AD8302 detector

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:

  1. two low-pass filters, identical to the ones used on the DDS boards
  2. a fixed 30dBm atenuators for the reference input
  3. two switched 30dBm atenuators for the DUT input
  4. a AD8302 detector
  5. a MAX11612 12bit ADC to read the detector
  6. LDO, passives and decupling for the active components
  7. transistors and diodes to drive the two atenuators switching relays

The detector board has 3 measurement modes:
1. The DUT atenuator is set to -30dBm -> the board measures +/- 30dBm around the reference signal
2. The DUT atenuator is set to -60dBm -> the board measures up to 60dBm of gain
3. The DUT atenuator is set to 0dBm -> the board can measure atenuation down to -60dBm

To use the full ADC range I set up AD8302 VREF as a reference input for the ADC.
On the initial power up and tests the detector had a lot of noise.
After switching the ADC’s REF input I deduced that where was something wrong when AD8302 was used as a reference.
For some reason the reference was oscillating at 5MHz. The problem was fixed by removing C22.
It looks like AD8302 does not like capacitive loading on the REF output pin.

The pcb is build on the same 100x50mm template where most of the bottom layer is just ground plane for RF shielding.
The rendered version is:

On the final board you can see the removed C22 and some other tentative changes – increasing the caps around the detector to improve LF response

Category: VNA
Comments are disabled