Improving System Performance And Frequency Stability With New Vcxos

By Author: Martin Lobo
Total Articles: 355
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Oscillators provide a consistent and stable timing reference for an electronic system or for a module(s) within the system. In some applications, the output frequency of the oscillator needs to be adjusted after the system is in operation. System designers can use a voltage controlled oscillator (VCXO) which has the capability to adjust its output frequency.

This frequency tuning technique can be used in many types of equipment including products developed for telecom, networking, wireless communications, instrumentation, audio/video and FPGA-based applications. One reason to use a VCXO is when the output frequency needs to be synchonized to an external reference frequency. For example, when data is transmitted from one end of a system to another (either wired or wirelessly), the data stream must synchronize on the receiving side. If the data does not match, a VCXO on the reciever side will use the embedded clock information to adjust or “pull” its frequency to synchronize with the exact frequency of the transitting side.

The VCXO is pulled ...
... by changing the control voltage. In quartz-based oscillators, this is typically done by placing a varactor diode on both sides of the quartz crystal oscillator and changing the capacitance by applying a reverse bias. In such devices, the total amount of frequency tuning may be limited by the characteristics of the crystal and the varactor diodes.

In oscillators that use micro electro-mechanical systems (MEMS) technology, a silicon resonator is used instead of a quartz resonator, and tuning is achieved through use of a phase locked loop (PLL). With any type of VCXO used in a high performance system, pull range, tuning slope (Kv), phase noise and frequency stability are important design considerations. Parameters for these key specifications vary considerably depending on the design and technology used for the timing device.

* Pull/tuning range: With quartz devices, the output frequency can only be adjusted up to a couple of hundred parts per million (+/- 100 to 200 ppm). In MEMS-based oscillators, the device is not limited with quartz crystals that have a small pull range. The frequency in MEMS VCXOs can be adjusted up to several hundred parts per million (± 25 to ± 1600 ppm), offering an extremely wide pull range.

* Tuning slope/linearity: With quartz devices, the varactor is non-linear and this translates to a non-linear slope in tuning gain especially close to 0V or Vdd. As voltage increases, frequency should increase at a constant rate. However, with quartz the slope deviates in the range of +/- 10%. In contrast, MEMS timing devices are extremely linear and have superior tuning slope consistency in the range of +/- 0.1 to 1%.

* Frequency stability: To achieve higher pull range in quartz VCXOs, a lower quality (low-Q) crystal must be used. Lower Q-factor quartz oscillators have more jitter. This negatively affects frequency stability and phase noise – two performance characteristics that are very important in applications that use VCXOs. With MEMS devices, this is not an issue because they do not use quartz crystals.

* Temperature drift: Voltage control can be used in temperature-compensated oscillators (TCXOs). Timing devices with this combined capability are called VCTCOXs. In quartz devices, allowances for temperature drift need to be made. Temperature drift is a problem with VCXOs since temperature varies over the voltage control range. Temperature-compensation helps to reduce drift.

In high performance systems, frequency stability and low phase noise are critical and have a direct impact on system performance. In quartz-based VCXOs, noise in the tuning slope negatively affects phase noise. Quartz devices are less flexible with limited pull range. Plus they have lower performance in terms of linearity, stability and drift. In contrast, MEMS-based VCXOs have a much wider pull range without the performance tradeoffs. In addition, MEMS timing devices across all categories – XOs, VCXOs, TCXOs and clock generators – have much higher reliability and more flexibility with customizable features. As designers look for new solutions to optimize their systems, they are implementing control techniques such as those employed in VCXOs, TCXOs, as well as using new MEMS technology to further enhance performance.

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