Frequency conversion circuit

Abstract: Within the delayed detection type demodulator, the received signal is first quantized by a limiter amplifier 10 and then subjected to frequency conversion by a frequency converter 50 including; an exclusive OR element 51; a shift average generator 52 consisting of a shift register 53 and an adder 54; and a comparator 55. In response to the output of the frequency converter 50, the phase comparator 60 outputs a relative phase signal representing the phase shift of the received signal after frequency conversion relative to the phase reference signal. The phase comparator 60 includes: an exclusive OR element 61; an absolute phase shift measurement means 62 consisting of an adder 63 and D flip-flop arrays 64 and 65; and a D flip-flop 66 serving as a phase shift polarity judgment means. Alternatively, the phase detection circuit 400 for generating the relative phase signal may include: a half-period detector means 901 consisting of a delay element 401 and an exclusive OR element 402; a phase reference signal generator means 902 consisting of a modulo 2N counter 403; and a phase shift determiner means 903 consisting of a phase inversion corrector 500 and a D flip-flop array 404. The delay element 40 delays the relative phase signal by one symbol period and the subtractor 41 outputs the phase difference signal representing the phase transition over each symbol period of the received signal. The judgment circuit 42 obtains the demodulated data from the phase difference signal.

Description: BACKGROUND OF THE INVENTION This invention relates to a frequency converter circuit used for exemple in a differential detection demodulator of a radio comminication system. A digital demodulator for DPSK signals is disclosed in European Patent Application EP-A-0 008 491. A conventional differential detection demodulator provided with a frequency converter and a phase comparator is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 64-12646, "DPSK demodulation system". Next, this differential detection demodulator is described by reference to Fig. 8. In Fig. 8, the frequency converter 20 includes a multiplier 21 and a low pass filter 22. The phase comparator 30 includes: a phase shifter 31 for shifting the phase of the local carrier (the phase reference signal) by π/2 radians; a multiplier 32 for multiplying the local carrier by the output of the low pass filter 22; a multiplier 33 for multiplying the output of the phase shifter 31 by that of the low pass filter 22; a low pass filter 34 for eliminating the high frequency components from the output of the multiplier 32; a low pass filter 35 for eliminating the high frequency components from the output of the multiplier 33; a sampler 36 for sampling the output of the low pass filter 34; a sampler 37 for sampling the output of the low pass filter 35; and a coordinate converter 38 for calculating and generating a relative phase signal from the outputs of the samplers 36 and 37. A delay element 40 delays the relative phase signal by one symbol period of the received signal. A subtractor 41 subtracts, in modulo 2 π, the relative phase signal delayed by one symbol period by the delay element 40 from the relative phase signal directly output from the coordinate converter 38. A devision circuit 42 outputs the demodulated data according to the values of phase transition over each symbol period of the received signal. Next the operation of the circuit of Fig.8 is described in detail. It is a common practice in the field of demodulators to convert the frequencies of the received signal to low frequencies using a frequency converter. This facilitates subsequent signal processing. The received signal is a differential phase shift keying (DPSK) signal. This received signal is input to the frequency converter 20, where the multiplier 21 multiplies it by the signal for frequency conversion. It is assumed that the frequency of the received signal is f 1 Hz and that of the frequency conversion signal f 2 Hz. Then the multiplied signal output from the multiplier 21 includes a high frequency component at f 1 + f 2 Hz and a low frequency component at f 1 + f 2 Hz. This multiplied signal output from the multiplier 21 is supplied to the low pass filter 22, where the high frequency component is suppressed and only the low frequency component is suppressed and only the low frequency component at f 1 + f 2 Hz is passed. The received signal thus undergoes the frequency conversion. After subjected to the frequency conversion by the frequency converter 20, the received signal is processed by the phase comparator 30. The multiplier 32 multiplies the received signal after the frequency conversion (output from the frequency converter 20) by the phase reference signal (the local carrier). The low pass filter 34 eliminates the high frequency components from the output of the multiplier 32, thereby obtaining the base band signal in phase with the local carrier (referred to as the in-phase base band signal). The phase shifter 31 shifts the phase of the phase reference signal or the local carrier by π/2 radians. The multiplier 33 multiplies the received signal after the frequency conversion (output from the frequency converter 20) by the output of the phase shifter 31. The low pass filter 35 eliminates the high frequency components from the output of the multiplier 33, thereby obtaining the base band signal in quadrature with the local carrier (referred to as the quadrature base band signal). The in-phase base band signal output from the low pass filter 34 is sampled by the sampler 36 and supplied to the coordinate converter 38. Similarly, the quadrature base band signal output from the low pass filter 35 is sampled by the sampler 37 and supplied to the coordinate converter 38. The coordinate converter 38 outputs the relative phase signal representing the phase shift of the received signal after frequency conversion relative to the local carrier, i.e. the phase reference signal. The value of the relative phase signal  is expressed by the values x and y of the sampled in-phase and quadrature base band signals as follows: = tan -1 (x/y) The relative phase signal output from the coordinate