Circuit arrangement for automatic frequency control in a radio telephone

A circuit for controlling a frequency of a frequency signal generator that provides a local oscillator signal to a mixer in a radio frequency section of a radio transceiver, said circuit comprising:

• a voltage controlled oscillator (VCO) connected to said frequency signal generator for generating a reference frequency signal;
• means for deriving an intermediate frequency signal from an input signal to said radio transceiver wherein said input signal is coupled to an input of said mixer and said intermediate frequency signal is outputted from said mixer;
• detector means for determining a signal strength of said intermediate frequency signal and for providing a signal strength indication thereof;
• phase detector means having first and second inputs;
• divider means responsive to said intermediate frequency signal and said reference frequency signal to derive therefrom equal frequency signals and to apply said equal frequency signals as inputs to said first and second inputs, respectively, said inputs causing said phase detector means to output a first control signal indicative of a phase difference between said equal frequency signals;
• switch means connected to receive said indication from said detector means and said first control signal, for selectively applying said first control signal to said VCO so as to maintain said reference frequency signal phase locked to said input signal, said switch means including memory means for storing values of said first control signal, said memory means comprising a latch circuit for storing digital values, an analog to digital converter connected to receive said first control signal and to apply a digital value corresponding to said first control signal to said latch circuit for storage, and a digital to analog converter connected to receive a stored digital value from said latch circuit, said switch means responsive to said signal strength indication exceeding or equalling a threshold to apply said first control signal to said VCO and to a decrease of said signal strength indication below said threshold, to cause said latch circuit to apply a stored value of said first control signal via said digital to analog converter to said VCO, said value of said first control signal stored in said latch circuit just prior to said decrease;
• constant voltage means connected to said switch means, said switch means applying during a start-up mode, an output from said constant voltage means to said VCO as a second control signal to set the reference frequency signal to a predetermined frequency;
• and
• means for controlling said switch means to apply said second control signal to said VCO if said signal strength indication from said detector means falls and remains below said threshold for a predetermined period.