2.6 GHz Fractional RF Frequency Synthesizer with 850 MHz Integer-N IF Frequency Synthesizer# Technical Documentation: LMX2364TM PLLatinum™ Low-Noise Frequency Synthesizer
Manufacturer : National Semiconductor (NS)
Document Revision : 1.0
Date : October 26, 2023
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## 1. Application Scenarios
The LMX2364TM is a high-performance, integer-N phase-locked loop (PLL) frequency synthesizer with an integrated voltage-controlled oscillator (VCO). It is designed for precision frequency generation in RF systems requiring low phase noise and high stability.
### 1.1 Typical Use Cases
* Local Oscillator (LO) Generation : Primary application in up/down-conversion stages of transceivers. The synthesizer generates stable LO signals for mixers in superheterodyne architectures.
* Clock Synthesis and Jitter Cleaning : Used in digital systems (e.g., FPGA, ASIC, high-speed data converters) to generate low-jitter clock references from a stable crystal oscillator.
* Channel Selection and Tuning : In multi-channel communication equipment like radios or scanners, the LMX2364TM enables precise, software-controlled selection of operating frequencies.
### 1.2 Industry Applications
* Professional Wireless Communications : Base stations, repeaters, and point-to-point microwave links where spectral purity is critical for minimizing adjacent channel interference.
* Test and Measurement Equipment : Signal generators, spectrum analyzers, and network analyzers utilize this synthesizer for their internal LO due to its low phase noise and fine frequency resolution.
* Satellite Communication Systems : Used in VSAT terminals and satellite modems for generating stable LO frequencies in L-band, S-band, and C-band transceivers.
* Broadcast Equipment : FM/VHF/UHF transmitters and exciter units requiring precise carrier frequency generation.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Integration : Combines a low-noise PLL and a VCO in a single package, reducing board space and simplifying design.
* Excellent Phase Noise Performance : Optimized for low close-in and far-out phase noise, crucial for meeting stringent communication standards.
* Wide Frequency Range : The integrated VCO typically covers a broad frequency range (specifics depend on the suffix/model variant), reducing the need for multiple components.
* Serial Interface Control : Simple 3-wire SPI-compatible interface allows for easy microcontroller integration and real-time frequency programming.
Limitations:
* Integer-N Architecture : Lacks the fine frequency resolution and lower phase noise potential of fractional-N synthesizers, which may be a constraint for some modern wideband standards.
* Power Consumption : As a high-performance RF component, its power consumption is typically higher than that of simpler clock generators or lower-frequency PLLs.
* External Loop Filter Required : Requires careful design and component selection for the external passive loop filter, which directly impacts lock time, stability, and spurious performance.
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## 2. Design Considerations
Successful implementation of the LMX2364TM requires careful attention to several critical design aspects.
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Poor Phase Noise and Spurs due to Inadequate Loop Filter Design.
* Solution : Model the loop filter meticulously using the manufacturer's PLL design software (e.g., National's WEBENCH® PLL Designer). Optimize for the desired lock time, phase margin (~45-55° is typical), and sufficient suppression of reference sidebands and VCO noise. Always prototype and measure.
* Pitfall 2: VCO Pushing/Pulling from Power Supply Noise.
* Solution : Implement exceptional power supply decoupling. Use a combination of bulk capacitors (e.g., 10µF), ceramic capacitors (0.1
