Figure 7: The spectrum of a remote keyless entry device uses FSK of a 433.9 MHz carrier to encode digital data to control entry into a vehicle. Image source: Digi-Key Electronics simple example is to see that of a continuous wave (CW) Morse code signal (Figure 5). CW signals encode data by turning an RF carrier on and off (on-off keying). On the spectrogram display the periods when the key is down – and the carrier is being transmitted – are indicated by the light blue-grey track on the display. The Morse character “V” (di di di dah) indicating testing can be seen in the signal track. Note that the software makes provision for receiving CW signals by supplying a beat frequency oscillator (BFO) labelled “CW shift” to provide a user-controlled audio tone to hear the code transmission. Since CW transmissions are narrowband, the receiver reduces the bandwidth to 300 hertz (Hz) as seen in the radio control pull-down menu. Keeping the receiver bandwidth to the minimum value needed for the mode being received minimizes the noise level in the channel.
shows dual peaks at about 433.9 MHz. Data encoding for this device uses frequency shift keying (FSK) where the carrier is shifted between two frequencies to indicate a digital one or zero. Other RKE fobs use amplitude shift keying (ASK) where the amplitude of a carrier is shifted between two levels, not too different from the CW signal. Conclusion The Adafruit 1497 SDR receiver opens up the whole world of VHF, UHF, and low microwave frequency bands to investigation hobbyists and professionals alike. It enables users to use a computer to tune into FM, TV, amateur radio, citizens band, weather, and short-wave broadcasts. It also can be used as a spectrum analyzer to verify the operation of a wide range of portable RF devices. The 1497 has also been used to create interferometers for radio astronomy – all at low cost.
Some measurement applications for an SDR receiver
In an increasingly linked world, there are many RF sources that need to be checked and serviced. An example is the verification of the update period of a remote weather station transmitter module (Figure 6). The spectrogram shows two RF bursts at the remote transmitter’s 433.93 MHz carrier frequency. The time scale on the spectrogram indicates that the FM bursts occur roughly 50 seconds apart. Automotive remote keyless entry (RKE) systems operate at either at 315 or 433 MHz, depending on where the vehicle is being used and the governing regulations. In this case, the user just needs to hold the key fob near the antenna and push one of the buttons to see the type of modulation used (Figure 7).
The spectrum of the RKE key fob
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