11-44
RF2514
Rev A5 040115
tor". The circuit consists of two D flip-flops whose out-
puts 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 inputs.
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
place the charge pump in a high impedance state,
maintaining the charge on the loop filter. The name "tri-
state comparator" comes from this. The main benefit of
this type of detector is the 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 correction. This type of
detector will lock under all conditions.
The charge pump consists of two transistors, one for
charging the loop filter and the other for discharging
the loop filter. The charge pump inputs are the outputs
of the phase detector flip-flops. If both amplifier inputs
are low, then the amplifier pair goes into a high imped-
ance state, maintaining the charge on the loop filter. In
the charge and discharge states, the loop filter inte-
grates the pulses coming from the charge pump to cre-
ate a control voltage for the voltage controlled
oscillator.
The 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 collectors and is comprised of internal
varactors and external inductance, which also provides
DC bias for the VCO. The varactor diodes are internally
configured for negative tuning. That is, a higher control
voltage results in a lower VCO frequency by reducing
the varactor reverse bias which correspondingly
increases the capacitance. The inductance is selected
by the designer for the desired frequency of operation.
Two inductor configurations are possible.
In the first configuration, two inductors are connected
in series between RESNTR- and RESNTR+. A resistor
is then used to provide the DC bias to the balanced
inductance node formed by the series connection of
the inductors. Ideally, the two inductors should be
equal in value, but a slight imbalance is acceptable if
necessary for VCO centering.
In the second configuration, a single inductor is placed
across RESNTR- and RESNTR+ and one resistor is
used to provide bias to the differential amplifier. The
resistor is connected in series from VCC to either
RESNTR- or RESNTR+. The inductor provides the DC
bias path for the other resonator pin. This configuration
has the advantage of lower cost and parts count, as
only one inductor is required; the disadvantage is
potentially suboptimal VCO centering due to limited
standard inductor values. For example, 20nH may be
the optimal inductance to center the VCO at the
desired operating frequency, but only 18nH and 22nH
inductors are available as standard values. However,
for the two-inductor configuration, both inductors can
be 10nH, thus giving the optimal 20nH of inductance.
Of course, the problem of optimization can also be
resolved by increasing (or decreasing) the inductance
of the traces running to the inductor in the single-induc-
tor configuration.
The output of the VCO is buffered and applied to the
prescaler circuit, where it is divided by either 32 or 64,
as selected by the designer, and compared to the ref-
erence oscillator frequency.
The transmit amplifier is a two-stage amplifier con-
sisting of a driver and an open collector final stage. It is
capable of providing 5dBm of output power into a 50
Ω
load while operating from a 3.6V power supply.
The lock-detect circuitry connects to the output of the
phase detector circuitry and is used to disable the
transmitter when the VCO is not phase-locked to the
reference oscillator. This is necessary to avoid
unwanted out-of-band transmission and to provide
compliance with regulatory limits during an unlocked
condition.
There are many possible reasons that the PLL could
be unlocked. For instance, there is a short period dur-
ing the start of any VCO in which the VCO starts oscil-
lating and the reference oscillator builds up to full
amplitude. During this period, the frequency will likely
be outside the authorized band. Typically the VCO
starts much faster than the reference oscillator. Once
both VCO and reference oscillators are running, the
phase detector can start slewing the VCO to the cor-
rect frequency, sliding across 200MHz of occupied
spectrum. In some competitive devices, the transmitter
output operates at full power under all of these condi-
tions.
The lock protection circuit in the RF2514 is intended to
stabilize quickly after power is applied to the chip and
to disable the base drive to the transmit amplifier. This
attenuates the output to levels that will be generally
acceptable to regulatory boards as spurious emis-
sions. Once the phase detector has locked the oscilla-
tors, then the lock circuit enables the MOD IN pin for
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