Transceiver Backplane Board

The Backplane Board is designed to fit in the HP-436A Power Meter chassis providing seven 64-pin DIN connectors for standard-sized 3.5″ x 7″ plug-in boards with 1″ spacing.

Referring to the Backplane Board Top View below, the left side of the board also includes connectors for:

  • Left & Right LCDs
  • Front Panel Board
  • Power Switch
  • Serial Ports 1, 2, & 3
  • Software Serial Port
  • PTT & External TR Switch

The right side of the Backplane Board has connectors for 13.8V DC power input, and power and bias control for the Pennywhistle PA module.

The Backplane Board includes power conditioning, power protection, and power switching circuitry. A link to the complete Backplane Board schematic is provided below.

Backplane Board schematic.

Backplane Board Top View
Backplane Board Bottom View

November 15th, 2021 Update. Dedicated Relay Driver bus connections for Tx Low Pass Filters were removed from the backplane to make room for a second I2C Bus on the Backplane Board. Tx Low Pass Filter selection is now accomplished by diode OR-ing the Band Pass Filter Relay Driver lines on the TX Low Pass Filter & Antenna Relay Board. The second I2C Bus was added to allow access to two I2C devices with same I2C address; specifically, Si5351 Clock Generators used on the Audio, QSD, SSB Gen., & BFO/Carr. Osc. Board, and the 1st Mixer, 2nd Mixer, Roofing Filter, & Post Mixer Amp. Board. No physical change was required to the Backplane Board, just redefinition of the bus connections. An Adafruit TCA9548A Breakout Board was added to the Arduino & I2C Relay Control Board to multiplex the Arduino I2C bus between I2C_0 and I2C_1 on the Backplane Board.

Referring to page 3 of the schematic below, the 64-pin bus connectors are J5 (far left) through J11 (far right). All bus connectors have a common set of functions for pins a1 – a12, a31, a32, and b1 – b32. The common functions across the bus connectors are as follows:

  • 13V Digital (a3 & b3)
  • 13V Analog (a13 & b13, except J5 a13)
  • 5V Digital (b4)
  • Ground (a1, a2, a31, a32, b1, b2, b31, & b32)
  • I2C_0 (b5 & b6)
  • I2C_1 (b15 & b16)
  • Band Pass Filter Relay Driver (b21 – b29)
  • T/R Relay Driver (b14 & b30)
  • RF Attenuator Relay Driver (a4 – a7)
  • Relay Driver Common (b20)
  • External T/R Switch Selection & Driver (a8 & a9)
  • RF Preamp Relay Driver (a10)
  • PTT (a12)
  • Pennywhistle PA (b10 & b11)
  • Power Management (a11, b7, b8, & b9)
  • Relay Driver Spares (b12, b17, b18, &b19)

In addition to common connections across J5 – J11, J5 has unique connections on a13 – a30 associated with the Arduino & I2C Relay Control Board for the front panel, serial ports, and other functions. As such, the Arduino & I2C Relay Control Board can only be installed in J5 location. The Arduino & I2C Relay Control Board includes PCF8575 I2C General Purpose IO Expander and ULN2803A Darlington Transistor Array to drive all the relay functions on the bus.

Backplane Bus Schematic

One of the objectives with the 2020 Transceiver build is to manage the power down function from the Arduino so that current settings can be saved and the transceiver powered down in a controlled fashion. The power control and conditioning circuitry on page 2 of the backplane schematic below is based on the Elecraft KS3.

13.8V power is connected to the Backplane Board at J1. DC Input filtering is provided by L1, C10 and C11, followed by D1 and D2 which provide over voltage and reverse voltage protection, respectively.

Power on/off switching for 13V_D, 13V_A, and 5V_D on the backplane bus operates as follows.

  • 13V_UNSW is connected to one terminal of the SPST front panel power switch
  • PWR_SW is connected to other terminal of the SPST front panel power switch
  • When the power switch is turned on, Q4 is biased ON which in turn biases U4 ON, applying 13V_D, 13V_A, and 5V_D to the bus.
  • Once started, the Arduino asserts PWR_HLD which causes Q3 to be biased ON which will now keep U4 biased ON regardless of the state of the power switch.
  • The Arduino monitors PWR_SENS to detect when the power switch is turned off.
  • When the Arduino detects that the power switch is turned off, a routine is executed to save current settings to EEPROM and perform any other shutdown functions before the Arduino releases PWR_HLD which removes 13V_D, 13V_A, & 5V_D from the bus, including the Arduino & I2C Relay Control Board.

Power connection for the Pennywhistle PA is made before D2 (reverse polarity protection diode) to avoid the diode voltage drop, providing full 13.8V for the PA. The Arduino controls power switching for the PA via Q2 and U3 by asserting PA_PWR_ON.

U1 is a INA219 breakout board from Adafruit that is used to monitor PA voltage and current via I2C bus connection to the Arduino. U1 was mounted on the bottom side of the backplane to avoid interference with plug in boards on the top side of the backplane.

U2 provides regulated 5V DC power for the INA219 breakout board. 5V DC from U2 is also provided on the backplane, however, this should only be used for low power, incidental requirements due to power dissipation limits of U2. Typically, plug in boards include regulation to convert 13V_D and 13V_A to required voltages.

The Arduino controls PA bias on/off switching via Q1 by asserting PA_ENB.

L3 provides RF isolation between 13V_D and 13V_A power rails on the backplane.

Backplane Board Installed in the HP-426A Chassis

© 2014 – 2021 Rod Gatehouse AD5GH

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s