converter 38 is supplied to the subtractor 41 and the delay element 40. At the delay element 40 the relative phase signal is delayed by one symbol period of the received signal and then is supplied to the subractor 41. The subtractor 41 subtracts, in modulo 2 π, the output of the delay element 40 from the output of the coordinate converter 38, and thereby obtains the phase shift difference signal (abbreviated hereinafter to phase difference signal). The phase difference signal output from the subtractor 41 represents the phase transition over each symbol period of the received signal. Upon receiving the phase difference signal from the subtractor 41, the devision circuit 42 obtains the demodulated relationship between the phase difference signal and the demodulated data. The above conventional differential detection demodulator has the following disadvantage. Since the frequency converter and the phase comperator circuits are composed of analog parts, integration of circuit parts into ICs is difficult. Thus, the adjustment or tuning of the circuits is indispensable. Further, it is difficult to reduce the size and the power consumption of the circuit. SUMMARY OF THE INVENTION It is therefore the object of this invention to provide a frequency converter consiting of digital circuit elements, such that the circuit can easily be integrated into ICs and hence the adjustment step of the circuit can be dispensed with and the size and the power consumption can be reduced. This object is solved according to the present invention by the features of claim 1. Prefered embodiments of the frequency converter circuit in complince with the invention are defined in the subclaims. The present frequency converter circuit for converting a frequency of a first 2-level quantised signal using a second 2-level quantised signal having a frequency distinct from the frequency of the first signal comprises: an exclusive OR element for obtaining a logical exclusive OR of the first and second signal; running average generator means, coupled to the exclusive OR element, for generating a signal corresponding to k times a running average of an output of the exclusive OR element, k being a positive integer; and hard devision means, coupled to the running average generator means, for converting an output of the running average generator means to a 2-level logical signal. Preferably, the running average generator means comprises: a shift register coupled to the exclusive OR element and having (2n + 1) stages to hold respective bits, where n is a positive integer and the output of the exclusive OR element is first supplied to a first stage of the shift register, the shift register shifting the bits held in the stages from the first toward (2n + 1)th stage in synchronism with a clock signal having a period shorter than periods of the first and second signals; and an adder means coupled to the shift register, for adding bits of the respective stages of the shift register, wherein an output of the adder constituting the output of the running average generator means. It ist still preferred that the running average generator means comprises: a shift register coupled to the exclusive OR element and having (2n + 2) stages to hold respective bits, where n is a positive integer and the output of the exclusive OR element is first supplied to a first stage of the shift register, the shift register shifting the bits held in the stages from the first toward (2n + 2) th stage in synchronism with a clock signal having a period shorter than periods of the first and second signals; a sign invertor coupled to the shift register, for inverting a polaritiy of an output bit of the (2n + 2) th stage; and adder coupled to the first stage of the shift register and the sign invertor; and a delay element having an input coupled to an output of the adder and having an output coupled to an input of the adder, the delay element delaying the output of the adder in synchronism with the clock of the shift register; wherein the adder adds outputs of the first stage of the shift register; the sign invertor; and the delay element, the output of the delay element constituting the output of the running average generator means. Preferably, the hard devision means compares the output of the running average generator means with a predetermined threshold level to convert the output of the running average generator means to the 2-level logical signal. BRIEF DESCRIPTION OF THE DRAWINGS The features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The structure and method of operation of this invention itself, however, will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing the circuit structure of a differential detection demodulator provided with a frequency converter according to this invention and a phase comparator; Fig. 2 is a timing chart showing waveforms within the frequency converter in the case where the shift register has five stages to hold respective bits; Fig. 3 is a timing chart showing the waveforms of the phase reference signal, the received signal after the frequency conversion, and the output of the exclusive OR element 61 of Fig. 1, in the two cases where the received signal after frequency conversion leads (show above) and lays (shown below) with respect to the phase reference signal; Fig. 4 is a timing chart showing the waveforms occurring within the absolute Phase shift measurement means 62 of Fig. 1 when the received signal after the frequency conversion leads relative to the phase reference signal, in the case where the clock frequency of the D flip-flop array 64 is 16 times the frequency of the phase reference signal. Fig. 5 is a timing chart showing the same waveforms as those of Fig.4, occurring when the received signal after the frequency conversion lags relative to the phase reference signal; Fig. 6 is a timing chart showing the waveforms related to the operation of the D flip-flop 66 of Fig. 1; Fig. 7 is a block diagramm showing the circuit structure of differential detection demodulator comprising another frequency converter according to this invention; and Fig. 8 is a block diagram showing a conventional differential detection demodulator provided with a