RF2514
RF2514 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 along with the growth in home security and automation and the advent of various remote control applications. Typi-
cally 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 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 superior in performance and comparable in cost to the traditional
SAW based designs. The RF2514 is an example of such a device. The RF2514 is targeted for applications such as 315, 433,
868 and 915MHz band remote keyless entry systems, wireless security systems, and other remote control applications.
The RF2514 Transmitter
The RF2514 is a low cost AM/ASK VHF/UHF transmitter designed for applications operating within the frequency range of
100MHz to 1000MHz. In particular, it is intended for 868 and 915MHz band systems (ETS 300 220 applications and FCC
Parts 15.231 and 15.249 transmitters) and remote keyless entry systems. 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 MLF16 plastic
package. This, combined with the low external parts count, enables the designer to achieve small-footprint, high-performance,
low-cost designs.
The RF2514 is designed to operate from a supply voltage ranging from 2.2V 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 +5dBm output power
into a 50Ω load and is intended to comply with FCC and ETSI requirements for unlicensed remote control transmitters. ESD
protection is provided on all pins except for OSCB, OSCE, RESNTR-, RESNTR+, TXOUT, and the two analog ground pins (1 and
9).
While this device is intended for OOK operation, it is possible to use narrowband FM. This is accomplished by modulating 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 transmission of voice and
other low data rate signals can therefore be accomplished. Refer to the Application Schematic in the data sheet for details.
RF2514 Functional Blocks
A PLL consists of a reference oscillator, a phase detector, a loop filter, a voltage controlled oscillator (VCO), and a programma-
ble divider in the feedback path. The RF2514 includes all of these internally except for the loop filter and the reference oscilla-
tor's crystal and two feedback capacitors.
The reference oscillator is a Colpitts type oscillator. Pins OSC B and 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 min-
imum.
The prescaler uses a series of flip-flops to divide the VCO frequency by either 64 or 32, 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.
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