SUMMARY OF THE PROBLEM (For more details about the hardware see my first log)
I build a (DIY) prototype of a 800-1600MHz FMCW GPR and I am not that happy with the results yet.
There could be many causes but I would like to start by getting your opinion on the performance of the front-end. If that is not ok, there is no point at continuing with more complicated stuff like antenna's.
THE MEASUREMENT
I attached a good coax cable + a series of attenuators between antenna input and output.
This way I should get an idea of the sensitivity of the device and if it is working.
Attenuators I used 10dB and 20dB inline attenuators ( see image). I increased the attenuation from 60dB to 110dB and the measured voltages behave exactly as expected ( 20dB attenuator -> voltage 10x lower). So confirms there are no measurement errors or stray signals leaking in.
A scan measurements with 80dB (4x20dB) attenuation between TX and RX input
A scan ( with hardware high pass / differentiator enabled).
The frequency is about what I would expect :
with a 20ns delay and a 800MHz/1ms sweep I expect a beat frequency of 20ns * 800MHz / 1ms = 16KHz. And there are about 16 cycles. Note : the FFT uses zero padding that is why the FFT peaks at 4x higher value.
A scan measurements with 110dB (4x20dB+2x10dB) attenuation between TX and RX input Amplifier gain is set higher to maximize signal.
What I would like to learn from experienced builders :
Is this front-end performance OK ? Do you need extra measurements to say ?
Would you expect a higher sensitivity ? Or a cleaner sine wave ?
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.
Fieldman