frequency converter and a phase comparator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the accompanying drawings, the preferred embodiments of this invention are described. Fig. 1 ist a block diagram showing the circuit structure of a differential detection demodulator provided with a frequency converter according to this invention and a phase comparator. A limiter amplifier 10 subjects the received signal to a 2-level quantization. A frequency converter 50 coupled to the limiter amplifier 10 effects a frequency conversion on the 2-level quantized received signal output from the limiter amplifier 10. The frequency converter 50 is organized as follows. An exclusive OR element 51 is coupled to the limiter amplifier 10 to obtain the logical exclusive OR of the output of the limiter amplifier 10 and the signal for frequency conversion (the frequency conversion signal). A running average generator 52 removes the high frequency components from the output of the exclusive OR element 51. The running average generator 52 includes: a shift register 53 for sequentially delaying the output of the exclusive OR element 51; and an adder 54 for adding the output bits of the shift register 53. A comparator 55 coupled to the adder 54 compares the output of the adder 54 with a predetermined threshold value. Further, a phase comparator 60 is coupled to the frequency converter 50 to compare the phase of output of the frequency converter 50 (the received signal after frequency conversion) and the phase of the phase reference signal. The phase comparator 60 is organized as follows. An exclusive OR element 61 coupled to the comparator 55 effects the logical exclusive OR operation upon the output of the comparator 55 and the phase reference signal. In response to the output of the exclusive OR element 61, and absolute Phase shift measurement means 62 determines the absolute value of the phase shift of the received signal after frequency conversion relative to the phase reference signal. The absolute phase shift measurement means 62 includes an adder 63 coupled to the exclusive OR element 61 and a pair of D flip-flops 64 and 65 coupled to the adder 63. The output of the D flip-flop arrey 64, delaying the output of the adder 63, is returned to the adder 63. The adder 63 adds the outputs of the exclusive OR element 61 and the D flip-flop array 64. The D flip-flop array 65 stores the output of the adder 63. The phase comparator 60 further includes a D flip-flop 66. In response to the output of the exclusive OR element 61, the D flip-flop 66 decides whether the phase of the received signal after frequency conversion leads or lags relative to the phase reference signal. The bits output from the D flip-flops 65 and 66 are combined to obtain the output of the phase comparator 60 (i.e., the relative phase signal). The output of the phase comparator 60 is supplied to the subtractor 41 and the delay element 40. At the delay element 40 the relative phase signal is delayed by one symbol period of the received signal and then is supplied to the subtractor 41. The subtractor 41 subtracts, in modulo 2 π, the output of the delay element 40 from the output of the phase comparator 60, and thereby obtains the phase difference signal. The devision circuit 42 obtains the demodulated data on the basis of the predetermined correspondence relationship between the phase difference signal and the demodulated data. Next, the operation of the circuit of Fig. 1 is described in detail. First, the limiter amplifier 10 shapes the received signal into a rectangular waveform of a constant amplitude. Namely, the limiter amplifier 10 acts as a 2-level quantizer for subjecting the received signal to the 2-level quantization, such that the output of the limiter amplifier 10 is quantized to logical "0" and "1". The 2-level quantized output of the limiter amplifier 10 is supplied to the frequency converter 50, where the exclusive OR element 51 effects the logical exclusive OR operation upon the output of the limiter amplifier 10 and the signal for frequency conversion (the frequency conversion signal) which also takes either the logical value "0" or "1." Bay the way it is noted that if the logical values "0" and "1" are converted to numerical values "1" and "-1", respectively, then the exclusive OR operation corresponds to the multiplication operation of corresponding numbers. Therefore the exclusive OR element 51 acts as a multiplier for multiplying the output of the limiter amplifier 10 (the 2-level quantized received signal) by the signal for frequency conversion. The output of the exclusive OR element 51 is then supplied to the shift register 53 having (2n + 1) stages to hold respective bits, where n is a positive integer. The frequency of the clock signal supplied to the shift register 53 is assumed to be higher than the frequency of the output of the limiter amplifier 10 and the frequency of the signal for frequency conversion. The (2n + 1) bits output from the respective stages of the shift register 53 are supplied to the adder 54. Let the period of the clock for the shift register 53 be T c . Further, let the value of the output of the exclusive OR element 51 at the time t = i • T c be represented by a oi , where i is an integer and a oi is either "0" or "1"; a oi ∈ {0, 1}. Furthermore, let the value of the mth bit of the shift register 53 at time t = i · T c be a mi , where m =1, ··· (2 n + 1), i is an integer, and a mi is either "0" or "1": a mi ∈ {0, 1}. Then, a mi = a o (i - m) Thus, the output b i of the adder 54 at the time t = i · T c is given by: Namely, the output bi of the adder 54 at the time t = i · T c is equal to (2n + 1) times the average of the (2n + 1) sequentially shifted values: a0 (i - 1) ··· a0 (i - 2n - 1), of the output of the exclusive OR element 51. The output of the adder 54 constitutes the output of the running average generator 52, which is supplied to the comparator 55. The comparator 55 compares the output of the running average generator 52 with the constand n . Depending on the value bi of the output of the running average generator 52 and the constand n , the value bi of the output signal of the comparator 55 is given as follows: d i = 0 ( b