Modulation rate/Frequency Response

     This is the rate at which the control voltage can change resulting in a corresponding frequency change.  It is measured by applying a sinewave signal with a peak value equal to the specified control voltage, demodulating the VCXO's output signal, and comparing the output level of the demodulated signal at different modulation rates.  While non-crystal controlled VCOs can be modulated at very high rates (greater than 1 MHz for output frequencies greater than 10 MHz), the modulation rate of VCXOs is restricted by the physical characteristics of the crystal.

Stability vs. Pullability

      A quartz crystal is a high Q device, which is the stability-determining element for the crystal oscillator.  It inherently resists being "pulled" (deviated) from its designed frequency.  In order to produce a VCXO with significant pullability, the oscillator circuit must be "de-Q'd".  This results in degrading the inherent stability of the crystal in terms of its frequency vs temperature characteristic, its aging characteristic, and its short-term stability (and associated phase noise) characteristic.  One particular scenario with VCXOs is that increased deviation results in degraded stability which can result in the need for still wider deviation, further degrading stability, resulting in a spiraling increase in the required deviation.  Therefore, it is in the user's best interest not to specify a wider deviation than that absolutely required.

Phase Locking Application

      When a VCXO is being used in phase lock loop application, the deviation should always be at least as great as the combined instability of the VCXO itself and the reference or signal onto which it is being locked.  However, if the open loop stability requirements of a system are more stringent than what a standard VCXO can provide, a TC-VCXO may be required. For the highest stability open loop requirements, the appropriate oscillators may be those TCXOs or OCXOs that incorporate a narrow deviation VCXO option.

Oscillator Output Frequencies

      Fundamental mode crystals (generally 10 - 35 MHz) permit the widest deviation, while 3rd overtone crystals (generally 30 - 125 MHz) allow deviation approximately 1/9^th of that which applies to fundamentals.  Therefore, all wide deviation VCXOs (greater than ±100 to ±200 ppm deviation) uses fundamental crystals.  On the other hand, narrower deviation VCXOs can use either fundamental mode or 3rd overtone crystals, and the selection of which often depends upon such specifications as linearity and stability.  It is rare that higher overtone, and therefore higher frequency crystals find applications in VCXOs.  Thus, VCXOs with output frequencies higher or lower than available frequencies from the appropriate crystals include frequency multipliers or dividers.

4. Temperature Compensated Crystal Oscillators

      The temperature characteristics of crystal oscillators are largely depending on those of crystal units used which are generally expressed by cubic curves of AT cut quartz blank.  The temperature compensating circuit, which must be custom-built for each unit, is used to tune the oscillator just enough to offset the uncompensated frequency change with temperature.  TCXO features excellent temperature characteristics, fast warm-up time (typically 50 to 1000 ms), low power consumption (10 to 150 mW), lightweight, compactness, and with a fraction of the cost of an OCXO’s.  It is ideally suited for various communications equipment such as cellular phones, two-way radios, cordless telephones, microwave communications equipment and satellite communications system, measurement instrument and many more other applications.