Optical Communications Equipment
Ubiquitous networking is rapidly progressing worldwide. The number of subscribers for xdigital subscriber line (xDSL), CATV and fiber to the home (FTTH) is increasing on access networks (subscribers' networks), with information technology (IT) utilized in advanced ways. As a result, large quantities of traffic are generated on the networks.
Trunk transmission networks should be upgraded accordingly. They should work at further higher speeds and handle larger quantities of data. Transfer speed for optical communications equipment is rapidly shifting from 2.4 Gbps to 10 Gbps to 40 Gbps.
The equipment used for these trunk transmission networks incorporates voltage-controlled crystal oscillators (VCXO) working at high frequencies on the clock and data recovery (CDR) circuits capable of regenerating data and clock signals from noise-carrying signals. The dejittering circuit also incorporates VCXOs working at high frequencies for minimizing jitters contained in clock signals.
A 600MHz VCXO
Traditionally, 155.52 MHz has mainly been used for these VCXOs at a transfer speed of 2.4 Gbps. At present, 622.08 MHz is used for transfer speeds of 10 Gbps and 40 Gbps. In optical communications equipment, the shift of transfer speed to higher frequencies raises the frequencies for VCXOs accordingly.
Moreover, the trend of size reduction and capacity enhancement in the equipment has influenced the size of VCXO. The package for VCXO is currently shifting from the classic 11.4 × 9.6 mm to 7.0 × 5.0 mm.
At present, more than one communication scheme are generally used: SDH / SONET that has a clock stability of ± 20 ppm and Ethernet, which has a clock stability of ± 100 ppm. Thus, a VCXO is required to be more widely variable (more than 150 ppm in certain cases) in frequency control range than ever.
The 9170D low-profile Oven Controlled Crystal Oscillator
To meet this requirement, NDK has developed a 7.0 × 5.0 × 1.8 mm VCXO based on a crystal unit whose fundamental frequency is 600 MHz. Compared with the conventional 11.4 × 9.6 mm VCXO, which provides 600 MHz achieved by multiplying a fundamental frequency of 150 MHz, the newer VCXO offers a good spectrum characteristic free from subharmonics (Fig. 1) and have phase jitters at 145 fs-rms (at an offset frequency of 12 kHz to 20 MHz) (Fig. 2).
Fig. 1 : Spectrum characteristic of a 600MHz VCXO
Fig. 2 : Phase noise characteristic of a 600MHz VCXO
The newer VCXO covers an absolute pulling range of ± 100 ppm. Signals on an Ethernet (frequency stability: ± 100 ppm) can be retracted with a phase-locked loop (PLL).