Selection of VCO frequency variable range

There are several points to consider regarding the selection of the VCO's variable frequency range. Even though the input frequency stability is ±20ppm, if the variable frequency range is set to ±20ppm, the following variations are not taken into account, causing the PLL to lose lock.

・Initial deviation: ±5ppm
・Frequency temperature characteristics: ±15ppm
・Power fluctuation: ±5ppm
・Secular change: ±5ppm/year
・Assuming that the life of the equipment is 10 years,
・±0.5ppm x 10 years = ±5ppm

becomes. These sum to ±30ppm, which is greater than the input frequency stability. If a VCO with a frequency variable range of ±20ppm is selected, the fluctuation factors will exceed the variable range and the PLL will not be able to lock. In the above case, considering input fluctuations, a variable width of at least ±50ppm is required.

Consider the VCO. The VCO becomes a resonant circuit with variable frequency. A VCO composed of L and C generally has a variable range of several thousand ppm, but the frequency may deviate greatly due to power supply fluctuations, fluctuations due to power supply noise may cause jitter, poor temperature stability, etc. It will be considered. A VCXO that uses a crystal has a frequency variable range of over 100 ppm, and is more stable than a VCO.

Crystals have an internal multiplier circuit because they cannot directly oscillate at high frequencies. Because it has a multiplier circuit, phase noise is inferior to VCSO using SAW (surface acoustic wave) that can directly oscillate. In addition, since it operates stably around 70℃, there are OCXO with an oven, TCXO that compensates for frequency fluctuations due to temperature with a digital circuit, and MCXO that compensates with the CPU inside the chip. Since it performs frequency correction, it has excellent frequency temperature characteristics, but it is more expensive than VCXO.

For the VCO used in the PLL, it is necessary to select the most suitable parts from VCO, VCXO, TCXO, OCXO, MCXO and VSCO depending on the required specifications.

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