i ≦ n ) 1 ( b i > n ) Namely, the comparator 55 acts as a hard devision means for converting the output bi of the running average generator 52 into a 2-level signal which takes either of the two logical values "0" or "1". The signal processing within the frequency converter 50 thus converts the frequency of the 2-level quantized received signal (output of the limiter amplifier 10). Namely, if the frequency of the 2-level quantized received signal is represented by f 1 Hz, that of the signal for frequency conversion by f 2 Hz, then the frequency of the received signal after frequency conversion (the output of the frequency converter 50) is: f 1 - f 2 Hz. Next, this is described in detail by reference to the waveform diagrams. Fig. 2 is a timing chart showing waveforms within the frequency converter 50 in the case where the shift register 53 has five stages to hold respective bits, namely where n = 2. At the top row is shown the time scale as measured by the periods of the clock for shift register 53 (the first through 25 th periods). The waveforms shown below the time scale are, from top to bottom: the clock supplied to the shift register 53; the output of the limiter amplifier 10 (2-level quantized received signal); the signal for frequency conversion (the frequency conversion signal); the output of the exclusive OR element 51; the first bit of the shift register 53; the fifth bit of the shift register 53; the output of the adder 54 (the inserted numbers representing the values of the output); and the output of the comparator 55. It is assumed that all the five bits of the shift register 53 are at logical "0" at time "1". Let the frequency of the clock supplied to the shift register 53 be f 0 Hz. Further, assume that the frequency f 1 of the 2-level quantized received signal (the output of the limiter amplifier 10 supplied to the frequency converter 50) and the frequency f 2 of the signal for frequency conversion are given by: f 1 = f 0 /4 f 2 = f 0 /6 Then, the frequency f 3 of the output of comparator 55 (i.e., the output of the frequency converter 50) is given by: f 3 = f 0 /12 From the above three equations, the frequency f 3 of the output of the frequency converter 50 (the received signal after frequency conversion), the frequency f 1 of the output of the limiter amplifier 10 (the 2-level quantized received signal), and the frequency f 2 of the signal for frequency conversion satisfy: f 3 = f 0 /12 = f 0 /4 - f 0 /6 = f 1 - f 2 Further, since the commutative law holds for the logical exclusive OR operation, the waveforms of the 2-level quantized received signal and the frequency conversion signal (the signal for frequency conversion) can be interchanged without affecting the waveforms of the output waveforms of the exclusive OR element 51, the shift register 53, the adder 54 and the comparator 55. Under such circumstances, the frequency f 0 of the clock signal for the shift register 53, the frequency f 1 of the 2-level quantized received signal, and the frequency f 2 of the signal for frequency conversion satisfy: f 1 = f 0 /6 f 2 = f 0 /4 Thus, the frequency f 3 of the received signal after frequency conversion are expressed in terms of the frequencies f 1 and f 2 as follows: f 3 = f 0 /12 = f 0 /4 - f 0 /6 = f 2 - f 1 The above relations can thus be summarized by the equation: f 3 = f 1 - f 2 In Fig. 2, the output of the exclusive OR element 51 includes a high frequency component at f 0 /2 Hz. However, the output of the comparator 55 does not include such high frequency components. Namely, the running average generator 52 consisting of the shift register 53 and the adder 54, and the comparator 55 acting as the hard devision means for converting the output of the running average generator 52 into a 2-level logical signal, function together as a low pass filter for removing the high frequency components from the output of the exclusive OR element 51. The output of the frequency converter 50 (the received signal after the frequency conversion) is supplied to the phase comparator 60. The exclusive OR element 61 effects the exclusive OR operation upon the received signal after the frequency conversion and the phase reference signal which is a 2-level signal taking either the logical "0" or "1". As in the case of the exclusive OR element 51 within the frequency converter 50, the exclusive OR element 61 acts as a multiplier for multiplying the received signal after the frequency conversion by the phase reference signal. The duration during which the output of the exclusive OR element 61 is continuously sustained at the logical 1 is proportional to the absolute value of the phase shift of the received signal after the frequency conversion relative to the phase reference signal. Next this is described in detail by reference to waveforms. Figur 3 is a timing chart showing the waveforms of the phase reference signal, the received signal after the frequency conversion, and the output of the exclusive OR element 61 of Fig. 1, in the two cases where the received signal after frequency conversion leads (shown above) and legs (shown below) with respect to the phase reference signal. The absolute value of the phase shift ϕ of the received signal after frequency conversion relative to the phase reference signal is expressed in terms of: the length of time τ between the rising or falling edges of the received signal after frequency conversion and the phase reference signal; and the period T of the phase reference signal. Namely, the absolute value of the phase shift ϕ is expressed as follows: | ϕ | = 2 π τ/T As understood from Fig. 3, the time τ between the rising or the falling edges of the phase reference signal and the received signal after frequency conversion is equal to the time during which the output of the exclusive OR element 61 is continuously sustained at logical "1". Thus, the duration by which the output of the exclusive OR element 61 is sustained at logical "1" is proportional to the absolute value of the phase shift of the received signal after frequency conversion relative to the phase reference signal. Consequently, the absolute value of the phase shift of the received signal after frequency converter relative