Digitally controlled frequency generator including a crystal oscillator

A digitally controlled oscillator for use in a radar receiver comprising:

• a crystal oscillator for providing an oscillator signal of a defined frequency;
• a digital divider for dividing said oscillator signal by a selectable number controlled by a digital word for providing a clock signal;
• a low frequency oscillator receiving said clock signal for providing a low frequency signal, said low frequency oscillator including a plurality of digitally controlled filter resonators for determining the frequency of said low frequency signal and having a center frequency dependent upon said clock signal;
• means for providing first and second pairs of quadrature phase shifted signals derived from said clock signal and said low frequency signal and from said oscillator signal, respectively;
• and
• an image reject mixer for mixing said first and second pairs of quadrature phase shifted signals for providing a single high frequency output signal which varies in accordance with said digital word.

A method of operating a digitally controlled oscillator for use in a radar receiver, said method comprising the steps of:

• providing a first oscillator signal of a defined frequency;
• dividing said first oscillator signal by a selectable number controlled by a digital word for providing a clock signal;
• receiving said clock signal at a filtering element located within a low frequency oscillator for controlling the center frequency of said filtering element;
• resonating said filtering element at said center frequency for determining the output signal frequency of said low frequency oscillator, said center frequency being dependent upon said clock signal;
• providing first and second pairs of quadrature phase shifted signals derived from said clock signal and said low frequency oscillator output signal and from said first oscillator signal, respectively;
• and
• mixing said first and second pairs of quadrature phase shifted signals for providing a single high frequency output signal which varies in accordance with said digital word.