A: The frequency pullability or deviation in VCXO is the change in the output frequency with respect to change in control voltage. Pullability is usually specified as minimum; however, in some applications, a maximum pulling value is also specfied to avoid circuit instability.
Please refer to figure 1 for Colpitts VCXO circuit:

Some major factors that affect the frequency deviation in VCXO:

• Load capacitance value Cl and C2.
• Frequency.
• Crystal characteristics (Cl, CO/C1, size)
• Varactor type and capacitance.
• Voltage control Vc.
• Operating temperature.
  The frequency pullability can be increased either by using a low capacitance with sharp slope varactor, connect in series another varactor or an inductor, adjusting load capacitor values Cl and C2, or increasing control voltage. Be very careful when considering any above methods because it may create circuit instability which has a severe effect on jitter, linearity, unwanted modes, frequency hysteresis, or frequency shift over temperature.

Q: What is the typical input impedance?

A: The input impedance is a function of modulation frequency. Its minimum input impedance is 50kW at 10kHz.

Q: What is the transfer function? A: The transfer function is the direction of change in frequency versus the change in control voltage. Most applications require a positive transfer function, which the frequency rises when increasing control voltage.

Q: What is linearity and what are the factors that affect it?

A: Linearity is the deviation from the best straight-line slope of the frequency versus control voltage curve. The typical linearity in Abracon VCXO is +10% maximum for standard pullability. Larger pulling may worsen the linearity.

Q: What is the modulation bandwidth?

A: The modulation bandwidth is the minimum +3dB bandwidth frequency, relative to a lkHz to IOkHz modulation frequency. Unless otherwise specified as default, other values of modulation bandwidth and frequency must be specified when ordering.

Q: What is the control voltage?

A: The control voltage is the external voltage applied to the input of the VCXO. It consists of a minimum, a maximum, and a center voltage. The center control voltage is the nominal voltage that sets the oscillation frequency to its minimal value.
Standard control voltages:
Vc min = 0.5Vdc; Vc max. = 4.5Vdc; Vc center = 2.5Vdc + 0.5V

Q: What is a TCXO?

A: A TCXO (Temperature-Compensated Crystal Oscillator) is a crystal oscillator that has a high-precision crystal, a temperature-compensated network. There are several methods to design a compensated network, which could vary from simple, less expensive to, complicate and very expensive:

• Method 1: Thermistor/Capacitor networks (Direct compensation). Lowest cost, no varactor needed.
• Method 2: Traditional thermistor network.
• Method 3: Analog Polynomial Generator
• Method 4: Digitally segmented analog
• Method 5: Digital compensation
• Method 6: Digital compensation with DAC voltage summers.

Q: Why and when we need to use a TCXO?

A: We need to use a TCXO when the frequency stability of the oscillator falls beyond the design limitation of a standard simple (fixed) crystal oscillator which is typically less than +5ppm over a standard or extended temperature window. The TCXO costs more du~o its complex circuit and manufacturing.

Q: Why there is an internal trimmer or control voltage on a TCXO?

A: The purpose of the internal trimmer (variable capacitor) or a control voltage is to re-adjust the frequency to its nominal frequency for aging compensation or initial setting. The internal trimmer is accessible via a hole on top of the TCXO can and can be adjusted with a special tool. The control voltage can be set with a voltage divider or an external voltage. Both methods of adjustment usually can not produce large frequency deviation rather than 5 to 15ppm enough to offset the frequency due to standard aging.

Q: How to specify frequency stability on a TCXO?

A: Unless otherwise specified, the frequency stability on a TCXO is specified as follows:

• Due to temperature change: + 2.5ppm @ -20°C to +70°C
• Due to aging: + lppm per year max.
• Due to supply voltage (+5%): + 0.3ppm max.