to the phase reference signal can be determined by measuring the duration in which the output of the exclusive OR element 61 is continuously sustained at logical "1". The output of the exclusive OR element 61 is supplied to the absolute phase shift measurement means 62, where the adder 63 adds the outputs of the exclusive OR element 61 and the D flip-flop array 64, the output of the adder 63 being supplied to the D flip-flop array 64 and the D flip-flop array 65. The frequency of the clock signal suppliced to the D flip-flop array 64 is selected at M times the frequency of the phase reference signal, where M is an even number not less than four. The D flip-flop array 64 acts as a delay element for storing the output of the adder 63. Thus, during the time when the output of the exclusive OR element 61 is sustained at "1", the output of adder 63 is incremented by one over each period of the clock signal of the D, flip-flop array 64. On the other hand, during the time when the output of the exclusive OR element 61 is at "0", the output of the adder 63 remains constant. The output of the adder 63 is also input to the D flip-flop array 65. The frequency of the clock signal of the D flip-flop array 65 is two times that of the phase reference signal, the rising edges of the clock of the D flip-flop array 65 coinciding with the rising or the falling edges of the phase reference signal. Further, the D flip-flop array 64 is reset at the rising edge of the clock for the D flip-flop array 65. Namely, the D flip-flop array 64 is reset at each half period of the phase reference signal. The output of the D flip-flop array 65 is thus equal to the integral part of the duration of logical "1" of the output of the exclusive OR element 61 during each half period of the phase reference signal, as normalised by the periods of the clock signal of the D flip-flop array 64. Namely, the output of the D flip-flop array 65 is obtained by dividing the duration of the logical "1" of the output of the exclusive OR element 61 by the length of the period of the clock signal of the D flip-flop array 64 and then discarding the fractional part of the quotient resulting from the division. Next, the operation of the absolute phase shift measurement means 62 is described by reference to the waveform diagrams. Fig. 4 is a timing chart showing the waveforms occurring within the absolute phase shift measurement means 62 of Fig. 1 when the received signal after the frequency conversion leads relative to the phase reference signal, in the case where the frequency of the clock signal of the D flip-flop array 64 is 16 times the frequency of the phase reference signal (namely, M = 16). Fig. 5 is a timing chart showing the same waveforms as those of Fig. 4, occurring when the received signal after the frequency conversion lags relative to the phase reference signal. In Figs. 4 and 5, from top to bottom are shown the waveforms of: the clock supplied to the D flip-flop array 64; the clock supplied to the D flip-flop array 65; the phase reference signal; the received signal after frequency conversion; the output of the exclusive OR element 61; the output of the D flip-flop array 64; the output of the adder 63; and the output of the D flip-flop array 65. The numbers shown at the last three waveforms are the values thereof at respective time intervals. As described above, the frequency of the D flip-flop array 65 is two times that of the clock of the phase reference signal. Further, the D flip-flop array 64 is reset at respective rising edges of the clock of the D flip-flop array 65. Furthermore, as described above, the output of the D flip-flop array 65 is obtained by normalizing the duration of the logical "1" of the output of the exclusive OR element 61 during each half period of the phase reference signal by the length of the period of the clock signal of the D flip-flop array 64 and then discarding the fractional parts of the normalized value. Let the value of the output of the D flip-flop array 65 be represented by µ, where µ is an integer ranging from 0 to M/ 2 (µ  {0, 1, ···, M/ 2}). Them, the following relationship holds among: the output µ of the D flip-flop array 65, the ratio M of the frequency of the clock of the D flip-flop array 64 to the frequency of the phase reference signal, and the absolute value of the phase shift ϕ of the received signal after frequency conversion relative to the phase reference signal: 2 π µ /M ≤ | ϕ | < 2 π (µ + 1)/M Namely, the value µ of the output of the absolute phase shift measurement means 62 is approximately equal to the absolute value of the phase shift of the received signal after frequency conversion relative to the phase reference signal, and the error is not greater than ± π/M. Thus, by selecting a large value of the ratio M of the frequency of the clock of the D flip-flop array 64 to the frequency of the phase reference signal, the measurement error of the absolute value of the phase shift can be reduced arbitrarily. The absolute value of the phase shift of the received signal after the frequency conversion relative to the phase reference signal is thus measured by the absolute phase shift measurement means 62. If the sign bit representing the positive or the negative sign is added to the measurement value µ in correspondence with the phase lag of the phase lead of the received signal after the frequency conversion relative to the phase reference signal, then the phase shift of the received signal after frequency conversion relative to the phase reference signal can adequately be respresented. As comprehended from Figs. 4 and 5, the value of the output of the exclusive OR element 61 at each rising edge of the clock signal of the D flip-flop array 65 corresponds to the lag or the lead of the phase of the received signal after frequency conversion relative to the phase reference signal. Namely, in the case of Fig. 4 where the phase of the received signal after frequency conversion leads relative to the phase reference signal, the output of the exclusive OR element 61 at the instant at which the clock signal of the D flip-flop array 65 rises is at logical "1". On the other hand, in the case of Fig. 5 where the phase of the