Direct conversion receiver

• US 5,584,068 A
• Filed: 08/28/1995
• Issued: 12/10/1996
• Est. Priority Date: 11/26/1992
• Status: Expired due to Term

• Current Assignee: ST-Ericcson SA (Telefonaktiebolaget LM Ericsson)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Mohindra, Rishi
• Primary Examiner(s): Faile, Andrew
• Assistant Examiner(s): Nguyen, Lee
• Application Number:
  US08/520,330
• Time in Patent Office: 470 Days
• Field of Search: 455/182.1, 455/182.2, 455/183.1, 455/183.2, 455/192.1, 455/192.2, 455/257, 455/259, 455/260, 455/265, 455/324, 455/256, 375/97, 375/80, 375/83, 375/84, 375/81
• US Class Current: 455/324
• CPC Class Codes: H03D 3/007 by converting the oscillations into two quadrature related signals H03D3/245 takes precedence
• CPC Class Codes: H03D 7/165 at least two frequency changers being located in different paths, e.g. in two paths with carriers in quadrature combined with amplitude demodulation H03D1/2245, combined with angle demodulation H03D3/007; N-path filters H03H19/002
• CPC Class Codes: H03J 7/02 Automatic frequency control H03J7/18 takes precedence
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A direct conversion receiver for FSK modulated digital signals has two quadrature signal paths (I, Q) and an a.f.c. loop for controlling a local frequency generator. A control signal for the a.f.c. loop is produced by an offset frequency detector, and is in the form of a unipolar series of pulses having an average value which is proportional to the offset of the local frequency with respect to the carrier frequency of the signal being received. The offset frequency detector derives such control signal based on the difference between phase-shifted cross-products of the signals in the I and Q paths. This achieves fast a.f.c. loop response, improving the performance of the receiver, and permits a simpler loop filter for achieving the requisite signal averaging. The receiver also includes an out-of-range detector for detecting when the local frequency is outside a predetermined window with respect to the carrier frequency of the received signal, in which case the a.f.c. control signal is derived from the average level of data in the received signal rather than from the offset frequency detector.

Claims

1. 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 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_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.

2. A direct conversion receiver as claimed in claim 1, wherein the received r.f. signal is digitally modulated, said first and second phase shifters are shift registers, and said first and second multiplier means are exclusive-OR logic circuits.

3. A direct conversion receiver as claimed in claim 1, wherein said first and second phase shifters, said first and second multiplier means and said subtractive combining means are all functionally implemented by a microprocessor.

4. A direct conversion receiver as claimed in claim 1, further comprising means coupled to said offset frequency detector for deriving said frequency control signal (ct) from said frequency offset signal (dct) by any of analog integration, digital integration, analog low pass filtering, and digital low pass filtering.

5. 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_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.

6. A direct conversion receiver as claimed in claim 5, wherein said decision circuit evaluates said control voltage with reference to whether it corresponds to a frequency within a predetermined frequency window.

7. 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.

8. 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.