Brainstorm: Time Device Technologies
What are the functionalities that time device technologies are helping to improve?
Walter Oliwa, Senior VP Research & Design, CTS Electronic Components
Any device or system that requires a clock or timing device to function will improve its performance as the quality of the timing device improves. Parameters that define the quality of a timing device include: phase noise, jitter, temperature stability, aging, PSRR, power dissipation, G-sensitivity, as well as other parameters that are sensitive to a particular application.
Some of the functionalities that timing devices are helping to improve include:
Lower Bit Error Rates: Telecom systems are using clock oscillators with jitter as low as 25 fs to provide the reference clock for their integrated chip sets. The lower jitter improves the transmission quality, and reduces the power required to process the same amount of data.
Longer Battery Life: Portable instrumentation, radios, and other remotely located equipment require a stable timing reference. This level of performance requires the use of an OCXO in order to meet the desired stability and phase noise. OCXOs packaged in small TO-8 assembly offer temperature stability ranging from 10 to 20 ppb, while dissipating only 120 mW of power. Reducing the power consumption allows the size, weight, and ultimately the cost of the equipment to be reduced.
Higher Bandwidth Optical Networks: The bandwidth requirements of our communication systems continue to increase. Data must be received and re-transmitted constantly throughout the network; the timing requirements are complex. To maintain synchronization it is required to use the clock signal associated with the incoming data stream as the reference frequency. In order for the clock signal to function properly, a “Clock cleaner”, or jitter attenuator, is required to effectively filter the noise from the recovered clock and provide a jitter-free signal that can be used for the retiming process. Jitter attenuators made specifically for this application provide over 80 dBc of jitter suppression, while generating an output signal with less than 50 fs of jitter.
Harsh Operating Environments: There are some applications where electronic systems are required to operate effectively while being subjected to mechanical shock and vibration. Oscillators are now constructed using assembly techniques that minimize the mechanical coupling to the sensitive elements providing a G-sensitivity of 5x10e-10.
Alison Steer, Product Marketing Manager, Mixed Signal Products, Linear Technology
In clock distribution circuits, it is important to maintain the quality of the clock signal, which is represented as either a jitter or phase-noise measurement. Phase noise is a frequency domain error, while jitter is a time domain error. In a sampling system, close-in phase noise will affect the spectral purity of the signal, while broadband noise will degrade the SNR. It’s important to notice whether clock jitter is specified as integrated jitter or as broadband jitter, which can provide a better figure of merit when comparing jitter specifications. The measurement is also dependent on the signal frequency.
The LTC6957 is a DC to 300 MHz dual output buffer/driver/logic translator, ideal for converting sine waves into low-phase noise, logic level signals in applications such as Gigabit wireless, gyroscopes, test and measurement, radio and satellite communications, medical imaging, military, and GPS. The LTC6957 converts any DC to 300 MHz reference frequency into dual LVPECL, LVDS, or CMOS outputs with low additive jitter of 45 fSRMS over the 12 kHz to 20 MHz integration bandwidth, and less than 200 fSRMS broadband total jitter for the entire family.
The LTC6957 can be used to convert any signal type to a logic level signal, including the buffering and distributing of the commonly used 10 MHz reference source. The device has a proprietary, selectable, input stage bandwidth-limiting feature, which improves the additive phase noise for slow slewing signals by up to 3 to 4dB. Choosing the appropriate input stage filter setting for a given input signal’s slew rate, increases the slew rate of the input signal while limiting the broadband noise. This keeps a lid on the total jitter at the output of the LTC6957, which is crucial in numerous applications where the reference signal is of limited amplitude or slew rate.
Neil Floodgate, Vice President, IQD Frequency Products
Although the phenomenon of Quartz as an accurate frequency source has been known since the late 19th Century, it was not until the advent of the second-world war that its use became prolific. At that time, the use of vacuum tubes was the methodology driving the quartz and produced stabilities of 50 ppm.
Subsequently, analogue mobile communications started to make an impact, which led to the development of oscillators with stabilities of sub 10 ppm. These were achieved by Temperature Compensated Crystal Oscillators (TCXO’s) and Oven Controlled Crystal Oscillators (OCXO’s).
Today’s oscillators are able to meet the requirements of the digital age where virtually everyone has a smartphone in their pocket. An example of how far the industry has developed is IQD’s CFPT-77series of 2.5 x 2.0 mm TCXVCO, which achieves a stability of 0.5 ppm over an operating temperature range from -30° to +85°C.
IQD’s IQOV-150 series OCXO delivers frequencies up to 1 GHz with a phase noise performance at -140 dBc/Hz at 100 kHz offset, with temperature stabilities down to 2 ppb.