave when overload signals are harvested. At this point, the harvested RF signal splits and flows down two paths, one for RF detection and frequency counting and the other for RF rectification into DC that powers all the circuits (there is no battery, everything is powered by the RF power collected during transmit).

The RF Power Harvesting circuit rectifies the incoming signal, storing it first in C3, the primary energy storage for 12 VDC.  D5 regulates the voltage around 12 VDC, providing VCC12 which is used to power the RF relay and the rest of the circuitry.  From there, the current flows through D1 and charges C2, the secondary storage capacitor, which provides the input voltage for U3, a low-dropout voltage regulator that supplies 3 VDC for the remaining logic. Based upon testing, C3 is capable of operating the RF relay (12 VDC @ 16.7 ma.) up to 5 times before being discharged fully, more than enough for this application. C2 provides stored energy to power the remaining low-power circuits, which sleep most of the time and when fully active, draw no more than 10 ma for brief periods (only when transmitting BLE via the 2.5 Ghz antenna, which is typically not required).  Nominal operating currents are just a few milliamps before the MCU goes back into a sleep state, whereby it draws around 1.1 microamps.

U5 takes a small sample of the harvested RF signal and converts it into a square wave, which is fed into U4, a PIC12F675 microcontroller’s clock pin. U4 runs a small program which divides its clock frequency by 1,000, thereby converting the RF signal from megahertz into kilohertz.  This prescaled signal is then passed onto U1, the J-Box controller, where it: a) wakes the sleeping MCU up each time RF transmit signals are detected, and b) feeds an internal counter, which is sampled periodically to calculate the RF signal’s frequency.  That frequency is then used to determine whether to switch the RF relay in or out, based upon this J-Box’s position in the antenna.

U1 provides overall J-Box control for the Software-defined Antenna.  This MCU is pre-programmed with the J-Box control program, which oversees J-Box operation and provides status information via the 2.5 Ghz BLE (bluetooth low energy) antenna. Each J-Box acts as a master node, advertising itself periodically for clients who may wish to connect. Clients include any BLE compatible device capable of operating over the BLE protocol, which is just about any wireless  device these days, including iPhones, iPad, Android phones and tablets, BLE enabled PC’s, BLE-enabled Arduino, Raspberry Pi or other BLE-enabled IoT devices.  In the future, J-Box Controllers will also be capable of accepting and applying software updates over the air (not available in early versions).

The J-Box Controller operates as follows.

When RF transmit occurs, the RF Harvest circuit collects RF energy, which drives:
1) energy harvesting circuit that powers the J-Box
2) detects RF signal is present
3) wakes up the MCU from lowest-power sleep state
4) MCU measures frequency of RF signal
5) detects RF signal stopped
6) MCU determines whether to open/close relay

Future functions
7) MCU advertises itself for up to 30 seconds in case a client wants to connect to it
8) If no client connects, MCU goes back to sleep.
9) if a client connected, MCU provides status information (capacitor voltages, relay state, etc.) and can be updated or reconfigured.

That’s pretty much it. There are a few more odds and ends I want to add to the initial design before going to PCB production on the first units, including:

  • JTAG connector and interface for attaching debugger and ICE to production units for initial programming and debugging
  • UART interface for attaching serial interface via USB adapter to PC for programming and configuration
  • Stubs for attaching optional super-capacitor, battery and/or solar-cells to supplement (or replace) RF harvesting for powering the J-Box
  • Stubs for other I/O pins that are currently unused, so this device can be leveraged for other uses in the future

Please reply with comments and suggestions to this post.