How to Select RF Modules
RF modules are partially finished circuits that can be incorporated into larger designs. They include receivers, transmitters, and transceivers. RF modules use several different modulation methods and radio techniques. On-off key (OOK) modulation turns a signal on or off. Amplitude modulation (AM) causes the baseband signal to vary the amplitude or height of the carrier wave to create the desired information content. Frequency modulation (FM) causes the instantaneous frequency of a sine wave carrier to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal. Amplitude shift key (ASK) is a modulation method that transmits data by varying the amplitude of the transmitted signal. Frequency shift key (FSK) is a digital modulation scheme that uses two or more output frequencies. Phase shift key (PSK) varies the phase of transmitted digital signals in accordance to the baseband data signal. Radio techniques limit localized interference and noise. Direct sequence spread spectrum techniques spread signals over a large band by multiplexing the signal with a code or signature that modulates each bit. Frequency-hopping spread spectrum techniques move signals through a narrow set of channels in a sequential, cyclical, and determined pattern.
Performance specifications for RF modules include sensitivity, output power, communication interface, operating frequency, measurement resolution, and maximum transmission distance. Sensitivity is the minimum input signal required to produce a specified output signal having a specified signal-to-noise (S/N) ratio. Output power is the maximum signal power that RF modules can transmit. There are two basic types of communications interfaces. Parallel interfaces handle multiple bits of data at the same time and include standards such as the general-purpose interface bus (GPIB). Serial interfaces transfer one bit after another. Examples include the universal serial bus (USB), RS232 and RS485. Operating frequency is the range of transmitted and received signals. Measurement resolution is the minimum digital resolution. Maximum transmission distance is the maximum distance that a transmitter and receiver can be separated.
There are several form factors for RF modules. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Flat pack (FPAK) devices have flat leads and are available in a variety of body sizes and pin counts. Stand alone devices are benchtop or floor-standing units with a full casing or cabinet and an integral interface. Rack-mounted fit in a standard 19” telecommunications rack and include hardware such as rail guides, flanges, or tabs.
RF modules use several types of connectors. Bayonet Neil-Concelman (BNC) connectors are used in applications to 2 GHz. Threaded Neil-Concelman (TNC) connectors are similar in size to BNC connectors, but feature a threaded coupling nut for applications that require performance to 11 GHz. Miniature coaxial (MCX) connectors provide broadband capability through 6 GHz and are used in applications where weight and physical space are limited. Ultra high frequency (UHF) connectors are designed with non-constant impedance for use in comparatively low voltage and low frequency applications. Subminiature-A (SMA) connectors directly interface the cable dielectric without air gaps. Subminiature-B (SMB) connectors snap into place and are used for frequencies from DC to 4 GHz. Subminiature-P (SMP) connectors are rated to 40 GHz and, depending on detent type, can withstand from 100 to 100,000 interconnect cycles. Reverse SMA (RSMA) and reverse TNC (RTNC) connectors have reverse polarity. Other connectors for RF modules include MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.
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