received signal after frequency conversion lags relative to the phase reference signal, the output of the exclusive OR element 61 at the instant at which the clock signal of the D flip-flop array 65 rises at logical "0". Thus, the output of the exclusive OR element 61 is input to the D flip-flop 66 which is supplied with the same clock signal as the D flip-flop array 65, such that the output of the D flip-flop 66 represents whether the phase of the received signal after frequency conversion lags or leads relative to the phase reference signal. Next, this is described by reference to waveform diagrams. Fig. 6 is a timing chart showing the waveforms related to the operation of the D flip-flop 66 of Fig. 1. From top to bottom in Fig. 6, are shown the waveforms of: the clock supplied to the D flip-flop 66; the phase reference signal; the received signal after frequency conversion; the output of the exclusive OR element 61; and the output of the D flip-flop 66. As described above the clock of the D flip-flop 66 is the same as the clock of the D flip-flop array 65. Namely, the frequency of the clock of the D flip-flop 66 is two times that of the phase reference signal, the rising edges of the clock of the D flip-flop 66 coinciding with the rising or the falling edges of the phase reference signal. It can be comprehended from Fig. 6 that when the phase of the received signal after frequency conversion lags relative to the phase reference signal, namely when the Position of the rising or the falling edge lags relative to the corresponding rising or falling edge of the phase reference signal, the D flip-flop 66 outputs the logical "0" for each half period of the phase Referenden signal. On the other hand, when the phase of the received signal after frequency conversion leads relative to the phase reference signal, namely when the Position of the the rising or the falling edge leads relative to the corresponding rising or falling edge of the phase reference signal, the D flip-flop 66 outputs the logical "1" for each half period of the phase reference signal. Thus, in response to the output of the exclusive OR element 61, the D flip-flop 66 decides at the edge of each half period of the phase reference signal whether the received signal after frequency conversion lags or lead relative to the phase reference signal. The output of the D flip-flop 66 constitutes the sign bit representing the polarity of the phase shift of the received signal after frequency conversion relative to the phase reference signal. The output of the D flip-flop 66 is combined with the output of the absolute phase shift measurement means 62 to form together the output of the phase comparator 60. Thus, the output of the phase comparator 60 is a combination of the outputs of the absolute phase shift measurement means 62 and the D flip-flop 66. The output of the phase comparator 60 is the relative phase signal which represents the phase shift of the received signal after frequency conversion relative to the phase reference signal. The relative phase signal output from the phase comparator 60 is delayed by the delay element 40 by one symbol period of the received signal, and then is supplied to the subtractor 41. The relative phase signal is also supplied to the subtractor 41 of modulo 2 π. Upon receiving the outputs of the phase comparator 60 and the delay element 40, the subtractor 41 subtracts, in modulo 2 π, the output of the delay element 40 from the output of the phase comparator 60, and thereby obtains the phase difference signal, which represents the phase transition over each symbol period of the received signal. The phase difference signal output from the subtractor 41 is supplied to the devision circuit 42. The devision circuit 42 obtains the demodulated data corresponding to the value of the phase difference signal, on the basis of the predetermined correspondence relationship between the phase difference signal and the demodulated data. The above description relates to the case where the received signal is modulated in accordance with the differential phase shift keying (DPSK). This invention can also be applied to MSK or GMSK modulation systems. Further, in the case of the above embodiment, the constand n serving as the parameter of the frequency converter 50 is equal to 2 (n = 2) and hence the output of the shift register 53 has five bit stages. However the constant n may be any positive integer. For example, it may be that n = 6 (namely the shift register 53 may have 13 bit stages) or n = 7 (namely the shift register 53 may have 15 bit stages). Furthermore, in the case of the above embodiment, the ratio M of the frequency of the clock of the D flip-flop array 64 to that of the phase reference signal is 16 (M = 16). However, the constant M may be any positive even number, such as 32 (M = 32) or 64 (M = 64). Fig. 7 is a block diagram showing the circuit structure of another differential detection demodulator according to this invention. The circuit is similar to that of Fig. 1 except for the structure of the running average generator 52a of the frequency converter 50a. The running average generator 52a includes: a shift register 53a provided with (2n + 2) stages (first through (2n + 2) th stages to hold respective bits), where n is a positive integer and the bits are sequentially shifted from the first toward the (2n + 2) th bit in synchronism with the clock of the shift register 53a: and an adder 54a for adding the first bit of the shift register 43a and the outputs of a sign inverter 56 and a D flip-flop 57. The sign inverter 56 inverts the polarity of the (2n + 2) th bit of the shift register 53a and supplies the result to the adder 54a. The D flip-flop 57 coupled to the output of the adder 54a serves as a delay element for storing the output of the adder 54a. The output of the D flip-flop 57 is supplied to the adder 54a. Next, the operation of the circuit of Fig. 7 is described. As in the case of the circuit of Fig. 1, the limiter amplifier 10 quantizes the received signal into a 2-level quantized signal taking either the logical "0" or "1". The 2-level quantized received signal output from the limiter amplifier 10 is supplied to the frequency