'Fractional' related articles 1

  1. 2007/04/10 CX72301 - Spur-Free, 1.0 GHz Dual Fractional-N Frequency
Applications
• General purpose RF systems
• Low bit rate wireless telemetry
• Instrumentation
• Specialized Mobile Radios (SMRs) and Private Mobile Radios (PMRs)

Features
• Spur-free operation
• 1.0 GHz maximum operating frequency
• 500 MHz maximum auxiliary synthesizer
• Ultra-small step size, 100 Hz or less
• High internal reference frequency enables large loop bandwidth implementations
• Very fast switching speed (e.g., below 100 µs)
• Phase noise to –96 dBc/Hz inside the loop filter bandwidth @ 950 MHz
• Software programmable power-down modes
• High-speed serial interface up to 100 Mbps
• Three-wire programming
• Programmable division ratios on reference frequency
• Phase detectors with programmable gain provide a programmable loop bandwidth
• Frequency power steering further enhances rapid acquistion time
• On-chip crystal oscillator
• Frequency adjust for temperature compensation
• Direct digital modulation
• 3 V operation
• 5 V output to loop filter
• 28-pin EP-TSSOP 6.4 x 9.7 mm package

Description
 Skyworks CX72301 direct digital modulation fractional-N frequency synthesizer provides ultra-fine frequency resolution, fast switching speed, and low phase-noise performance. This
synthesizer is a key building block for high-performance radio system designs that require low power and fine step size.
 
 The ultra-fine step size of less than 100 Hz allows this synthesizer to be used in very narrowband wireless applications. With proper temperature sensing or through control channels, the synthesizer’s fine step size can compensate for crystal oscillator or Intermediate Frequency (IF) filter drift. As a result, crystal oscillators or crystals can replace temperature- compensated or ovenized crystal oscillators, reducing parts count and associated component cost. The device’s fine step size can also be used for Doppler shift corrections.
 
 The CX72301 has a phase noise floor of –95 dBc/Hz up to 1.0 GHz operation as measured inside the loop bandwidth. This is permitted by the on-chip low noise dividers and low divide ratios provided by the device’s high fractionality. Reference crystals or oscillators up to 50 MHz can be used with the CX72301. The crystal frequency is divided down by independent programmable divider ratios of 1 to 32 for the main and auxiliary synthesizers. The phase detectors can operate at a maximum speed of 25 MHz, which allows better phase noise due
to the lower division value. With a high reference frequency, the loop bandwidths can also be increased. Larger loop bandwidths improve the settling times and reduce in-band phase noise.
Therefore, typical switching times of less than 100 µs can be achieved. The lower in-band phase noise also permits the use of lower cost Voltage Controlled Oscillators (VCOs) in customer applications.

 The CX72301 has a frequency power steering circuit that helps the loop filter steer the VCO when the frequency is too fast or too slow, further enhancing acquisition time. The unit operates with a three-wire, high-speed serial interface. A combination of a large bandwidth, fine resolution, and the threewire, high-speed serial interface allows for a direct frequency
modulation of the VCO. This supports any continuous phase, constant envelope modulation scheme such as Frequency Modulation (FM), Frequency Shift Keying (FSK), Minimum Shift
Keying (MSK), or Gaussian Minimum Shift Keying (GMSK). This capability can eliminate the need for In-Phase and Quadrature (I/Q) Digital-To-Analog Converters (DACs), quadrature
upconverters, and IF filters from the transmitter portion of the radio system.

CX72301-11 PH00-D102

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