CX72301 Datasheet, PDF (3/20 Pages) List of Unclassifed Manufacturers – Spur-Free, 1.0 GHz Dual Fractional-N Frequency Synthesizer

DATA SHEET • CX72301
Technical Description
The CX72301 is a fractional-N frequency synthesizer using a ∆Σ
modulation technique. The fractional-N implementation provides
low in-band noise by having a low division ratio and fast
frequency settling time. In addition, the CX72301 provides
arbitrarily fine frequency resolution with a digital word, so that the
frequency synthesizer can be used to compensate for crystal
frequency drift in the RF transceiver.
Serial Interface
The serial interface is a versatile three-wire interface consisting of
three pins: Clock (serial clock), Data (serial input), and CS (chip
select). It enables the CX72301 to operate in a system where one
or multiple masters and slaves are present. To perform a
loopback test at start-up and to check the integrity of the board
and processor, the serial data is fed back to the master device
(e.g., a microcontroller or microprocessor unit) through a
programmable multiplexer. This facilitates hardware and software
debugging.
Registers
There are ten 16-bit registers in the CX72301. For more
information, see the Register Descriptions section of this
document.
Main and Auxiliary ∆Σ Modulators
The fractionality of the CX72301 is accomplished by the use of a
proprietary, configurable 10-bit or 18-bit ∆Σ modulator for the
main synthesizer and 10-bit ∆Σ modulator for the auxiliary
synthesizer.
Main and Auxiliary Fractional Units
The CX72301 provides fractionality through the use of main and
auxiliary ∆Σ modulators. The output from the modulators is
combined with the main and auxiliary divider ratios through their
respective fractional units.
VCO Prescalers
The VCO prescalers provide low-noise signal conditioning of the
VCO signals. They translate from an off-chip, single-ended or
differential signal to an on-chip differential Current Mode Logic
(CML) signal. The CX72301 has independent main and auxiliary
VCO prescalers.
Main and Auxiliary VCO Dividers
The CX72301 provides programmable dividers that control the
CML prescalers and supply the required signals to the charge
pump phase detectors. Programmable divide ratios ranging from
38 to 537 are possible in fractional-N mode and from 32 to 543 in
integer-N mode.
Reference Frequency Oscillator
The CX72301 has a self-contained, low-noise crystal oscillator.
This crystal oscillator is followed by the clock generation circuitry
that generates the required clock for the programmable reference
frequency dividers.
Reference Frequency Dividers
The crystal oscillator signal can be divided by a ratio of 1 to 32 to
create the reference frequencies for the phase detectors. The
CX72301 has both a main and an auxiliary frequency synthesizer,
and provides independently configurable dividers of the crystal
oscillator frequency for both the main and auxiliary phase
detectors. The divide ratios are programmed through the
Reference Frequency Dividers Register.
NOTE:
The divided crystal oscillator frequencies (which are the
internal reference frequencies), Fref_main and Fref_aux,
are referred to as the reference frequencies throughout
this document.
Phase Detectors and Charge Pumps
The CX72301 uses a separate charge pump phase detector for
each synthesizer which provides a programmable gain, Kd, from
31.25 to 1000 µA/2π radians in 32 steps programmed using the
Control Register.
Frequency Steering
When programmed for frequency power steering, the CX72301
has a circuit that helps the loop filter steer the VCO using the
LD/PSmain signal (pin 9). In this configuration, the LD/PSmain
signal can provide a more rapid acquisition.
When programmed for lock detection, internal frequency steering
is implemented and provides frequency acquisition times
comparable to conventional phase/frequency detectors.
Lock Detection
When programmed for lock detection, the CX72301 provides an
active low, pulsing open collector output using the LD/PSmain
signal (pin 9) to indicate the out-of-lock condition. When locked,
the LD/PSmain signal is three-stated (high impedance).
Power Down
The CX72301 supports a number of power-down modes through
the serial interface. For more information, see the Register
Descriptions section of this document.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
101090H • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • July 21, 2004
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