I have two CDM324 Radar motion sensors.

I want to run the two sensors close to each other.

The transmitted frequency is roughly 24 GHz and I am afraid that a small shift in one sensor transmitted frequency could enter the received path of the other sensor and it can be interpreted as a reflected signal (motion) even if there is no motion at all.

For example: the first and second sensor transmit at 24,000,100 Hz, 24,000.000 Hz, respectively. The second sensor can detect a doppler frequency of 100 Hz even if there is no motion.

I run the two sensors next to each other and I have no interference at all. Is it a chance? Could an interference happen? A false detection would be a problem for me?

  • 2
    \$\begingroup\$ "A false detection would be a problem for me?" Are you asking us or telling us? \$\endgroup\$
    – winny
    Commented Oct 1, 2019 at 14:43

1 Answer 1


A CDM324 module is a very simple device.

There isn't even a crystal or any other component on the board (see here) which takes care that the 24 GHz frequency is accurate. So the frequency will not be accurate. That means that the module will need to work over a wide frequency range. That also means that any other device within range operating at 24 GHz or a frequency close to 24 GHz can disturb this device.

Your only chance is that two modules are using such a significantly different frequency (like 23.5 GHz and 24.5 GHz) that the frequency difference is so large that this cannot be picked up by the boards.

But there is no guarantee that two (randomly selected) modules will have such significantly different frequencies that this will work.

The safest option is to only use one board at any moment (time-share).

  • \$\begingroup\$ I am thinking to use "the variation of the oscillation frequency versus the power supply". For example, running the two senors at 4.75v and 5.25v respectively. There is a 20 MHz variation for that 0.5 v of supply voltage. Is it (20 MHz) safe? could a spectrum analyzer approve the concept? \$\endgroup\$
    – mmyname
    Commented Oct 1, 2019 at 14:27
  • \$\begingroup\$ I don't know what the IF roll-off is but the difference frequency for reasonable closing/opening velocities is only a few kHz so the two units might have to be within a few hundred kHz for them to interfere. If so it is quite probable that they would not do so. \$\endgroup\$ Commented Oct 1, 2019 at 15:56
  • \$\begingroup\$ 20MHz in 20GHz would be 1 part in 1000, corresponding to a target moving at 0.001C or 186 miles per second. We don't know what your target's characteristics are, so you'll have to decide if that's safe... \$\endgroup\$
    – user16324
    Commented Oct 1, 2019 at 15:59
  • \$\begingroup\$ Even if my IF roll-off is only a few KHz, I am afraid from the instability of the internal oscillator. for example: CDM324 evaluated by spectrum analyzer and its peak goes 3 MHz around 23.913.150 GHz. Further, the temperature issue is to be considered. So I guess I should buy a new sensor with a different band.i.e. 10 GHz. \$\endgroup\$
    – mmyname
    Commented Oct 2, 2019 at 7:05
  • \$\begingroup\$ I am afraid from the instability of the internal oscillator. Why? These "radars" work by looking at the frequency delta so the absolute value of the RF frequency is irrelevant. Going for a 10 GHz solution makes no difference if the circuit is similar. If you need to have a stable frequency (you would need to provide a good reason why you need that) then a circuit with a PLL (synthesizer) is needed making the circuit much more complex and expensive. \$\endgroup\$ Commented Oct 2, 2019 at 7:10

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