What is a Differential Oscillator? Where can it be used in Electronics?

There are two major kinds of electrical signaling –Single-ended (LVTTL, TTL, LVCMOS, CMOS, etc.) and differential (LVDS, LVPECL, HCSL, etc.). Differential signaling makes the use of 2 signals that are exactly opposite in phase to each other, thus eliminating common mode noise and resulting in a higher performance system. Differential signaling is used by many high performance protocols such as SATA, SAS, FibreChannel, 10G Ethernet, etc.

Differential oscillators are usually used to provide higher frequencies in very high performance systems where single-ended clocks do not perform well, such as the examples listed above. Typically, differential oscillators are used at frequencies above 100 MHz, because the rise times of differential clocks are usually much faster, and can support these high frequencies. However, it is not unusual to see differential clocks at even 25 MHz. Differential oscillators can output frequencies as high as 1 GHz.

One of the main reasons why differential oscillators are used in electronic systems is that they offer more robustness against power supply noise (and therefore, a higher PSRR) and reduce common mode noise coupling in the system. This is especially crucial for very high speed circuits, typically above 6 Gigabits per second data rates.

Historically, LVPECL protocols have been very popular in differential signaling. However, recently, LVDSs signaling has started to gain in popularity, driven by the lower power consumption of this protocol. Since the output frequencies of differential oscillators are very high, they have typically operated at 3.3V and higher voltages. However, newer differential oscillator devices from Silicon MEMS timing companies have offered differential oscillators at voltages as low as 1.8V.

Typical quartz-based differential oscillators are offered in industry standard, 6-pin footprints, either 7x5mm or 5×3.2mm. Silicon MEMS-based differential oscillators also fit in these footprints, ensuring that they can replace differential quartz oscillators with no changes in design or layout. Some newer devices are also available in extremely small, 3.2×2.5mm packages.

Silicon MEMS-based differential oscillators are usually programmable, i.e. their frequency, voltage, stability, drive strength, and other features can be customized exactly to the required specification, which is different than what quartz differential oscillators can achieve. The ability to customize is an extremely important feature that can be useful in a variety of scenarios –such as –reducing EMI, bit error rates, higher performance, higher throughput, etc.

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Silicon Oscillator

What is a VCXO? Why is a VCXO used in Electronics?

A VCXO - Voltage Controlled Oscillator- is a reference timing component that is commonly used in telecom, consumer and industrial electronics. A VCXO differs from an XO (Oscillator) in that it offers an capability to "fine-tune" the output frequency within a certain range, after the device is already populated in a system. This capability is not available on Xos.

Typically, a VCXO is used wherever a clock and data recovery function is performed. Usually, that involves receivers such as wireless base stations and other telecom equipment as well as consumer electronic devices such as Set Top Boxes (STB). The VCXO is part of a control loop that involves a PLL - Phase Locked Loop - that synchronizes the recovered clock on the receiver with the transmitted clock - to ensure that the two devices are locked and data integrity is maintained.

Pull range and Linearity are two key specs for VCXOs. Typically, crystal oscillator based VCXOs will offer a pull range of up to ±200 PPM of the output frequency. The maximum value of this "fine-tuning" capability is limited in a crystal-based VCXO because of the type of crystal and circuit that is used. This control is implemented on the analog oscillator circuit using varactors or switched capacitor arrays, which switch in and out as needed. In addition, a pullable crystal is also required - which is more expensive and not as easily available as a standard crystal resonator.

Pulling is achieved by an analog control voltage that is provided as input into the VCXO. The variation of the output frequency with this control input voltage is measured as linearity - the more linear a VCXO, the better control and characteristics it exhibits.

A silicon based VCXO offers much better pull range and linearity than a crystal-based VCXO. It does so because the "pulling" is done using Phase Locked Loops instead of varactors and pullable crystals - thus achieving a far larger pull range at a far better linearity. VCXOs that are based on Silicon MEMS will offer pull range of up to ±1600 PPM, as well as linearity of less than 1%, at comparable phase noise and jitter as crystal-based VCXOs. Thus, they are becoming the VCXOs of choice in electronics applications.

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TCXO Oscillator

VCXO Oscillator