converter 50a, in which the exclusive OR element 51 effects logical exclusive OR operation upon the output of the limiter amplifier 10 (the 2-level quantized received signal) and the signal for frequency conversion (the frequency conversion signal) which also takes either the logical value "0" or "1". As in the case of the circuit of Fig. 1, the exclusive OR element 51 acts as a multiplier for multiplying the output of the limiter amplifier 10 (the 2-level quantized received signal) by the signal for frequency conversion. The output of the exclusive OR element 51 is supplied to the first stage of the shift register 53a, from whence it is shifted toward the (2n + 2) th stage in synchronism with the clock of the shift register 53a. The frequency of the clock of the shift register 53a is substantially greater than the frequencies of the 2-level quantized received signal and the signal for frequency conversion. The first bit of the shift register 53a is input to the adder 54a. On the other hand, the (2n + 2) th bit of the shift register 53a is input to the sign inverter 56, where the sign or polarity of input signal is inverted and then supplied to the adder 54a. The output of the D flip-flop 57 is also supplied to the adder 54a. Thus, the adder 54a adds the first bit of the shift register 53a, the output of the sign invertor 56, and the output of the D flip-flop 57, and outputs the result to the D flip-flop 57. The D flip-flop 57 acts as the delay element for storing the output of the adder 54a. The clock of the D flip-flop 57 be the same as that of the shift register 53a. Let the output of the D flip-flop 57 and the respective bits of the shift register 53a be at logical "0" at the initial state. Let the period of the clock of the shift register 53a and the D flip-flop 57 be represented by T c . Further, let the output of the exclusive OR element 51 at the time t = i · T c· , where i is an integer, be represented by ( a 0i ∈ {0, 1}). Furthermore, let the first and the (2n + 2) th bits of the shift register 53a at the time t = i · T c· be represented by p i and q i (p i ∈ {0 , 1}). Then, taking into consideration that all the bits of the shift register 53a are at logical "0" at the initial state (i.e., at the time t = 0), the following relationships hold, depending upon the value of i: p i = 0 (i = 0) a 0(i-1) (1 ≦ i) q i = 0 (0 ≦ i ≦ 2n+1) a 0(i-2n-2) (2n + 2 ≦ i) As described above the sign inverter 56 inverts the polarity of the (2n + 2) th bit output from the shift register 53a. Thus, if the output of the sign inverter 56 at the time t = i · T c is represented by r i ∈ {-1, 0}), then r i is given, depending on the value of i, by; r i = - q i = 0 (0 ≦ i ≦ 2n + 1) - a 0(i-2n-2) (2n + 2 ≦ i) Further, the output s i of the D flip-flop 57 at the time t = i · T c is represented by: s i = p i + r i + s i-1 The output s 0 of the D flip-flop 57 at the initial state (i.e., at the time is equal to 0 (s 0 = 0). Further, the output r i of the sign inverter 56 is also equal to 0 (r i = 0) during the time t ≤ (2n + 1) T c . The output s i of the D flip-flop 57 for 1 ≤ i ≤ 2n + 1 is thus expressed as: Next, the above equation is proved for arbitrary 1 ≤ i ≤ 2n + 1 by mathematical induction. First, for i = 1, the equation holds since: s 1 = p 1 + r 1 + s 0 = p 1 = a 0 0 Next, assume that the equation is true for i = j . Then the equation is satisfied for i = j + 1 because: Thus, it has been proved that the equation holds for all integers i in the range: 1 ≤ i ≤ 2n + 1 (QED). Thus, the output s 2n+1 of the D flip-flop 57 at the time t = (2n+1) T c . is given by: Namely, the value s 2n+1 is equal to (2n + 1) times the average of the preceding (2n + 1) output values: a00, a01, ···, a0(2n) , of the exclusive OR element 51. From this it can be shown that the following relation holds for t ≥ (2n +1) T c · : Next, the above equation is proved by mathematical induction. First, for i = 2n + 1 the equation holds since: Next, assume that the equation holds for i = j. Then the equation is satisfied for i = j + 1 since: Thus, the equation has been proved for all integer i not less than (2n + 1): i ≥ (2n + 1). (QED) In summary, it has been shown that the output s i of the D flip-flop 57 is equal to (2n + 1) times the average of preceding (2n + 1) output values, a0(i-2n-1 ), a0(i-2n) , ···, a0(i-1) , of the exclusive OR element 51. This output s i of the D flip-flop 57 constitutes the output of the running average generator 52a. Thus, after the time t = (2n +1) T c· , the running average generator 52a functions similarly to the running average generator 52 of Fig. 1. By the way, the number of the signals input to the adder 54a is three, irrespective of the number of the stages of the shift register 53a. In the case of the circuit of Fig. 1, the number of signals input to the adder 54 is equal to the number of stages, (2n + 1), of the shift register 53. Since n is greater than one (n ≥ 1) and hence (2n + 1) ≥ 3, the number of signals input to the adder 54a is not greater than (and generally substantially less than) the number of signals input to the adder 54 in the circuit of Fig. 1. Thus, compared to the embodiment of Fig. 1, the circuit of the embodiment of Fig. 7 is simplified. The output of the running average generator 52a is supplied to the comparator 55. The comparator 55 compares the output of the running average generator 52a with the constant n . Depending on the value s i of the output of the running average generator 52a and the constand n , the value d i of the output signal of the comparator 55 is given as follows: d i = 0 ( s i ≦ n ) 1 ( s i > n ) Namely, the comparator 55 acts as a hard devision means for converting the output s i of the running average generator 52a into a 2-level signal which takes either the logical value "0" or "1". Thus, the signal processing within the frequency converter 50a subsequent to the running average generator 52a is identical to that subsequent to the running average generator 52 in Fig. 1. Further, the running average generator 52a acts in a similar manner as the running average generator 52 of Fig. 1. Thus, as in the case of the embodiment of Fig. 1, the running average generator 52a, consisting of the