System for Detecting Signal Quality of a Phase-Modulated Wave

This invention relates to a system for detecting the signal quality of a digital phase-modulated wave.

In a case of detecting the quality of a digital modulated wave deteriorated by noise and, in particular, interference among signal elements due to its amplitude distortion and its phase distortion, it has heretofore been the practice to employ the principle such that a predetermined, particular signal pattern is transmitted so that a deviation of the transmitted specified signal pattern from a predetermined pattern to be received is measured by means of a meter or a cathode-ray tube. However, since the specified signal pattern is necessary in this method, it is impossible to monitor the signal quality of a communication circuit in the operating condition and low in efficiency.

An object of this invention is to provide a system for detecting the signal quality of a digital phase-modulated wave, which is capable of detecting the signal quality irrespective of whether the sent-out code has a specified pattern or a random pattern and to achieve monitoring the signal quality of a communication circuit in the operating condition.

In accordance with the principle of this invention, the phase positions of a digital phase-modulated input wave are detected by a digital phase detector having the number of output bits composed of higher digits representative of the number of quantum phase positions of the input wave and at least one lower digit so that the lower digit is taken out as signal quality information.

The principle, construction and operations of this invention will be clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which

FIG. 1 is a vector diagram of an eight-phase phase-modulated wave including noise or interference to be measured in accordance with this invention;

FIG. 2 is a diagram showing the phase distribution of the wave shown in FIG. 1;

FIG. 3 is a diagram showing the relationship between the phase positions and binary code units in the case where phase differences are represented by binary numbers;

FIG. 4 is a block diagram illustrating an example of this invention;

FIG. 5 is a block diagram showing an example of a digital phase detector employed in the example shown in FIG. 4;

FIG. 6 is a block diagram illustrating another example of this invention;

FIG. 7 is a block diagram showing an example of a digital phase detector employed in the example shown in FIG. 6; and

FIGS. 8 and 9 are block diagrams illustrating examples of digital indicators employed for indicating measured results of this invention.

With reference to FIGS. 1 and 2, the signal quality of a phase-modulated wave to be measured in accordance with this invention will first be described. In FIG. 1, vectors to be directed at eight quantized phase positions indicated by solid line arrows are distributed due to noise or interference in the hatched ranges. FIG. 2 shows the vector distribution in FIG. 1, with the phase being represented on the abscissa. The vectors are distributed at the vicinity of eight positions each of which is an integer multiple of .pi./4. The signal quality can be commonly evaluated in accordance with the magnitude of the dispersion of this distribution for all the quantized phase positions.

Illustrated in FIG. 3, is an example of the digital detection of the phase difference between a reference wave and a received wave. Let it be assumed that one period (2.pi.) of, for example, one eight-phase phase-modulated carrier wave is represented with 6 binary bits. At the phase position 0 of the reference carrier wave, a scale-of-64 counter is started to count clock pulses of a repetition frequency 64-times higher than that of the carrier wave. It is further assumed that transistion instants of the received wave are delayed by a phase angle .theta. with respect to the reference wave as shown in FIG. 3. At this time, the count contents of the scale-of-64 counter are 001101 and this six-bit code unit indicates the phase angle .theta.. In these 6 bits, the three lower digits (M.sub.2, M.sub.1 and M.sub.o) commonly indicate a deviation from the center of distribution groups having ideal positions represented by the three high bits M.sub.3 - M.sub.5 (the eight ideal positions, each being an integer multiple of .pi./4) as shown in FIG. 2. Namely, a digit M.sub.2 represents the direction of deviation, while digits M.sub.1 and M.sub.o indicate the magnitude of deviations (when M.sub.2 = 1, M.sub.1 and M.sub.o are given in the form of complements).

FIG. 4 illustrates an example of this invention, in which a reference numeral 1 indicates a digital phase detector; 2 designates a carrier wave regenerating circuit; 3 identifies a D-A converter; and 4 represents a low-pass filter. A reference number 100 indicates a received signal. The phase difference between the received signal 100 and a reference carrier wave 102 regenerated by the carrier wave regenerating circuit 2 is detected by the digital phase detector 1. The carrier regeneration is performed by the use of the received wave 100 and, in some cases, by the use of a detector output 103. A reference numeral 101 designates a signal representing only an error of the aforementioned phase difference from the phase of the received wave 100. In the digital indication of the phase position, if a phase range 0 to 2.pi. is divided into parts 2.sup.n to indicate binary information of n channels, the error is indicated by lower digits of the output bits added with at least one lower digit, which is other than higher digits representing the number of quantized modulation phases. The digital signal 101 indicating the error is converted by the D-A converter 3 into an analogue signal and then applied to the low-pass filter 4 to provide a signal 105 reresenting the signal quality of the received wave 100. The signal 105 thus obtained is applied to a volt-meter or a lamp for indicating the signal quality. In this case, if a nonlinear amplifier such as a square-law circuit or the like is inserted after the D-A converter 3 or the low-pass filter 4, the scale of the meter can be modified to provide a nonlinear characteristic suitable for a particular purpose. A reference numeral 106 indicates modulation timing pulses, which are employed for sampling and holding the phase difference.

