Hello, my name is Taro. This time, I would like to write about Voltage Controlled Oscillator (VCO) which is one of the components of PLL.
Please refer to this page for the relationship between PLL and VCO.
Overview of VCO
VCO is an oscillator that outputs a frequency corresponding to an input voltage. A rough diagram of the relationship between input voltage and output frequency is shown below.
VCO, a component of the PLL, controls the output frequency according to the error signal coming from the PFD / CP through the LPF. For more information on the components of the PLL, please click here.
The output frequency range of the VCO can vary. Below are the output frequency ranges of VCOs used in IDT's products.
Types of VCOs
VCOs can be broadly classified into the following three categories
・Ring-VCO
・LC-VCO
・VCXO
These VCOs are used differently depending on the application.
Ring-VCO
Ring-VCO is an oscillation circuit with an odd number of inverter stages connected in a ring shape. The number of inverter stages and gate delay are used to control the oscillation frequency.
Because of its wide output frequency range, the VCO used in general PLLs is a Ring-VCO.
However, compared to LC-VCOs and VCXOs, the frequency stability is inferior.
As shown in the above figure, the frequency is controlled by current, which is affected by power supply noise.
However, low-noise clock generators are used in general PLLs because noise can be suppressed by technology to reduce the effect of power supply noise and by LPF design, and because integration is possible.
LC-VCO
LC oscillators can oscillate by connecting a coil and a capacitor and satisfying oscillation conditions. The oscillation conditions are shown below.
By connecting a varactor to the capacitor, it can be used as a VCO. A varactor is a variable capacitance diode whose capacitance can be varied according to voltage.
LC-VCOs have lower output phase noise characteristics than ring-VCOs, so they are used in PLLs for phase noise-sensitive applications.
For example, the Zero Delay Buffer for audio applications uses a PLL with an LC-VCO.
LC-VCOs are also used in PLLs that require high frequencies because of their high frequency output capability.
VCXO
The VCXO consists of an LC-VCO coil replaced by an XTAL (crystal resonator).
When used as a VCO in a PLL, the VCXO always provides a nearly constant output, even when the REF clock is interrupted.
Despite these advantages, the disadvantages are a narrow output frequency range and a larger circuit scale compared to Ring-VCO / LC-VCO.
VCXO-based PLLs are used in Jitter Attenuators.
The jitter attenuator reduces jitter by outputting the clock signal back through the PLL. However, as mentioned above, the circuit scale of the XTAL is large, so the XTAL may need to be mounted externally.
Conclusion
Application Examples
・Ring-VCO: General consumer electronic devices such as media players, digital cameras, POS terminals, etc.
・LC-VCO: Applications requiring low phase noise such as audio
・VCXO: Applications with strict Jitter accuracy requirements such as multiple access communication systems
IDT has a wide lineup of timing products, so a variety of VCOs are used for each application.
When selecting a timing product that uses a PLL, please refer to this article and pay attention to the VCO as well.
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