The 3’rd in the series is the control board.
It’s purpose is to allow easy interface from a single-board computer that has one SPI and one I2C port.
Also, it provides a common reference clock and a synchronous update signal to the two DDS boards, as required by AN-587
The SPI port is level-shifted to a MCP23S17-E/SP SPI to parallel expander.
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Category Archives: VNA
The second board in the VNA is the phase / frequency detector.
For this one I chose an AD8302 chip.
It will measure in the range I’m interested in.
It might not be the best part for this, but for the initial design it’s enough, as it works out-of-the box.
The board contains:
- two low-pass filters, identical to the ones used on the DDS boards
- a fixed 30dBm atenuators for the reference input
- two switched 30dBm atenuators for the DUT input
- a AD8302 detector
- a MAX11612 12bit ADC to read the detector
- LDO, passives and decupling for the active components
- transistors and diodes to drive the two atenuators switching relays
The signal generator is based on the AD9851 DDS chip
The board contains:
- AD9851 chip and passives
- 2 5V LDO’s for the digital and analog DDS power supplies
- footprint and passives for a local 30MHz oscillator, for stand-alone use
- 50MHz low-pass filter copied from the N2PK VNA project:
- several footprints for atenuators
- a 1:2 50ohm power spliter
- 2 gain blocks based on the MSA-0286 MMIC amplifiers
- different connectors for RF / CLOCK / power / DDS data
Hello
While working on RF stuff I got to the point where I had to tune filters and measure frequency response of amplifiers and other tuned circuits.
This implies having access to a VNA.
As something like that is out of reach, and buying an existing one is out of my budget I started out designing one from scratch.
For that I’ve used the N2PK VNA as a starting point with some small changes that I though will be useful.
The entire system will have:
- Two AD9851 based generators, each with 2 buffered outputs
- One AD8302 based detector that measures phase and amplitude differences
- One system control board, that offers to the driving computer a SPI and an I2C interface
Each board has a 12V input and local LDO’s for all needed voltages.
12V was chosen because the value of the MMIC’s bias network was high enough to require just a generic inductor.
I’ve used 5V for all internal supplies as all chips used that voltage.
Also, I’ve used a common PCB template – so the boards can be stacked with M3 spacers.
That will also simplify making some shielded cases, if needed – just one case template.
The overall system architecture looks like this:
