In the 2018 Update, the VFO and LO Board was replaced with a VFO, LO, and BFO Board. The original concept for the Express Receiver was that one day a companion Express Transmitter would be added, and the VFO, LO and BFO signals generated in the Receiver would used in the Transmitter to avoid duplication for the frequency generation circuitry.
The new board retained the Si570s for VFO and LO signal generation, and added an Si5351A for BFO generation.
The Si570 signals were split through 2-way power dividers for the Receiver and Transmitter. Two clocks from the Si5351A were used for Receiver and Transmitter BFO signals, leaving the 3rd clock spare.
To simplify the use of the S15351A, an Adafruit Si5351A breakout board was used to mount the Si5351A and associated circuitry on the VFO, LO and BFO Board.
Note that to make the BS170 level converters work reliably with Si570s and the Arduino I2C bus, it was necessary to increase the pull-up resistors to 10K ohms as shown in the schematic below.
In the original VFO and LO Board, a general purpose digital I/O line was used to select routing of the I2C bus to either the VFO or LO Si570. For this new board, an ATtiny85 microcontroller was added to eliminate the need for the general purpose digital I/O line. The ATtiny85 has a unique slave address on the I2C bus, and listens for commands to switch the I2C bus to either the VFO or LO Si570 via the CD4052 analog switch. Software for the ATtiny85 is available on GitHub at the following link: https://github.com/RodNewHampshire/Express-Receiver-VFO-LO-BFO-Board-ATTiny85
The ExpressPCB file can be provided upon request.