Distance (Not Speed!) Measurement Using CDM324 Radar Module

[@basvaningen4439]

Is it possible to measure distance of a static object with this radar module?

[@TechSponsorTV]

The simple answer is no. There is some chance of using super-low resolution FMCW (second part of answer) for stationary objects. And for moving objects there is a better option to measure distance: There is a capacitor near FET transistor under metallic shield. If drain-to-source capacitor is removed (or replaced by a smaller value cap), then frequency of this module will respond to supply voltage fast enough to perform FSK modulation. This is the most viable option, because measuring range of ~25 meters will require only 3mhz frequency step, e.g., from F1=24110 MHz to F2=24113 MHz. For modules with quadrature output max. unambiguous distance doubles to ~50 meters z=C/(2*df). C - speed of light, df - FSK modulation frequency step. Possible implementation: some development board with DAC output and ADC input. DAC connected to operational amplifier capable of outputting enough current to CDM324 and constantly outputs meandered signal voltages V1/V2 to achieve switching between two frequencies F1/F2. ADC is connected to CDM324 signal output and synchronized to DAC: samples acquired during DAC low voltage V1 go to array SIGNAL1[k], and during high DAC voltage V2 go to array SIGNAL2[k]. Then, we perform a FFT on both signals and find two complex spectrums SPECTRUM1[k] and SPECTRUM2[k]. We can extract amplitude and phase values for each k-th element of these arrays. E.g., find amplitude spectrum AMP1[k] for SPECTRUM1[k], find amplitude peaks (targets) indexes AMP1[m]. E.g., m=33, 114 and 120 - three targets: slow 33 and two fast 114 and 120. For each target bin "m" we find phases PHASE1 and PHASE2 of SPECTRUM1[ m] and SPECTRUM2[m] using ATAN2 function (google ATAN2). Find difference between phase DPHASE=PHASE1-PHASE2. And convert DPHASE to distance, e.g., -180...0 and 0..180 degrees maps to targets at distances 0..~25 meters moving in different directions (in case of CDM324). For radars with quadrature output unambiguous range 0..~50 meters for a 3MHz FSK modulation bandwidth. Another type of modulation FMCW would be difficult to implement using this module, because it requires much greater tuning range in order of 50MHz or better for usable resolution. It is hard to achieve such wide tuning range, and even if it is possible S21 phase of NE3210S01 and similar FETs used in CDM324 clones may go crazy, and amplitude too. This means that we will have huge variation of amplitude over FMCW sweep which is not good. And frequency may respond too rapid to some tuning voltages - hard to linearize it for FMCW. To sum up, FSK or FMCW can be used after small hardware modification. Unfortunately, FSK can be used only to measure moving targets. There is an option to perform scan of static objects by moving around, so every static object moves relative to the radar, e.g., we can measure distance to roadside trees and other objects if radar moves with a vehicle. But because everything moves, spectrum will have lots of overlapped signals and results may be far from ideal. FMCW: The main problem with FMCW is small achievable modulation bandwidth with CDM324, so range and resolution probably will have ridiculous values for practical use. Btw, older RFBeam radar modules with FSK modulation used exact same approach (and in some cases even same FET transistor), and parallel feedback oscillator is based on very similar microstrip half-wavelength resonator.

[@Jinguapingi]

@@TechSponsorTV Amazing reply; thanks!!!

[@TechSponsorTV]

@@Jinguapingi Thank you too for letting me know!

[@trevinsmall]

Is there any way to get positional data of the moving object, not just distance? Would this require multiple Rx antennas so that some kind of triangulation could be performed with the multiple range measurements?

[@TechSponsorTV]

