Communication system designers cite several key demands for frequency translator modules including smaller packaging, lower cost and higher functionality. The challenge is to improve performance while reducing size, say frequency control manufacturers. The good news is that next-generation devices coming to market address many of these concerns.
Companies like Connor-Winfield, SaRonix and Vectron are developing new frequency translators and timing modules that incorporate new design features. While shrinking package size, these products also feature autonomous mode, alarms, automatic free run or holdover functions, and a variety of low-voltage positive-emitter-coupled logic (LVPECL) and CMOS outputs while shrinking the package size.
Manufacturers have also improved jitter performance at high frequencies of 622 MHz and 670 MHz by replacing crystal oscillators with surface-acoustic wave (SAW) devices in their new module designs.
Frequency control manufacturers are finding that they need to shift SAW technology for higher frequencies up to 800 MHz. At output frequencies up to 155 MHz, they continue to use economical high-frequency fundamental-mode crystals, which provide low jitter and tight stability.
Although there is a tradeoff between losing the inherent accuracy of a crystal oscillator and sacrificing free run and holdover specifications, there is a work around, said Ed Miguel, engineering manager for timing products group for Connor-Winfield Corp. (Aurora, Ill.)
Miguel said an inexpensive 20-ppm crystal oscillator can be used to lock the frequency translator to it whenever free-run mode is selected to preserve the accuracy of the frequency. He believes it's a good solution because it reduces the package size and cost, while simplifying the design.
One of Connor-Winfield's most recent new product developments is its new family of 3.3-volt digital synchronous clock generators (DSCGs) for line card applications. The company will launch the new line in the latter part of second quarter 2004. These modules include new features such as holdover (or hold last), which typically isn't incorporated into frequency translators and autonomous mode.
The digital synchronous clock generators (DSCGs)with digital PLL control from Connor-Winfield are designed as a reference input for OC-192 and SERDES.
This is the company's first foray into the digital filter-based SCG market. Historically, digital filters have been fairly large and there isn't a lot of space on the line card, Miguel said. "With DSCGs you can design in more features such as holdover and automatic operation. If it loses a reference it switches automatically to another reference, and if you lose both references it goes into holdover or free-run mode," he said
The DSCG, measuring 1.02 x 1.02 x 0.265-inch, provides a high precision phase-locked loop (PLL) frequency translator for telecommunications applications. It generates a LVPECL output at 622.08 MHz with a disable function. Two other LVPECL outputs can be dividers such as 155.52 MHz or 77.76 MHz. A CMOS output offers a flexible output between 8 kHz to 155.52 MHz. They can be used in line cards and service termination cards to provide reference, phase locked, synchronization for TDM, PDH, SONET and SDH network equipment.
Other key specs include dual input references (8k Hz to 77.76 MHz), and 20-ppm free run and holdover accuracy. Each reference has a loss of reference and loss of lock alarm. The device supports both manual and autonomous modes, and can be configured to provide a jitter attenuated internal reference.
Connor-Winfield also plans to introduce several new series (CSA1 to CSA8) of 3.3-volt clock smoother modules that are typically used to attenuate jitter on an output, to be launched in second quarter. Samples will be available in March or April. All of the devices are jitter-smoothing PLLs that use a voltage controlled SAW oscillator (VCSO). They offer LVPECL, low voltage differential signaling (LVDS) or CMOS clock inputs and LVPECL or LVDS differential outputs. Available in a 14.22 x 11.68 x 3.56-mm surface-mount package, the devices can be used in SONET/SDH applications.
Exhibiting low jitter generation and low phase noise, the new CSA1 to CSA4 devices offer output frequencies up to 800 MHz and lock to specific input frequencies from 19.44 MHz to 200 MHz, while the CSA5 to CSA8 modules offer output frequencies up to 311.04 MHz and lock to specific input frequencies from 2.43 MHz to 77.76 MHz.
In December 2003, Connor-Winfield introduced a new synch timing solution in a single chip with the introduction of its STC3500 device. The company also launched two new timing modules including the Stratum 3 synch module and Stratum 3E synch module in a 2.05 x 1.25 x .76 inch package.
"When we started in the frequency translation business in 1997, the timing modules were relatively large about 2-inches square and they didn't have a lot of functionality," Miguel said. "The primary function was to attenuate jitter and switch between one or two references but it was directly controlled. Now, we've added more features and increased the inputs to 4 and 8 inputs, that can accept any one of eight pre-selected input frequencies automatically," he said.
