DIY 800-1600MHz FMCW ground radar. Are the front-end measurements looking ok ? - Page 1
Forum section: electronics / rf-microwave

author: DIY9140
author_rank: Newbie
timestamp: August 23, 2025, 07:19:50 am
content: I build a working prototype of a 800-1600MHz FMCW ground penetrating radar, but I am not sure if it works properly.Could be a lot of things wrong: RF front-end not good enough, antenna reflections, instable ground coupling, wrong frequency range for the application,…I would like to detect buried brick & tiles up to 75cm deep in normal sandy loam ground.First I would like to know what you think of the front-end, without looking at the antennas.I put a 5m coax + 110dB attenuators between input and output and measured the mixers output. See images below.I created a Hackaday page with more details about the project & result of this measurement.The project page :https://hackaday.io/project/203825-diy-8001600-mhz-fmcw-gpr-in-progressThe front-end test log :https://hackaday.io/project/203825/log/242929-question-does-the-frontend-perform-good-enoughWhat do you think ? is the front end good enough for my application ?thank you!

author: szoftveres
author_rank: Regular Contributor
timestamp: August 23, 2025, 07:30:33 pm
content: The mixer is very noisy (best NF = 11dB); once you measure the isolation between Tx and Rx antennas (in real-life conditions, e.g. your device hovering above the ground), you can calculate how much room you have to add an Rx antenna LNA without overdriving the mixer and the rest of the chain with signals picked up directly from the Tx antenna.In theory, pushing the NF down by only ~6dB with an LNA could get you ~1.4x increase in range.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 24, 2025, 03:31:31 am
content: Do you have a spectrum analyzer, or any way of verifying if your frequency sweep is actually linear (and around the same power)?

author: DIY9140
author_rank: Newbie
timestamp: August 24, 2025, 08:10:47 am
content: Yes, I tuned the VCO and filter voltage sweeps that are produced by the microcontroller to get a constant output power and a linear frequency sweep. Both verified with a spectrum analyser.

author: DIY9140
author_rank: Newbie
timestamp: August 24, 2025, 08:45:15 am
content: you are correct, normally the AD8347 is always used with a LNA in front.The isolation between the TX and RX antenna's is not that good. With the 2 antenna's on the ground I measured the isolation across the 800-1600MHz range in different separations/orientations.* flat on ground , TX and RX same orientation , 5cm in between : 50dB (800MHz) to 35dB(1600MHz) isolation* flat on ground , cross polarized TX and RX , 5 cm in between : about 50dB isolation across the band.But when putting the antenna's on short grass, the isolation varies a lot when the distance to the ground varies a few cm.What I see is that in real life with these antenna's the internal AD8347 gain has to be set much lower than its maximum to avoid saturation. Making its NF even worse.I guess that FMCW is not the best idea for a ground radar, because there is such a large and variable coupling between the antenna's ...

author: szoftveres
author_rank: Regular Contributor
timestamp: August 24, 2025, 03:41:58 pm
content: When leakage occurs from Tx to Rx in an FMCW radar, the mixing product is always either DC (in-phase leakage with no time delay) or lowest baseband frequencies (800Hz is the lowest in your system) due to very nearby reflections. The level of these mixing product is always the strongest signal you'll ever encounter in your system - reflections from targets 50cm deep are many tens of dB weaker.It's important to see that when a target is up close, usually the DSP (FFT) process giain combined with the dynamic range of a 12bit ADC are perfectly enough to detect it; this AD8347 mixer has flat 40dB built-in baseband gain, which is great for telecomm applications, but in this case it simply overloads itself when the target is right nearby or when leakage occurs.So all you need to do is to get a mixer and LNA combination that will just not yet saturate due to leakage, and then use range compensation (4th order high-pass amplifier with 0dB gain at low frequencies, you refer to it on your schematic as 'differentiator') to amplify the higher frequencies.For the mixer, I would use something like an ADE-11X+ (preceded by an Rx antenna LNA, and driven by strong LO), followed immediately by the range compensation. When the target is close, a ~20 dB LNA can bring the signal level up enough so that the ADC and FFT will have no problem detecting it even if the baseband gain at these low frequencies is 0dB. Beyond that, the range compensation amplifier takes over the amplification, so that the (theoretical) output signal level remains constant and never brings the following stages into saturation.Out of curiosity, what type of varicaps are you using in your tracking filter?

author: DIY9140
author_rank: Newbie
timestamp: August 24, 2025, 07:54:05 pm
content: Thank you for the elaboration.I use SMV1234-079LF varicaps ( bought at Digikey)My first prototype used a PGA-105+ low noise preamplifier and ADE-30 mixer, but I don't remember why I switched to the AD8347 ....

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 26, 2025, 06:33:02 am
content: I've built a 2.4 GHz FMCW radar for fun before.What is the bandwidth of the signal that you are receiving? If it's in the audio range, you should get an audio codec to digitize your signal and get way more dynamic range. In my case the frequencies were under 500 kHz, so I used this chip from Analog Devices:https://www.analog.com/media/en/technical-documentation/data-sheets/adar7251.pdfHaving a better ADC will let you get away with less gain, and therefore no saturation. With audio it's pretty common to have 16 or even 24 bits. Even if your signal is 10 times smaller than full scale, a 16 bit ADC will be able to better detect it compared to a 12 bit ADC (at least in the ideal case).I also second having a high pass filter. The signal path should be LNA -> mixer -> transistor/op amp baseband amplifier -> high pass filter -> adjustable amplifier -> ADC. Getting a separate LNA chip and a separate mixer chip will be useful. Try to keep the noise of the baseband amplifier, but it's not critical since the LNA mostly affects the noise figure. In your example, you have 13 - 35 = -22 dBm at the input of the RX antenna worst case. The LNA will have 20dB roughly, so -2 dBm will go into the mixer. That will drop to about -9 dBm. (Generally the mixer starts saturating above 0dBm, so you still have some margin, and you can always pick a lower gain LNA). The op amp/transistor amplifier can have another 10dB voltage gain, and then it will go through the high pass filter. Then you can have another adjustable amplifier before the ADC. From a noise figure perspective, the LNA noise dominates, but if you are not careful with how you design the baseband amplifier, you can easily make the noise worse. The LNA + mixer result in 13 dB gain, and even low noise op amps have about 10dB noise figure. (A 50 ohm resistor has 0.9 nV/sqrt(Hz) noise, and 10dB means multiplying that by sqrt(10) to get 2.8 nV/sqrt(Hz), which is in line with low noise op amps). You can design a baseband amplifier with discrete transistors, just pick one with a low base spreading resistance. The filter can be constructed as either a LC filter, or two RC filters spread out throughout the system. You can also get a 3dB improvement by using both I and Q. In your program, you will just do the FFT on complex numbers instead of real numbers (so for every instance put I + j*Q into the FFT). If you use an audio codec you get the second channel for free, so there is no point in not using it.