Preliminary
11-31
RF2514
Rev A2 010215
11
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. 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 ele-
ments. This approach has been attractive because the
SAW devices have been readily available and a trans-
mitter, for example, could be built with only a few addi-
tional 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 fre-
quency control. These devices are superior in perfor-
mance 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 transmit-
ter designed for applications operating within the fre-
quency 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 sys-
tems. It can also be used as a local oscillator signal
source. The integrated VCO, phase detector, pres-
caler, 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 bipo-
lar-CMOS process and packaged in an industry stan-
dard 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 volt-
age ranging from 2.2V 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 +5dBm output power into a
50
Ω load and is intended to comply with FCC and
ETSI requirements for unlicensed remote control trans-
mitters. 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 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.
RF2514 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
RF2514 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 OSC B and OSC E provide connections to a tran-
sistor that is used as the reference oscillator. The Col-
pitts 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 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 refer-
ence frequency.
The RF2514 contains an onboard phase detector and
charge pump. The phase detector compares the phase
of the reference oscillator to the phase of the prescaler
output. The phase detector is implemented using flip-
flops in a topology referred to as either "digital phase/
frequency detector" or "digital tri-state comparator".
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