FIG. 5 shows an example of the construction of the digital phase detector 1 shown in FIG. 4. A reference numeral 11 designates a hard limiter; 12 represents a transition detector; 13 shows a counter; 14 refers to a clock pulse generator; 15 denotes a signal converter; and 16 designates a register. The received wave 100 is reshaped by the hard limiter 11 into a rectangular wave and its transition instants are detected by the transition detector 12. The counter 13 starts at the transition instant of the reference wave 102, that is, at an instant t.sub.1 shown in FIG. 3 to count clock pulses 112 and stops at the transition instant pulse 111 of the received signal at an instant t.sub.2 in FIG. 3. In the count contents of counter 13 the lower digits 113 (i.e. three digits M.sub.o, M.sub.1 and M.sub.2) are applied to the signal converter 15 to rectify positive and negative deviations thereof and then converted into code units commonly representative of only the magnitude of the deviation for each of the quantized phase positions. The code units are applied to the register 16 and sampled-and-held in the register 16 by the application of the modulation timing pulses 106 thereto.

FIG. 6 illustrates another example of this invention, which employs a self oscillator in place of the carrier regenerating circuit. In FIG. 6, reference numerals 1a, 3 and 4 designate a digital phase detector, a D-A converter and a low-pass filter, respectively, as shown in the example of FIG. 4. A reference numeral 2a indicates an oscillator which generates a frequency very close to the frequency of the carrier wave. In this case, an output 101a from the phase detector 1a is not the phase difference between the received wave and the reference carrier wave but the phase difference between two successive elements of the received wave, unlike the example of FIG. 4. This phase difference is also distributed at the vicinity of the integer multiples of .pi./4, as shown in FIG. 2. Accordingly, an output 105, which is obtained by D-A conversion and low-pass filtering of the binary numbers representing the phase difference in the same manner as in the example of FIG. 4, has a voltage value corresponding to the signal quality of the received wave.

FIG. 7 illustrates an example of the construction of the phase detector 1a in the example of FIG. 6. Reference numerals 11, 12, 13, 14 and 15 respectively identify a hard limiter, a transition detector, a counter, a clock pulse generator and a signal converter as shown in the example of FIG. 5. A reference numeral 17 designates a register, and 18 indicates a subtracter. An output 113a of the counter 13 represents the phase difference between the reference carrier wave 102a and the received wave 100 as described with reference to the example of FIG. 5. In this case, however, the reference carrier wave 102a has a phase position independent of the received wave. The phase difference 113a between them is stored in the register 17 to be delayed for a period of time corresponding to one signal element. A signal 113a, which is representative of the phase difference between the next signal element of the received wave and the reference carrier wave, is subtracted in the subtracter 18 from a signal 114 representative of the delayed phase difference. As a result of this, the phase of the reference carrier wave is cancelled so that the result represents the phase difference between successive two signal elements of the received wave. An output 115 is composed of the lower digits of the subtracted result and digitally rectified by the signal converter as described with reference to the example of FIG. 5.

The foregoing description has been given with regard to the case where a phase error represented in a digital form is converted into an analogue signal and then indicated by a meter or another type of indicator. However, it is also possible that the phase error representated in a digital form be indicated in that form.

FIG. 8 shows one example of the digital indication, in which a reference numeral 22 designates a decoder. Upon application of the signal 101 in FIG. 4 or 101a in FIG. 6 to the decoder 22, this decoder 22 produces an output at any one of its outputs 116 corresponding to the phase error code unit. The outputs 116 are connected to indicators such as lamps or the like.

FIG. 9 illustrates another example of digital indication, in which a reference numeral 21 designates an adder; 19 indicates a register; and 20 identifies a divider. These three elements 19, 20 and 21 constitute a digital smoothing circuit. An output 117 of the adder 21 is stored in the register 19 and is thereby delayed. The divider 20 divides an output 118 of the register 19 by an input 119 so that the divided output 120 is added by the adder 21 to the error signal 101 (or 101a). An output 117 of the adder 21 is a smoothed function of the input 101 (101a) and is a code unit equivalent to an analogue output obtained by the application to an analogue CR smoothing circuit, and its smoothing time constant is dependent on the input 119 to the divider 20. The output 117 is applied to a numeral indicator or the like to provide an indication.

As has been described in the foregoing, in the present invention, since a phase signal is represented in a digital form with binary digits and commonly indicated only by the lower digits on a meter, a numeral indicator or the like, it is possible to monitor the signal quality of a phase-modulated wave with a simple device irrespective of any pattern being transmitted. Therefore, the use of the present invention facilitates monitoring of the quality of a communication circuit and adjustment of a manual equalizer therefor.