A key feature of the timing modules is its master/standby (master/slave)cross-connect capability. With it, either one of the two timing cards to be used in a system can be in the master or standby mode. The timing modules also incorporate a serial port interface, which provides access to 57 control-and-feedback registers to the host processor including monitoring reference qualification and adjusting phase offsets. In addition, they offer three outputs: one for up to 77 MHz, one selectable for T1 or E1 frequencies and one for 8 kHz to cross-connect to another module for master/standby control.
Following its flagship SFT100 series of synchronous frequency translator modules that cover the output frequency range of 1.544 to 77.76 MHz, SaRonix Inc. (San Jose, Calif.) will launch two frequency translator families this month, the SFT300 and SFT400, that expand the frequency output (LVPECL) range from 77.76 MHz to 670 MHz range.
With the introduction of the SFT400 frequency translator modules, SaRonix's product portfolio spans the input frequencies of 8 kHz to 84 MHz and output frequencies of 1.544 MHz to 670 MHz.
In addition, the translators will be available in two packaging options: true surface-mount pads or elevated standoffs that allow for board cleaning as well as automated pick and place assembly.
Other key features include an 8 kHz or 19.44 MHz user-selectable input reference frequency, ±20 ppm maximum holdover stability, less than 1 picosecond jitter in free-run mode, and alarm and status outputs.
The company said these modules accommodate one or more user-selectable input reference signals and can accommodate more than one frequency value that is a user-defined value at any given time. In addition, the modules provide various outputs that indicate the operating mode and status of the module. These outputs can be fed into the platform's processor for further action.
A key difference between the two product lines is that the SFT300 is a fundamental-mode voltage-controlled crystal oscillator (VCXO)-based module that can translate a low frequency reference clock signal of 8 kHz to 84 MHz to a higher frequency of 77.76 MHz to 168 MHz. On the other hand, the SFT400 is based on a fundamental-mode VCXO with SAW-based filtering and can translate low-frequency reference clock signals from 8 kHz to 84 MHz to a higher frequency range of 311 MHz to 670 MHz. Both devices are designed for synchronous networking and telecommunications applications.
SaRonix has incorporated a SAW VCXO in its new translators to produce a low-jitter output, but has incorporated an additional quartz crystal to
stabilize the performance of the device upon loss of the input reference signal, according to Craig Taylor, vice president of frequency control products for SaRonix, a Pericom subsidiary. "Even with a 600- to 700-MHz output, if the input signal is lost our device will maintain 20
ppm holdover indefinitely and will operate within the Sonet/SDH minimum clock requirements."
"Upon realizing that there is no input signal the device will automatically revert to free-run mode. The voltage control is maintained and held fixed internally so the customer doesn't need to worry about applying a voltage control to the internal oscillator. The unit will revert to this holdover mode and achieve a free-run accuracy of ±20 ppm over life even with a 622 MHz output, which can't be achieved by just using a SAW oscillator," Taylor said.
"One of the disadvantages of the SAW device is when you lose the input reference frequency, the frequency stability of the device doesn't meet the SONET/SDH requirements. SAWs have a very lose frequency stability of ±100 ppm or more so if you lose the input reference frequency, the output frequency of the translator could drift outside of operational limits," Taylor said.
SaRonix's next-generation modules will provide a 70% reduction in size, intelligent input frequency selection and Stratum 3 holdover stability of 4.6 ppm
Vectron International (Hudson, N.H.) is also active in the design of frequency translators. Next up is the new FX-701, with an input frequency range of 8 kHz to 80 MHz and output frequency range of 200 MHz to 800 MHz. The chipset solution includes a 7 mm x 5 mm crystal-based FX-700 and a 7 mm x 5 mm SAW-based frequency multiplier. Samples will be available in the second quarter.
Preliminary specs for the FX-730 SAW-based frequency translator, available in the third quarter, include an input frequency range of 19 MHz to 800 MHz and output frequency range of 125 MHz to 800 MHz. Vectron is targeting the industry's smallest form factor, 7 x 5 x 2 mm, for this device.
The FX-730 will incorporate the functionality of Vectron's FX-104 and to a certain extent the FX-102 in a small form factor. The company said the integrated translator could replace up to six or eight components (like the phase detector, amplifier, crystal VCXO or SAW VCXO, feedback divider) with a single chip.
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