System overview

Block diagram RF FRONT-END

Complete schematic

General info

• VCO is swept linearly in 1ms from 800MHz to 1600MHz. VCO non linearites are compensated.
• Microcontroller averages 16 sweeps, does analyses (FFT) and sends results 30 times/s to smartphone app through a Bluetooth serial link.
• Large VCO harmonics are reduced with a tunable low pass filter that is swept synchronously with the VCO.  Resulting in clean constant power output, with second harmonic at least 35dB lower then main frequency.
• Special care is taken to isolate the input/output circuit and reduce VCO pulling. 
• The AD8347 is a quadrature demodulator, but currently I am only using the Q output
• The Q output is connected to 2 LF amplifiers, both are measured by the microcontroller 12bit ADC inputs.
  • the first LF amplifiers only amplifies
  • the second LF amplifier has a high-pass filter. The idea is that the further away a reflection is the weaker it will be and the higher the frequency is.  So it makes sense to amplify higher frequencies more. In software I can choose which amplifier to use.
• The AD8347's amplification is controlled by the microcontroller ( using a I2C DAC)

Smartphone app 

With the Android app I can :

• set the sensitivity (amplifier gain)
• view live A scan's ( average of 16 sweeps). 

• view B scan ( time - depth )

• control many settings

I will add more about the antennas  & performance later,  but first I would like to find out if the electronics perform good enough or not. That is my first question in a next post.