Simplifying Radio Access Network Clocking Using Proven DSPLL Technology

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Carriers deploying low-cost, low-power RAN nodes typically fall into one of the following categories:

  • Wireless service providers under pressure to expand network capacity and coverage to support bandwidth-intensive services.
  • Operators deploying a heterogeneous network (HetNet) of small, low-power Radio Access Network (RAN) nodes:
    • Remote Radio Heads (RRHs) in 4G/LTE base stations increase network coverage and capapcity.
    • Small Cells improve coverage in dead spots, and offload wireless data to fixed wireline IP networks closer to the end user.
    • Distributed Antenna Systems (MDAS) complement RRHs and small cells to extend network coverage and ensure seamless "five-bar" service in office parks, hotels, and public areas like arenas and concert facilities.

RAN Nodes are Optimized for Size, Power, and Performance

RAN designs are space constrained, relatively inexpensive compared to other networking equipment, and may be deployed in non-temperature controlled locations.

They must be optimized for size, power, performance, and cost.

This ebook covers a proven timing technology that...

  • reduces current timing BOM footprint 66%
  • reduces power cosumption by 30%
  • increases design flexibility
  • reduces system costs

RAN Timing Requirements Diverse and Demanding

Despite the fact that RAN devices must be small, affordable, and reliable in outside elements, the RF timing specifications remain stringent due to wireless phase noise requirements.

RAN devices require an increasing mix of frequencies to support system-level functions, some of which are shown on the next page.

These combined requirements drive increased complexity, power consumption, and footprint because traditional timing solutions are inflexible, power hungry, and require multiple discrete components.

 

RAN System Requirements Drive Timing Complexity

RF timing specifications remain very stringent. An increasing mix of frequencies is required to support system-level functions without sacrificing performance.

 

Application Example Frequencies
4G/LTE data converters 30.72 MHz
61.44 MHz
122.88 MHz
245.76 MHz
491.52 MHz
983.04 MHz
RF tranceivers 25 MHz
40MHz
100 MHz
Ethernet PHYs 25 MHz
125 MHz
156.25 MHz
322.265625 MHz
System clocks 25 MHz 66.666 MHz

Introduction to DSPLL vs. Current Timing Architectures

Traditional RAN designs used cascaded, two-stage phase-lock loop (PLL) technology for high-performance, low phase noise clock synthesis.

Cascaded, two-stage PLLs provide excellent phase noise performance, but suffer from limited frequency flexibility, high power, sensitivity to noise and vibration, and a large, discrete footprint including external VCXOs and loop filters.

DSPLL timing technology addresses RAN's timing needs.

RAN designs need smaller, more flexible, better timing solutions to meet today's requirements.

DSPLL addresses these challenges in a single IC that offers low phase noise, greater frequency flexibility, lower
power, and better immunity to noise and vibration effects
than current solutions.

While this new technology may sound too good to be true...

 

DSPLL is Already Widely Deployed in Telecom Apps

...in reality DSPLL has already been widely adopted and deployed in demanding telecommunications and networking applications within core, metro and access equipment, including cutting-edge 100G/400G SerDes clocking applications.

  • >50 Million DSPLL-enabled Nodes Already Deployed in High-Performance, Demanding Telecom and Networking Applications
  • Hundreds of Millions of Reliable Operating Hours

 

Legacy, Cascaded, Two-Stage PLL Architecture

  • A cascaded, two-stage PLL is the traditional analog architecture for RAN designs.
  • It delivers low phase noise but only for a restricted set of output frequencies
  • The output frequency must be integer-related to its input frequency

This architecture uses a discrete, analog, narrowband VCXO-based PLL to implement jitter cleaning in its first stage. The VCXO must be used as the PLL's VCO to achieve low phase noise in conjunction with an external loop filter.

The 2nd stage PLL relies on an analog wideband PLL to provide integer clock multiplication. This requires an additional external loop filter.

Pros and Cons of Cascaded, Two-Stage PLLs

Pros

Cons

*Low phase noise for integer-related frequencies to its input reference *Multiple devices required to support non-integer-related frequencies
*Benefits and design requirements well understood by many designers *Requires large footprint, discrete VCXOs, loop filters, LDOs
*Design drawbacks and guidelines also well understood by many designers *High power
  *Large footprint
  *Each external node in the PLL is susceptible to noise
  *Susceptible to vibration

DSPLL Architecture Overview

  • The DSPLL architecture integrates and improves upon the traditional solution.
  • DSPLL technology provides system clocks such as Ethernet and baseband...
  • ...and the RF ultra-low phase noise clocks that meet demanding 4G/LTE phase noise requirements with margin.

 

DSPLL Architecture Explanation

The inner loop acts as a digitally-controlled oscillator for the outer loop, providing fractional clock synthesis with low phase noise without LC-oscillators and discrete VCXOs.

The outer loop digitally tunes the operating frequency of the inner loop, synchronizes to external reference clocks, attenuates jitter, and generates clocks.

All elements of the DSPLL are integrated on-chip, eliminating the need for discrete filter components and enabling easy PLL bandwidth adjustments to optimize phase noise performance.

 

Simplified Device Programming

Traditional cascaded PLL calculations and settings are done manually; a cumbersome, error-prone process.

ClockBuilder Pro is a simple, easy to use software tool that customizes the DSPLL in minutes.

  • Enter the input and output frequencies,
  • set the DSPLL bandwidth and other operating parameters,
  • and generate a device configuration.

This allows simple, efficient testing, and speeds time to market.

ClockBuilder Pro Software is Simple to Use

Performance: DSPLL vs. Cascaded, Two-Stage PLL

DSPLL technology performance meets or exceeds that of traditional solutions.

DSPLL Immune to Vibration Effects

RAN equipment is often deployed in external environments where wind, building or mechanical equipment can cause vibration.

DSPLL devices are highly immune to these effects.

DSPLL is 66% Smaller than Cascaded, Two-stage PLL

  Cascaded PLL in mm2 DSPLL in mm2
DUT 81 81
VCXO 151.2 -
Crystal - 8
Loop filters 9.3 -
Power supply filtering 22.8 3.7
Other 17.7 1.5
Total 282 mm2 94.3 mm2

The table shows components footprints and does not take into account PCB spacing or routing rules, further improving DSPLL's size advantage.

DSPLL makes design easier, because it integrates sensitive, large components on-chip, including the VCXO and power supply filtering components

DSPLL is 30% Lower Power than Cascaded, Two-stage PLL

Description Cascaded PLL DSPLL
Supply Voltage 3.3 V 3.3 V analog + 1.8 V core
Supply Current 600 mA 300 mA
Total Power 1.9 W 1.3 W

Improves RAN equipment design by reducing current consumption and heat in small enlosures.

Designs with Cascaded, Two-stage PLL

RRH, small cell and DAS designs require a combination of low-phase noise RF clocks plus baseband and Ethernet clocks. These clock domains are non-integer related, forcing traditional solutions to use a cascaded, two-stage PLL and a discrete clock generator.

Designs with DSPLL

RRH, small cell and MDAS designs require a combination of low-phase noise RF clocks plus baseband and Ethernet clocks. These clock domains are non-integer related. This poses no problem for DSPLL.

Smaller, Lower Power, Better, Faster to Market

Silicon Labs' proven DSPLL technology saves space, power, and cost, and is easier to design with, speeding time to market.

DSPLL performs better than legacy solutions.

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