This invention relates to digitally tuned voltage controlled oscillators and in particular to digitally tuned voltage controlled oscillators for use in the microwave frequency range.
A microwave frequency tunable oscillator requires accurate tuning, fast settling time, stability and reasonable cost. The prior art systems include a direct synthesizer with frequencies derived from discrete oscillators or a phase lock loop synthesizer. The phase lock loop system is accurate and cost effective but lacks the required tuning speed. The direct synthesizer is fast and accurate but is very expensive. The digitally tuned voltage controlled oscillator is very fast and inexpensive but is susceptible to frequency inaccuracies due to environmental conditions. However, because a tunable microwave frequency oscillator requires linearity of better than =0.25% over a 25% bandwidth. the voltage controlled oscillator must be insensitive to inaccuracies caused by temperature change and immune to the effects of aging.
The prior art voltage collected oscillator system traditionally used a digitally tuned oscillator with an array of two or more programmable read only memories. PROMs are typically programmed at specified temperatures in order to cause a digital to analog converter that drives the digitally tuned oscillator output to keep the voltage controlled oscillator within a window of linearity and frequency. The tolerance is specified within a straight line. The percent linearity is determined by one half the window bandwidth divided by the full band width times 100.
The voltage controlled oscillator and sometimes the digital to analog converter that is used to provide control voltages to the voltage control oscillator must be heated in order to maintain a specified linearity and frequency accuracy over an operational temperature range. However, heating only compensates for environmental factors. The PROMs which are keyed to the initial voltage controlled oscillator accuracy and the voltage controlled oscillator are characterized by their vast linearity versus DC control voltage at specified temperatures. A PROM is programmed such that when addressed the PROM will provide on its output a digital representation of a voltage to drive the voltage controlled oscillator via the digital analog converter. The PROM programming is typically performed at a single temperature. This temperature is usually chosen at the mid range of the desired operating temperature range or at a temperature based on a heater's heating characteristic. Needless to say, the vast linearity of frequency is at a signal temperature. Degradation is expected over the remaining temperature range and an erorr is typically expected in the microwave range of 1 megahertz per degree centigrade. Heaters therefore become a prime requirement and must keep up with the environmental changes. However, there is also a point where the heater power is not practical.
A method of compensating for the effects of aging is to periodically reprogram the PROMs that drive the digital to analog converter to more accurate values.