Infrared and RF are similar in that both are forms of electromagnetic radiation. The visible spectrum is shown below. Just below it is a bigger picture showing the broader electromagnetic spectrum.
Near-infrared (NIR) is at frequencies just below humanly-visible light (0.7-5 micron wavelength), mid-infrared is 5-~30 micron in wavelength and far-infrared (~30-~200 micron wavelength) is at the lower end of the infrared spectrum. At wavelengths below 1.1 microns, infrared is not thermal but is similar to humanly-visible light. Wavelengths longer than 1.1 microns are thermal with temperature inversely related to wavelengths. Humans radiate IR in the 8-14 micron range while far-infrared is very cold.
A typical Consumer IR (CIR) remote control has an IR Emitter Diode (IRED) that emits electromagnetic energy with a frequency of 319,928,146,808,510Hz and a wavelength of about 940nm (0.00000094m 0.94 microns). A typical Consumer IR receiver has a photo diode or photo transistor that is most sensitive to the same 940nm wavelength. Because there is also ambient IR (from sunlight, incandescent lights, etc.), the 940nm IR is usually modulated by a constant carrier frequency in the 32-40KHz range and CIR receivers incorporate a bandpass filter tuned to the carrier frequency. This allows the receiver to discriminate between the modulated IR signal and any ambient, unmodulated IR. In effect, the IR receiver is double tuned both to the wavelength of the fundamental IR and also to the lower carrier frequency.
Three factors influence CIR range. In order of decreasing importance they are: the power level of the IR emitter, the IR wavelength, and the carrier frequency. An IR emitter's output is proportional to the current through the emitter. Increasing the current will increase the power. Because the duty cycle is usually 50% or less, the emitter can be driven with very high currents. For optimal range, the IR wavelength of the emitter and receiver should match. An IrDA emitter with a CIR receiver is a mismatch as is a CIR emitter with IrDA receiver. Such mismatches have major effects on range. RF receivers usually have a fairly wide bandpass, enabling them to receive and decode IR with a carrier within ±5kHz of their nominal carrier frequency. Of course, the closer the carrier is to the center frequency, the better.
IrDA was designed to be very short range. (1-2m) IrDA does not use any secondary carrier but directly modulates 850nm IR with the data. It is hard to distinguish from ambient IR so range is very limited. Even when an IrDA emitter is further modulated with a 32-40KHz carrier, range is still limited because the IrDA emitter usually is lower power than a CIR emitter and because the CIR receiver uses a 940-950nm detector which does not respond as well to the 850nm IR. IrDA also limits the beam angle to ±15° while most CIR emitters are ±40° or greater.
Agilent makes an IrDA transceiver that uses 885nm which falls between IrDA's 850nm and CIR's 940nm wavelength. The transmitter has sufficient power to be used for CIR at distances of 25-30 feet (7-8 meters). It (or a similar transceiver) is used in some PDAs (e.g. Dell Axim x3).
Radio Frequency (RF) waves are lower in frequency and longer in wavelength than Infrared. At 300MHz the wavelength is 1m (39.37") while Consumer IR wavelengths are just under 1 millionth of a meter. Most RF remotes use a carrier in the 300-1000MHz range. An RF receiver only needs to be tuned to the carrier frequency used by the remote - there's no need for the double-tuning of the IR receiver¹. RF remotes and their receivers are tuned to a fixed frequency. The FCC allows unlicensed, low power use of 300MHz-1000MHz as well as some higher frequency bands.
As a general rule, the codes are comprised of pulses and spaces with durations of 0.3-1.5ms which is an audible signal in the 500-2000Hz range. The IR and RF receivers output the demodulated code waveform. The only difference is that IR receivers output an active low (inverted) signal while RF receivers output an active high signal.
Other than range, it really makes no difference whether the data signal is used to modulate an RF carrier, an IR carrier, an ultrasonic carrier, a laser beam, or smoke. At the receiving end, the demodulated signal carries the same information. X-10 used a chip developed by NEC for IR, uPD6121, in a since discontinued keychain remote and uses the same protocol for all of its RF devices.
Capturing any IR or RF code is fairly trivial. All you need is an IR or RF receiver module and a way to record the demodulated output pulse stream. The Panasonic PNA4602M is an inexpensive IR receiver tuned for 38kHz carrier. The Panasonic PNA4614M has the same pinout and is tuned to 56.9kHz. Vishay makes several IR receivers that use the same pinout in carrier frequencies from 30-56kHz. The Vishay TSOP1100 has a different pinout but can handle 33-43kHz. The Panasonic PNZ323B PIN Photo Diode can be used to measure the carrier frequency. For RF, the process is the same except you need an RF receiver tuned for the carrier frequency. Small, inexpensive RF receivers in the 300-1000MHz range are available from many sources.
Outside of North America, most RF remotes use 433.92MHz but many different frequencies are used in the USA and Canada. All RF transmitters sold in the USA are required to undergo FCC certification tests and FCC rules also require an FCC ID number on the device. The FCC database can be searched for the FCC ID number to determine the RF carrier frequency. If only one frequency is listed for both lower and upper frequency, the device uses ASK. If separate lower and upper frequencies are listed, the device uses FSK.
For RF control, both the transmitter and receiver need to be tuned to the same carrier frequency and need to use the same type of modulation. Most RF remotes use ASK (Amplitude Shift Keying) or OOK (On-Off Keying). OOK is really just a special case of ASK. OOK is also called CPCA (Carrier Present, Carrier Absent). All of the illustrations above depict ASK. FSK (Frequency Shift Keying) uses two different carrier frequencies to denote two different states. FSK is illustrated below.
Philips ProntoPro and Marantz touchscreen remote controls can be programmed to send either IR or RF. They transmit RF using double modulation similar to the way IR remotes work. The receivers use an RF receiver tuned to the RF carrier frequency, the output of which, feeds an Atmel U2538B chip tuned to the approximate 38kHz subcarrier frequency, giving them added noise immunity as they ignore any RF that is not also modulated at the subcarrier frequency. This requires both a high quality transmitter and receiver that are capable of switching at the subcarrier frequency.
¹ One manufacturer does double modulate their RF. Philips uses 36kHz with their Pronto RF extenders which pass the output of the RF receiver to a second receiver that is tuned to 36kHz.
Copyright: No part may be reproduced except as authorized by written permission. This restriction extends to reproduction in all media.