A novel approach to improving the stability of TCVCXO temperature performance

The output frequency of a temperature compensated voltage controlled oscillator is determined by the sum of an internally generated compensation voltage and an externally applied adjustment voltage. Without correction, non-linearities in the frequency vs voltage characteristic of the VCXO result in the frequency vs temperature characteristic of the device being dependent on the adjustment voltage. This is known as ´trim skew´. Additionally, the slope of the frequency vs voltage curve will be a function of temperature due to the temperature dependence of the crystal C₁ and of the varactor diode, limiting the temperature stability achievable. Both these effects are minimised in C-MAC´s latest high performance TCXO ASIC, code named ´Pluto´. Pluto has an additional circuit block inserted before the VCXO. The weighted sum of the compensation and adjustment voltages form the input to this novel circuit block, while the output is the voltage applied to the varactor, adjusting the frequency of the VCXO. As described in patent application EP1209812/US2002060597, the non-linear transfer function of the block can be digitally programmed to accurately compensate for the non-linearity of the VCXO´s frequency adjustment. A temperature sensor provides an additional input to the block. This is used to control the overall gain of the circuit in such a way as to correct for the temperature dependence of the crystal C1 and the varactor. The result is that a highly linear VCXO is achieved with constant voltage sensitivity over the temperature range. This in turn allows exceptional frequency vs temperature stability to be achieved over the whole adjustment voltage range. The digital control of the multiplying DACs used in the linearisation allows individual settings to be made, giving optimum results for each oscillator.