Yes, the most popular method is to use multiple Rx antennas with phase monopulse method. E.g., two Rx antennas spaced one-wavelength apart can be used for unambiguous angle measurements in -30 to +30 degrees range or maxAng=2*Arcsin(lambda/(2*d))=60 when d=lambda. This will provide angle PHI to the target. And the distance R is measured using FMCW, FSK, Multiple-frequency FSK (MFSK). Then we can obtain target 2D coordinates as x=R*Sin(PHI); y=R*Cos(PHI). Here is how it works for FMCW: Single Tx antenna emits microwave signal with frequency jumping between F1 and F2 at 100kHz, which also limits unambiguous range measurement to maxDist=C/(2*(F2-F1)). Signal bumps into some target travelling at speed V and reflects with some Doppler shift. This reflected signal will be received by both antennas Rx1 and Rx2. If target is on the left, then Rx1 receives signal a little earlier than Rx2, and exact target angular position can be calculated from phase difference between receiving mixer outputs of antennas Rx1 and Rx2. Note that mixer outputs will provide crazy meandered FSK signal at 100kHz, and before working with it one more thing should be done. Older radars used sample-and-hold circuit that switched at 100kHz (or something like that) to separate received signal to two non-meandered signals. Newer radars simply use ADC working at 100kHz and synchronized with FSK transmitter switching time. ADC sampling frequency can have an additional offset dt=0.5*1/100kHz in time to ensure that we do not sample the part during frequency transition. If each receiving antenna has a single-ended mixer, then down-converted signals Rx1 and Rx2 will be transformed to Rx1_F1, Rx1_F2 and Rx2_F1, Rx2_F2. If there are quadrature (IQ) mixers, then we will have Rx1_F1_I, Rx1_F1_Q, Rx1_F2_I, Rx1_F1_Q and Rx2_F1_I, Rx2_F1_Q, Rx2_F2_I, Rx2_F1_Q. Next step is to perform FFT. We can perform FFT on single antenna signals to find a distance. E.g., we can take FFT(Rx1_F1), find peak spectrum_Rx1_F1[N] with maximum amplitude. And find distance to the target from phase difference (phase(spectrum_Rx1_F1[N])-phase(spectrum_Rx1_F2[N])). Phase is found using ATAN2 function on bin [N] of complex spectrum. We can find the same distance using data from second antenna Rx2, perform averaging, etc. Next step is to find angle to the target: Now we compare phases of spectrum_Rx1_F1[N] and spectrum_Rx2_F1[N] (signal that was extracted for F1 frequency). The same angle can be found using spectrum_Rx1_F2[N] and spectrum_Rx2_F2[N]. Again, it may be beneficial to use all available data to find this angular value multiple ways and then perform averaging or something like that. If we have multiple targets with close speeds, then we will start observing various glitches: e.g., if FFT amplitude peaks spectrum_Rx1_F1[N1] spectrum_Rx1_F1[N2] are close, but targets angular positions or distances are very different, we will start to observe "averaged" ghost targets that are traveling somewhere in-between. That's why both FMCW and FSK are often replaced with Multiple-frequency FSK in many modern automotive radars. Here is another example that is easier to understand. An obscure method is to use two FSK radars placed at some distance apart. We can measure distance to the same target using two different radars: R1 and R2. If we draw two circles, one with radius R1 with center at (X1, Y1) and another with radius R2 with center at (X2, Y2), then these circles will intersect at target position. There may be multiple intersections, and detection performance can be somewhat optimized by increasing number of radars and their position. Because we also know radiation pattern of each radar, we can filter out circle intersections that occur behind the radar. About method from this video: it uses purely unmodulated radar with a single antenna. While very limited, it provides shockingly good results if targets have straight trajectories and constant speeds (or if there is a known model of target movement, that can be written as a formula to use for spectrogram pattern matching). Georgy M.

[@TechSponsorTV]

See how this algorithm works to measure distances to vehicles here: kzbin.info/www/bejne/r4S3eXh9o96WfK8

[@TuralMontin-w9k]

Hi, can you recommend fmcw radar, for 10-20 m distance range

[@TechSponsorTV]

Probably one of the best performance at 24 GHz are old UMRR series, for example UMRR-11 by smartmicro. But it's an MFSK. What are you going to measure? Generally,I would recommend any radar front end capable to control VCO in wide enough bandwidth B, because resolution R=C/(2B) can be rediculously low. With SLOW FMCW waveforms it is problematic to have good resolution and stay in allowed bands. FAST FMCW waveforms are used in modern 70GHz automotive radars, maybe you can find some model with documentation.. I am not sure, need some more info on your task. For example, with 250 MHz bandwidth resolution is around 0.6 meters. In real 10GHz FMCW traffic counter allowable bandwidth was lower, and as I recall there was only 2 fft points per lane, meaning resolution around 1.5 meters. Do not know any good affordable modules lately.