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RF2512 Datasheet(PDF) 5 Page - RF Micro Devices

Part No. RF2512
Description  UHF TRANSMITTER
Download  14 Pages
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Maker  RFMD [RF Micro Devices]
Homepage  http://www.rfmd.com
Logo RFMD - RF Micro Devices

RF2512 Datasheet(HTML) 5 Page - RF Micro Devices

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11-15
RF2512
Rev B9 010509
11
RF2512 Theory of Operation
Introduction
The RF2512 is a low cost FM/FSK UHF transmitter
designed for applications operating within the fre-
quency range of 300MHz to 1000MHz. In particular, it
is intended for 315/433/868 MHz band systems, remote
keyless entry systems, and FCC Part 15.231 periodic
transmitters. It can also be used as a single- or dual-
channel local oscillator signal source. The integrated
VCO, phase detector, prescaler, and reference oscilla-
tor require only the addition of an external crystal to
provide a complete phase-locked loop.
TheRF2512isprovidedina24-pinSSOP-24 package
and is designed to operate from a supply voltage rang-
ing from 2.2V to 5.0 V, accommodating designs using
three NiCd battery cells, two AAA flashlight cells, or a
lithium button battery. The device is capable of provid-
ing upto15mWoutputpower intoa50
Ω load
(+11.8dBm) and is intended to comply with FCC
requirements for unlicensed remote control transmit-
ters.
RF2512 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
RF2512 includes all of these internally except for the
loop filter and the reference oscillator's crystal and two
feedback capacitors.
The reference oscillators are Colpitts type oscillators.
Pin1(OSC B2),pin 2(OSCE), and pin3 (OSC B1)
provide connections to the internal transistors that are
used as the reference oscillators. The Colpitts configu-
ration is a low parts count topology with reliable perfor-
mance and reasonable phase noise. Alternatively, an
external signal could be injected into the base of either
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 user sets which oscillator is operational by setting
pin 24 (OSC SEL) either high or low. This allows the
implementation of two channel systems.
The prescaler divides the Voltage Controlled Oscilla-
tor (VCO) frequency down by either 64/65 or 128/129,
using a series of flip-flops, depending upon the logic
level present at pin 15 (DIV CTRL). A high logic level
will select the 64/65 divisor. A low logic level will select
the 128/129 divisor. This divided signal is then fed into
the phase detector where it is compared with the refer-
ence frequency.
In addition to the DIV CTRL setting, one also sets the
prescaler modulus by setting pin 14 (MOD CTRL)
either high or low. A high logic level will select the 64/
128 divisor. A low logic level will select the 65/129 divi-
sor.
Pin 12 (PRESCL OUT) provides access to the pres-
caler output. This is used for interfacing to an external
PLL IC.
The RF2512 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
the reset on each flip-flop. The outputs of the flip-flops
are also connected to the charge pump. Each flip-flop
output signal is a series of pulses whose frequency is
related to the flip-flop input frequency.
When both inputs of the flip-flops are identical, the sig-
nals are both frequency and phase locked. If they are
different, they will provide signals to the charge pump
which will either charge or discharge the loop filter or
enter into a high impedance state. This is where the
name "tri-state comparator" comes from.
The main benefit of this type of detector it's ability to
correct for errors in both phase and frequency. When
locked, the detector uses phase error for correction.
When unlocked, it will use the frequency error for cor-
rection. This type of detector will lock under all condi-
tions.
The prescaler and the phase detector bias voltage is
brought out through pin 13 (VREF P). This allows
bypassing of the of these two circuits to filter the refer-
ence crystal frequency.
The charge pump consists of two transistors, one for
charging the loop filter and the other for discharging
the loop filter. It's inputs are the outputs of the phase
detector flip-flops. Since there are two flip-flops, there
are four possible states. If both amplifier inputs are low,
then the amplifier pair goes into a high impedance
state, maintaining the charge on the loop filter. The


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