Preliminary
11-39
RF2516
Rev A10 010613
11
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 applications. 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 superior 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 transmit-
ter designed for applications operating within the fre-
quency range of 100MHz to 500 MHz. 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 stan-
dard power-down mode, the chip also includes an
automatic lock-detect feature that disables the trans-
mitter output when the PLL is out-of-lock.
Thedeviceismanufacturedona25GHz SiliconBipo-
lar-CMOS process and packaged in an industry stan-
dard 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 volt-
age ranging from 2.0V to 3.6V, accommodating
designs using three NiCd battery cells, two AAA flash-
light cells, or a lithium button battery. The device is
capable of providing up to +10dBm output power into a
50
Ω load, and is intended to comply with FCC require-
ments 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 modulating the reference oscillator rather than
applying the data to the MOD IN input pin. The MOD
IN pinshould be tiedhighto cause thedevice totrans-
mit. 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.
The RF2516 Functional Blocks
A PLL consists of a reference oscillator, a phase detec-
tor, a loop filter, a voltage controlled oscillator (VCO),
and a programmable divider in the feedback path. The
RF2516 includes all of these internally, except for the
loop filter and the reference 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 refer-
ence frequency.
The RF2516 contains an onboard phase detector and
charge pump. The phase detector compares the
phase of the reference oscillator to the phase of the
VCO. The phase detector is implemented using flip-
flops in a topology referred to as either “digital phase/
frequency detector” or “digital tri-state comparator”.
The circuit consists of two D flip-flops whose outputs
are combined with a NAND gate which is then tied to
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