RF2513
the charge pump to create a control voltage for the
voltage controlled oscillator.
VCC
The voltage controlled oscillator (VCO) is a tuned
differential amplifier with the bases and collectors
cross-coupled to provide positive feedback and a 360°
phase shift. The tuned circuit is located in the collec-
tors. It is comprised of internal varactors and two exter-
nal inductors. The designer selects the inductors for
the desired frequency of operation. These inductors
also provide DC bias for the VCO. The output of the
VCO is buffered and applied to the prescaler circuit,
where it is divided down and compared to the refer-
ence oscillator frequency.
C2
C1
X1
This type of oscillator is a parallel resonant circuit for a
fundamental mode crystal. The transistor amplifier is
an emitter follower and the voltage gain is developed
by the tapped capacitor impedance transformer. The
series combination of C1 and C2 act in parallel with the
input capacitance of the transistor to capacitively load
the crystal.
The PLL and VCO circuitry can be enabled by setting
applying a “high” logic level to pin 4 (PLL ENABL).
Conversely, the PLL and VCO circuitry will be turned
off if the level is tied “low”.
The nominal capacitor values can be calculated with
the following equations.
The transmit amplifier is a two stage amplifier con-
sisting of a driver and an open collector final stage. It is
capable of providing 8dBm of output power into a 50Ω
load while operating from a 3.6V power supply.
60 Cload
-----------------------
freqMHz
1
--------------------------
and C2 =
C1
=
1
1
------------- ------
–
Cload C1
The output power is adjustable by the setting of pin 7
(LVL ADJ). This analog input allows the designer a
15dB range of output power. As the LVL ADJ voltage is
reduced, the output power and current consumption
are reduced. LVL ADJ must be low when the transmit-
ter is disabled.
The load capacitance is usually 32pF. The variable freq
is the oscillator frequency in MHz. The frequency can
be adjusted by either changing C2 or by placing a vari-
able capacitor in series with the crystal. As an exam-
ple, assume a desired frequency of 14MHz and a load
capacitance of 32pF. C1=137.1pF and C2=41.7pF.
Additionally, the transmitter circuitry can be disabled
entirely by applying a “low” logic level to pin 11 (TX
ENABL). During transmission, this pin should be tied
“high”. This pin controls all circuitry except for the PLL
circuitry.
11
These capacitor values provide a starting point. The
drive level of the oscillator should be checked by look-
ing at the signal at pin 2 (OSC E). It has been found
that the level at this pin should generally be around
500mVPP or less. This will reduce the reference spur
During transmission the transmitter is enabled and the
impedance of the output pin, pin 8 (TX OUT), is low.
When the transmitter is not enabled, the impedance
becomes high.
levels and reduce noise from distortion. If this level is
higher than 500mVPP then decrease the value of C1.
The values of these capacitors are usually tweaked
during design to meet performance goals, such as
minimizing the start-up time.
The RF2513 contains onboard band gap reference
voltage circuitry which provides a stable DC bias over
varying temperature and supply voltages.
Additionally, by placing a variable capacitor in series
with the crystal, one is able to adjust the frequency.
This will also alter the drive level, so it should be
checked again.
Designing with the RF2513
The reference oscillator is built around the onboard
transistor at pins 1, 2 and 3. The intended topology is
of a Colpitts oscillator. The Colpitts oscillator is quite
common and requires few external components, mak-
ing it ideal for low cost solutions. The topology of this
type of oscillator is as seen in the following figure.
An important part of the overall design is the voltage
controlled oscillator (VCO). The VCO is configured as
a differential amplifier. The VCO is tuned via the exter-
nal inductors, capacitor and varactor. The varactor
capacitance is set by the loop filter output voltage
through a 4kΩ resistor.
11-28
Rev B8 010509
RFMD [ RF MICRO DEVICES ]
