ISM Radar Testbed: Doppler Experiments



In summary the following video demonstrates the use of a continuous-wave transmission, the reception of the received Doppler-shifted echo, and its mixing with a copy of the transmitted waveform, known as the correlation L.O.

This type of Doppler radar can detect the presence and velocity of a body in motion. Distance, however, is not measurable with traditional pulsed radar techniques due to the short distances involved. Target motion can be measured at baseband, close to D.C., the frequency of which indicative of the target velocity.

Demonstrated in the video is basic motion detection as well as an example of Doppler signature, in this case a house fan both stationary and oscillating.






The ISM Radar Testbed hardware used is a transmitter consisting of a VCO, power divider (for the correlation L.O.), and an amplifier capable of +17dBm. The amplifier was turned off for the first experiment thus the output power was -5dBm. The receiver consists of an LNA and mixer. Baseband filtration is intended to be provided externally, however was not included in the initial bringup. I also laid out two simple patch antennas which offer about 3dB of gain each in a bistatic configuration. I plan a series of antenna respins to experiment with beam forming and gain improvement.

ISMR_BD

Early models show that with a vehicle whose RCS is 32dBsm, at 5 meters this system is capable of outputing a receive echo at -25.1dBm. With a decent baseband filter the system noise floor is better than -85dBm thus making the system sensitive enough to detect vehicles 100 meters away, provided the following conditions are met:

1. Multipath is managed through narrower beamwidths and sidelobe shrouding
2. Backend processing SNR requirements are:
a. If performed in the time domain, 25dB SNR is sufficient
b. If an FFT or Goertzel algorithm is used, detection may go beyond the noisefloor however processing time must be minimized
3. All spurs and interferers lie 25dB below the carrier if time-domain processing is used

Also explored in the video are the various sources of noise in the system, both fixed frequency spurs and “wideband” noise floor bumps. The main culprit in this system is a cheap lab supply, which utilizes full-wave rectification and poor filtration which splatters the baseband spectrum with 120, 240, 360Hz, etc, spurs. A prominant 60Hz spur also exsists in the laptop sound card.

Turning off the transmitter makes all spurs but 60Hz disappear. This makes me believe that the transmit supply is bad and is generating sidebands which make their way to the receiver via the L.O. Of course direct coupling through the ground plane is possible, however knowing the source is key to suppression.

Proper filtration techniques shall be employed, as well as alternative power sources, such as batteries. The system is intended to be deployed on a vehicle, which one would think is safe due to battery operation, however alternators are a famous source of noise.

Multipath cannot be discounted as a source of interference in this demonstration, as well as the fact that the transmit VCO is not disciplined with a phase lock loop. Also and most unfortunate, the wifi on the laptop was left on during this experiment. The drift in the L.O.is very likely to be operating near the wifi band of operation which has two effects:

1. The first is visible in the noise floor of baudline, where through reciprocal mixing the wifi occupies enough noise bandwidth to raise the entire FFT noise floor.

2. In the Doppler signature section of the video, we see the laptop wifi drop out briefly as I approach the radar. I am possibly inducing another multipath mode which may very well affect the wifi receiver.

Although wifi radios employ OFDM modulation to maintain operations in environments of high traffic, noise, and multipath, the effects of a 20dBm CW tone generated 18 inches from the wifi receive LNA cannot be discounted. No matter how sophisticated the modulation and DSP routines, no amplifier is impervious to a blocker that puts the amplifier in saturation, thus causing all intended receive signals to distort. This blocker can even be out of band as long as it puts the amp in compression. Reciprocal mixing may also be a factor.

All of the above theories and solutions are easily provable through experimentation, which I intend to do shortly and publish. After that, the real fun begins with LFCW, where target range can be extracted, even at extremely close distances.

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