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RF2514
Rev A5 DS040115
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.
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". 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 inputs. Each flip-flop output signal is a series of pulses whose frequency is related to the flip-flop input fre-
quency. When both inputs of the flip-flops are identical, the signals 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 impedance state, maintaining the charge on the loop filter. In the charge and discharge states, the loop filter
integrates the pulses coming from the charge pump to create 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 capaci-
tance. The inductance is selected by the designer for the desired frequency of operation. Two inductor configurations are pos-
sible.
In the first configuration, two inductors are connected in series between RESNTR- and RESNTR+. A resistor is then used to pro-
vide 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 giv-
ing 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-inductor 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 reference oscillator frequency.
The transmit amplifier is a two-stage amplifier consisting 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 pro-
vide 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 during the start of any
VCO in which the VCO starts oscillating and the reference oscillator builds up to full amplitude. During this period, the fre-
quency 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 correct frequency, sliding across
200MHz of occupied spectrum. In some competitive devices, the transmitter output operates at full power under all of these
conditions.
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