Direct conversion receiver with dithering local carrier frequency for detecting transmitted carrier frequency

A receiver for receiving a frequency shift keyed (FSK) signal on a radio-frequency carrier, which includes a frequency deviation, comprising:

• first and second channel paths to which the received FSK signals are applied, each channel path including a mixer circuit, a low-pass filter connected to the mixer circuit, and a limiting amplifier connected to the low-pass filter;
• a first voltage-controlled oscillator for generating a first local carrier;
• a phase shifter for producing local carriers in quadrature phase relationship from the local carrier supplied from the first voltage-controlled oscillator and respectively applying the quadrature local carriers to the mixer circuits;
• a frequency detector coupled to one of the limiting amplifiers for generating a signal representative of a variation frequency of the received FSK signal, said variation frequency equaling the deviation frequency of said radio-frequency carrier minus an offset between the radio-frequency carrier and the local carrier;
• an average circuit for averaging the signal from the frequency detector and producing a signal representative of the deviation frequency of the received radio signal;
• a comparator circuit for detecting whether the signal from the frequency detector lies in or outside of the neighborhood of a frequency represented by the signal from the average circuit;
• and
• a ramp generator for periodically generating a ramp voltage of constant amplitude when the signal from the frequency detector is detected as lying outside of said neighborhood and holding said ramp voltage at an instantaneous value when said signal is detected as lying in said neighborhood, said voltage-controlled oscillator being representative to an output voltage of said ramp generator to control the frequency of said local carrier.

A receiver for receiving a frequency shift keyed (FSK) signal on a radio-frequency carrier, comprising:

• first and second channel paths to which the received FSK signal is applied, said first and second channel paths including respectively first and second mixer conduits, first and second low-pass filters respectively connected to the first and second mixer circuits, and first and second limiting amplifiers respectively connected to the first and second low-pass filters;
• voltage-controlled oscillator means for generating a local carrier;
• phase shifter means for producing local carriers in quadrature phase relationship from the local carrier supplied from the voltage-controlled oscillator and respectively applying the quadrature local carriers to the first and second mixer circuits;
• frequency control means connected to said first limiting amplifier for detecting frequency fluctuations in said local carrier generated by the oscillator means with respect to a representative of said frequency fluctuations and supplying said control signal to said oscillator means to remove said frequency fluctuations;
• and
• demodulator means coupled to said first and second limiting amplifiers for demodulating said received FSK signal to produce a demodulated signal.