RF2516
RF2516 Theory of Operation
Introduction
Short range radio devices are becoming commonplace in today’s environment. The most common examples are the
remote keyless entry systems popular on many new cars and trucks, and the ubiquitous garage door opener. Other
applications are emerging with the growth in home security, automation and the advent of various remote control applica-
tions. Typically these devices have been simplex, or one-way, links. They are also typically built using surface acoustic
wave (SAW) devices as the frequency control elements. This approach has been attractive because the SAW devices
have been readily available and a transmitter, for example, could be built with only a few additional components. Recently
however, RF Micro Devices, Inc. (RFMD), has introduced several new components that enable a new class of short-
range radio devices based on the use of crystals and phase-locked loops for frequency control. These devices are supe-
rior in performance and comparable in cost to the traditional SAW-based designs. The RF2516 is an example of such a
device. The RF2516 is targeted for applications such as 315MHz and 433MHz band remote keyless entry systems and
wireless security systems, as well as other remote control applications.
The RF2516 Transmitter
The RF2516 is a low-cost AM/ASK VHF/UHF transmitter designed for applications operating within the frequency range
of 100MHz to 500MHz. In particular, it is intended for 315MHz to 433MHz band systems, remote keyless entry systems,
and FCC Part 15.231 periodic transmitters. It can also be used as a local oscillator signal source. The integrated VCO,
phase detector, prescaler, and reference oscillator require only the addition of an external crystal to provide a complete
phase-locked loop. In addition to the standard power-down mode, the chip also includes an automatic lock-detect feature
that disables the transmitter output when the PLL is out-of-lock.
The device is manufactured on a 25GHz Silicon Bipolar-CMOS process and packaged in an industry standard SSOP-16
plastic package. This, combined with the low external parts count, enables the designer to achieve small-footprint, high-
performance, low-cost designs.
The RF2516 is designed to operate from a supply voltage ranging from 2.25V to 3.6V, accommodating designs using
three NiCd battery cells, two AAA flashlight cells, or a lithium button battery. The device is capable of providing up to
+10dBm output power into a 50Ω load, and is intended to comply with FCC requirements for unlicensed remote control
transmitters. ESD protection is provided on all pins except VCO and TX OUT.
While this device is intended for OOK operation, it is possible to use narrowband FM. This is accomplished by modulat-
ing the reference oscillator rather than applying the data to the MOD IN input pin. The MOD IN pin should be tied high to
cause the device to transmit. The deviation will be set by pulling limits of the crystal. Deviation sufficient for the transmis-
sion of voice and other low data rate signals can therefore be accomplished. Refer to the Application Schematic in the
data sheet for details.
The RF2516 Functional Blocks
A PLL consists of a reference oscillator, a phase detector, a loop filter, a voltage controlled oscillator (VCO), and a pro-
grammable divider in the feedback path. The RF2516 includes all of these internally, except for the loop filter and the ref-
erence oscillator’s crystal and two feedback capacitors.
The reference oscillator is a Colpitts type oscillator. Pins 1 (OSC B) and 2 (OSC E) provide connections to a transistor
that is used as the reference oscillator. The Colpitts configuration is a low parts count topology with reliable performance
and reasonable phase noise. Alternatively, an external signal could be injected into the base of the transistor. The drive
level should, in either case, be around 500mVPP. This level prevents overdriving the device and keeps the phase noise
and reference spurs to a minimum.
The prescaler divides the VCO frequency by either 64 or 32, using a series of flip-flops, depending upon the logic level
present at the DIV CTRL pin. A high logic level will select the 64 divisor. A low logic level will select the 32 divisor. This
divided signal is then fed into the phase detector where it is compared with the reference frequency.
Rev A17 060712
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RFMD [ RF MICRO DEVICES ]
