Wireless Telemetry is a technology that allows the remote measurement and reporting of information of interest to the system designer or operator. The word is derived from Greek roots tele = remote, and metron = measure.
Many methods of generating messages have evolved over the years, here are a few:
- Analog AM – (Amplitude Modulation) – The amplitude (strength) of a fixed frequency is changed, to represent the desired signal.
- Analog FM – (Frequency Modulation) – The wavelength of a signal, at an instant in time, represents the amplitude of the original signal.
- Digital OOK – (On-Off Keyed) – This is a simple method where the presence of a frequency is a logic 1, and the lack of a frequency is a logic 0.
- Digital ASK – (Amplitude Shift Keyed) – In this method, a large signal is a logic 1, and a small signal is a logic 0.
- Digital FSK – (Frequency Shift Keyed) – This method rapidly switches between two distinct frequencies to represent logic 1, and logic 0.
Several bands have been reserved for Industrial, Scientific, and Medical (ISM) uses. Of these, the 900MHz and 2.4GHz bands are the most popular. Many methods have been developed to provide widespread use of these bands to minimize cross-talk and interference. To achieve this, some radios hop frequencies inside a band, to reduce the chance of two radios sharing the same frequency at the same time. These are called Frequency Hopping Spread Spectrum (FHSS) radios. Protocols including 802.11, Bluetooth, and ZigBee have been devised to provide standards, allowing interoperability between manufacturers.
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