Frequency Details

The frequency range that most of the signals rtl_433 can handle is on the 433MHz ISM band. The center frequency of the band is 433.920 MHz and this is what most of the devices write that they use. This doesn't mean that the device actually use this frequency. In the device a crystal is used to generate a clock source for the transmitter. Imperfections in the crystal or temperature changes will change the actual frequency that the signal is transmitted on. Design choices in the hardware could also move signal in the available range (433.050-434.790). By default rtl_433 uses a 433.920 MHz center frequency with a 250kHz sample rate. This means that it is able to pick up signals that are in a range of 433.92MHz. What the exact range is unknown as the crystal imperfections are also valid for the rtl receiver. But as the sample rate is 250kHz we still get a decent range. The further away we get from the tuned frequency the lower the signal strength will be. The choice of 250kHz as sampling rate is to reduce the amount of samples to process.

Another factor that is a little bit unknown but should be valid is that the tuner in the stick (e4000, r820t, etc) sends a 8MHz wide signal to the rtl demod. The rtl demod has a filter that band limit the signal to around 2MHz. As we are sampling at 250kHz we should get aliasing of the signals that are outside our sampling range but still inside the 2MHz range. In most cases getting these aliasing signal components will destroy the received signal. For OOK signals this is not the case so we get more range "for free".

(Originally posted to the Google Group by Benjamin Larsen)