Direct conversion receiver

A direct conversion receiver comprising a local frequency generator coupled to a pair of quadrature related signal paths which include mixers for mixing down a received modulated radio frequency (r.f.) signal to derive a pair of quadrature related signals (I, Q) in said signal paths, and an automatic frequency control (a.f.c.) loop coupled to said signal paths for deriving from the quadrature related signals a frequency control signal (ct) for setting the local frequency. generator to the carrier frequency of the received r.f. signal;

• characterized in that said a.f.c. loop includes an offset frequency detector for deriving from the quadrature related signals a frequency offset signal (dct) having an average level which is proportional to an offset of the local frequency generator relative to the carrier frequency of the received r.f. signal, said offset frequency detector comprising:
• a first phase shifter and first multiplier means connected in series to a first of said quadrature related signal paths, said first phase shifter having a nonlinear phase shift vs. frequency characteristic;
• a second phase shifter and second multiplier means connected in series to a second of said quadrature related signal path, said second phase shifter also having a nonlinear phase shift vs. frequency characteristic;
• the first multiplier means having a control terminal connected to the second signal path, so that a signal (Q) in the first signal path is phase shifted and the phase shifted signal is multiplied by a signal (I) in the second signal path, the second multiplier means having a control terminal connected to the first signal path, so that a signal (I) in the second signal path is phased shifted and the phase shifted signal is multiplied by a signal (Q) in the first signal path;
• and
• means for subtractively combining the multiplied signals produced by said first and second multiplier means, the combined signal so produced being said frequency offset signal (ct), wherein the received r.f. signal is digitally modulated and further comprising:
• a demodulator coupled to the quadrature related signal paths for recovering data (dta) from the received digitally modulated r.f. signal;
• a data filter coupled to said demodulator for digitally filtering the recovered data;
• switching means coupled to said data filter and to said offset frequency detector for controllably selecting, as between the filtered recovered data and the frequency offset signal, which to use for producing said frequency control signal (ct);
• an averaging filter coupled to said data filter for deriving a control voltage V.sub.fdta corresponding to an average voltage level of the filtered data;
• and
• a decision circuit coupled to said averaging filter for receiving said control voltage and which controls said switching means based on evaluation of whether said control voltage corresponds to a frequency offset which is within a predetermined range of the carrier frequency of the received digitally modulated r.f. signal, the switching means being controlled to select said frequency offset signal for frequency control when the frequency offset is within said predetermined range and to select said control voltage for frequency control when the frequency offset is outside said predetermined range.

A direct conversion receiver comprising a local frequency generator coupled to a pair of quadrature related signal paths which include mixers for mixing down a received modulated radio frequency (r.f.) signal to derive a pair of quadrature related signals (I, Q) in said signal paths, and an automatic frequency control (a.f.c.) loop coupled to said signal paths for deriving from the quadrature related signals a frequency control signal (ct) for setting the local frequency generator to the carrier frequency of the received r.f. signal;

• characterized in that said a.f.c. loop includes an offset frequency detector for deriving from the quadrature related signals a frequency offset signal (dct) having an average level which is proportional to an offset of the local frequency generator relative to the carrier frequency of the received r.f. signal, said offset frequency detector comprising:
• a first phase shifter and first multiplier means connected in series to a first of said quadrature related signal paths, said first phase shifter having a nonlinear phase shift vs. frequency characteristic;
• a second phase shifter and second multiplier means connected in series to a second of said quadrature related signal paths, said second phase shifter also having a nonlinear phase shift vs. frequency characteristic;
• the first multiplier means having a control terminal connected to the second signal path, so that a signal (Q) in the first signal path is phase shifted and the phase shifted signal is multiplied by a signal (I) in the second signal path, the second multiplier means having a control terminal connected to the first signal path, so that a signal (I) in the second signal path is phased shifted and the phase shifted signal is multiplied by a signal (Q) in the first signal path;
• and
• means for subtractively combining the multiplied signals produced by said first and second multiplier means, the combined signal so produced being said frequency offset signal (ct), wherein the respective quadrature paths each include a bandpass filter and a high pass filter which together constitute a stopband filter for a.f.c. out-of-range extension, and the first and second phase shifters have a non-linear phase shift characteristic which approaches a predetermined phase shift with increasing frequency beyond a predetermined frequency offset relative to the carrier frequency of the received r.f. signal.

A direct conversion receiver comprising a local frequency generator coupled to a pair of quadrature related signal paths which include mixers for mixing down a received modulated radio frequency (r.f.) signal to derive a pair of quadrature related signals (I, Q) in said signal paths, and an automatic frequency control (a.f.c.) loop coupled to said signal paths for deriving from the quadrature related signals a frequency control signal (ct) for setting the local frequency generator to the carrier frequency of the received r.f. signal;

• characterized in that said a.f.c. loop includes an offset frequency detector for deriving from the quadrature related signals a frequency offset signal (dct) having an average level which is proportional to an offset of the local frequency generator relative to the carrier frequency of the received r.f. signal, said offset frequency detector comprising:
• a first phase shifter and first multiplier means connected in series to a first of said quadrature related signal paths;
• a second phase shifter and second multiplier means connected in series to a second of said quadrature related signal paths;
• the first multiplier means having a control terminal directly connected to the second signal path, so that a signal (Q) in the first signal path is phase shifted and the phase shifted signal is multiplied by a signal (I) in the second signal path, the second multiplier means having a control terminal directly connected to the first signal path, the control terminals of the first and second multiplier means thereby being directly cross-coupled to respective second and first signal paths, so that a signal (I) in the second signal path is phased shifted and the phase shifted signal is multiplied by a signal (Q) in the first signal path;
• and
• means for subtractively combining the multiplied signals produced by said first and second multiplier means, the combined signal so produced being said frequency offset signal (ct), wherein the respective quadrature paths each include a bandpass filter and a high pass filter which together constitute a stopband filter for a.f.c. out-of-range extension, and the first and second phase shifters have a non-linear phase shift characteristic which approaches a predetermined phase shift with increasing frequency beyond a predetermined frequency offset relative to the carrier frequency of the received r.f. signal.