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I need to design a system able to identify which frequency is being used by remote keyless entry (RKE). I found that the wireless carrier frequency can be 315MHz in the US/Japan and 433.92MHz (ISM band) in Europe and can be ASK or FSK. A reference to this can be found at:

https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3395.html

This was the most reliable document that I found, but this document is old, from 2005, and I dont't know exactly if there are newer frequencies now. I know there are receivers and transmitters operating in these frequency bands , but for now, I need only to identify in which frequency the RKE is. So I have two questions.

  1. There are another frequencies that are not 315MHz and 433.92 MHz to remote keyless entry?
  2. Someone know a circuit or an IC able to identify these frequencies ?

Any advice would be helpful.

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  • \$\begingroup\$ Consider interfacing to an SDR dongle and using it as a crude spectrum analyser. This can cover the unlikely bands as well as the likely ones. \$\endgroup\$
    – user16324
    Commented Feb 26, 2020 at 15:53
  • \$\begingroup\$ @BrianDrummond, what is a SDR means in this context ? I tried to find an example of a SDR dongle able to do this but I didn't found, would you give me an example ? \$\endgroup\$
    – Daniel
    Commented Feb 26, 2020 at 16:34

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Most devices that work in the 315 MHz and 433 MHz bands use a transmitter that transmits at a precise frequency (there are certain channels that can be used, see LDP433) but the receivers are not that precise as they don't need to be (and that makes them cheaper).

To distinguish one transmitter from the other (to prevent your garage door being opened by any other transmitter at the same frequency), the identification of the receiver is not the frequency but in the code (a number) that is sent. The transmitters transmit a certain sequence using OOK modulation and that code must then be recognized by (a microcontroller in) the receiver.

What you could do is simply buy two receivers, one for 315 MHz and one for 433 MHz. In combination with an microcontroller and some software (using an Arduino comes to mind as there are ready made libraries for use with keyfobs etc.) you can easily retrieve the data that was transmitted.

If you only need to know if the device is 315 MHz or 433 MHz then you might only need the two receivers and for example an LED at the output.

In addition to 315 MHz and 433 MHz, some devices use 866 MHz, or 915 MHz, if you can find a receiver for that what I say above also applies. But I think 315 MHz and 433 MHz are the most commonly used. 833 MHz is somewhat less common.

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    \$\begingroup\$ Many(most?) receivers in such systems are now using low IF superhets rather than superregen such as this: "img.ozdisan.com/ETicaret_Dosya/453934_1928180.pdf". They provide much better noise and EMC performance as well as superior frequency control. \$\endgroup\$ Commented Feb 26, 2020 at 15:18
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    \$\begingroup\$ @KevinWhite I agree that the superhets are more sensitive, I recommend them for most applications. However, that might be irrelevant for he application OP has in mind as there the transmitter will very likely be in close proximity of the receiver antenna. So good receiver performance isn't that relevant. \$\endgroup\$ Commented Feb 26, 2020 at 15:22
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    \$\begingroup\$ my comment was mainly to expand on your statement regarding the receivers not being all that precise. \$\endgroup\$ Commented Feb 26, 2020 at 15:25
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    \$\begingroup\$ @Daniel You will need a receiver for each frequency that you want to be able to detect. If you want to detect all of them then yes, you need 4 receivers. Fortunately they're not expensive. \$\endgroup\$ Commented Feb 26, 2020 at 16:52
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    \$\begingroup\$ I will need a different circuit to different types of modulation ? No, only if you want to extract the data that the transmitter is sending. The cheap modules usually only work with ASK, and may have issues with FSK. However if you only need to know what carrier frequency the transmitters are using, ASK or FSK is irrelevant, the cheap modules can still detect that there is a transmission even if they can't demodulate the data. \$\endgroup\$ Commented Feb 27, 2020 at 14:57

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