shift register 53a, the adder 54a, the sign inverter 56, and the D flip-flop 57, and the comparator 55 acting as the hard devision means for converting the output of the running average generator 52a into a 2-level logical signal, function as a low pass filter for removing the high frequency components from the output of the exclusive OR element 51. Thus, as in the case of the embodiment of Fig. 1, the 2-level quantized received signal output from the limiter amplifier 10 is subjected to the frequency conversion by means of the signal processing within the frequency converter 50a. Namely, if the frequency of the 2-level quantized received signal is represented by f 1 Hz and that of the signal for frequency conversion by f 2 Hz, then the frequency of the received signal after frequency conversion output from the comparator 55 is f 1 - f 1 Hz. The received signal after frequency conversion output from the frequency converter 50a is supplied to the phase comparator 60, which is the same as in Fig. 1. Thus, the phase comparator 60 outputs the relative phase signal representing the phase shift of the received signal after frequency conversion relative to the phase reference signal. The relative phase signal output from the phase comparator 60 is delayed by the delay element 40 by one symbol period of the received signal. At the same time, the relative phase signal is input to the subtractor 41, to which the relative phase signal delayed by one symbol period by the delay element 40 is also input. In response to the outputs of the phase comparator 60 and the delay element 40, the subtractor 41 outputs the phase difference signal which is obtained by subtracting in modulo 2 π the relative phase signal delayed by one symbol period from the relative phase signal output from the phase comparator 60. The phase difference signal output from the subtractor 41 represents the phase transition over one symbol period of the received signal. The devision circuit 42 obtains the demodulated data corresponding to the value of the phase difference signal, on the basis of the predetermined correspondence relationship between the phase difference signal and the demodulated data. The above description of circuit of Fig. 7 relates to the case where the received signal is modulated in accordance with the differential phase shift keying (DPSK). The principle of the invention can also be applied to MSK or GMSK modulation systems. Further, in the case of the above embodiment of Fig. 7, the constant n serving as the parameter of the frequency converter 50a is equal to 2 (n = 2) and hence the shift register 53 has six stages to hold the respective bits. However, the constand n may be any positive integer for example, it may be that n = 6 (namely the shift register 53a may have 14 bits) or n = 7 (namely, the shift register 53a may have 16 bits).

Claims: A frequency converter circuit (50, 50a) for converting a frequency of a first 2-level signal using a second 2-level signal having a frequency distinct from said frequency of said first signal, characterized by an exclusive OR element for obtaining a logical exclusive OR of said first and second signal; running average generator means (52, 52a), coupled to said exclusive OR element (51), for generating a signal corresponding to k times a running average of an output of said exclusive OR element (51), k being a positive integer; and hard decision means (55), coupled to said running average generator means (52, 52a) for converting an output of said running average generator means (52, 52a) to a 2-level logical signal. A frequency converter circuit as claimed in claim 1, wherein said running average generator means (52) comprises: a shift register (53) coupled to said exclusive OR element (51) and having (2n+1) stages to hold respective bits, where n is a positive integer and said output of said exclusive OR element (51) is first supplied to a first stage of said shift register (53), said shift register (53) shifting said bits held in said stages from said first toward (2n+1)th stage in synchronism with a clock having a period substantially shorter than periods of said first and second signals; and an adder means (54) coupled to said shift register (53), for adding bits of said respective stages of said shift register (53), wherein an output of said adder (54) constituting said output of said running average generator means (52). A frequency converter circuit as claimed in claim 1, wherein said running average generator means (52a) comprises: a shift register (53a) coupled to said exclusive OR element (51) and having (2n+2) stages to hold respective bits, where n is a positive integer and said output of said exclusive OR element (51) is first supplied to a first stage of said shift register (53a), said shift register (53a) shifting said bits held in said stages from said first toward (2n+2)th stage in synchronism with a clock having a period shorter than periods of said first and second signals; a sign invertor (56) coupled to said shift register (53a), for inverting a polarity of an output bit of said (2n+2)th stage; an adder (54a) coupled to said first stage of said shift registesr (53a) and said sign invertor (56); and a delay element (57) having an input coupled to an output of said adder (54a) and having an output coupled to an input of said adder (54a), said delay element (57) delaying said output of said adder (54a) in synchronism with said clock of said shift register (53a); wherein said adder (54a) adds outputs of: said first stage of said shift register (53a); said sign invertor (56); and said delay element (57), said output of said delay element (57) constituting said output of said running average generator means (52a). Frequency converter circuit as claimed in claim 1, wherein said hard decision means (55) compares said output of said running average generator means (52, 52a) with a predetermined threshold level to convert said output of said running average generator means (52, 52a) to said 2-level logical signal.

Inventors: KOJIMA TOSHIHARU, KOJIMA, TOSHIHARU, Kojima, Toshiharu, c/o Mitsubishi Denki K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H04L27
IPC codes: H03D7, H04L27, H03D13