JPS5811084Y2 - Optimal threshold setting device for polyphase phase demodulator - Google Patents

Description

[Detailed description of the invention] This invention makes it possible to completely grasp and quickly set the optimal threshold for identification, which averages the conditions of each phase of the demodulation board pull-in phase in the so-called multiphase PCM communication system. The present invention relates to an optimal threshold setting device for a polyphase phase demodulator.

In recent PCM communication systems using n-phase PSK differential phase modulation, typified by four-phase phase modulation, an n-phase demodulator is used for demodulation.

For example, the 4-phase demodulator is based on Yamamoto et al.'s "Design and Characteristics of Carrier Synchronization System for High-Speed QPSK," Research and Practical Application Report, Vol. 24, No. 1.
No. 0 (1975), pp. 2347 to 2366 In particular, it is configured as shown in Fig. 1, and the four-phase phase modulated wave is generated from a vCO that is squarely crossed by two phase detectors. Detection is performed using a regenerated carrier wave.

However, it is extremely difficult to perfectly match the characteristics of two phase detectors or to obtain completely orthogonal carrier waves using a phase shifter due to variations in components, temperature changes, stray capacitance in circuit networks, etc. .

Therefore, the phase difference between the four phases cannot be made completely 90°.

On the other hand, as mentioned above, it is known that there are four pull-in phases (reference local) of the recovered carrier wave from the CO.

The aforementioned phase detector detects a four-phase phase modulated wave using these four types of reference locals, so depending on where the reference local is located, even though the signals are of the same phase,
The phase detected waveforms will have some differences.

Generally, the demodulated output uses timing information extracted from the input wave, is reread and shaped, and is then passed through a waveform regenerator (discriminator) for output.

At this time, the input waveform is 1.0 at the appropriate timing point in the regenerator. A threshold must be established to determine which of the two levels of O is at.

The influence of setting this threshold on line quality is that when noise increases on the line, noise is added to the detected output waveform, so even under this noise arrival condition, , has a very important meaning in order to make a 0 level judgment without error.

As mentioned earlier, the problem here is that there is a slight difference in the demodulated waveform depending on the pull-in phase of the reference local signal during demodulation.

In other words, the optimum voltage for the above-mentioned threshold value, which is very important, differs slightly depending on the pull-in phase.

In order to find the optimal threshold value as a whole under this condition, conventionally, as shown in Fig. 1, a curve of threshold bias voltage versus bit error rate P8 is plotted for each n pull-in phase, and the tendency is Central value■.

Determine the relevant bias voltage■.

I often used the method of setting

FIG. 1 is an example of a four-phase PSK differential phase modulation wave.

Although this method is accurate, it requires complicated operations and takes a lot of time to make a decision, is not suitable for mass production in a factory, and is practically impossible for maintenance work during actual line operation.

The purpose of this invention is to eliminate the drawbacks of the conventional method by automating the procedure without essentially changing the measurement principle described above.
Multi-phase PSK differential phase system that allows the operator to immediately set the bias voltage to the optimum point while observing the cathode ray tube oscilloscope by wobbling the threshold bias voltage using a staircase wave or sawtooth wave. An apparatus for setting an optimal threshold value for a modulator/demodulator is provided.

In other words, the conventional work of plotting the corresponding bit error rate on a graph while successively changing the setting value of the variable resistor for setting the threshold bias voltage of the demodulator can now be done using a staircase waveform or sawtooth waveform. Automatically off-ring the threshold voltage (
Wobbling) L, the operator changes the center value of the off-ring with a variable resistor, plots the bit error rate for this off-ring as a waveform on a cathode ray tube oscilloscope, and selects the optimum waveform from these waveforms corresponding to the basic center voltage. Set the bias point.

Also, by using this off-ring drive signal, the input of the demodulator is controlled on and off at a predetermined period to automatically create n different pull-in conditions.

Furthermore, when observing the curve of threshold bias voltage versus bit error rate in a state where the bit error rate is very good, count the number of error pulses, obtain the timing pulse for the counter from the off-ring signal, and use D-A. After conversion and level adjustment, all operating systems can be automatically driven, such as displaying dotted curves using a cathode ray tube oscilloscope.

FIG. 2 shows an embodiment of the optimal threshold setting device for a PSK phase demodulator according to this invention. In this example, when applied to a 4-phase PSK modulated wave, the transmitter board 12 is It is driven to create a 4-phase PSK modulated wave.

This four-phase PSK modulated wave is sent to the receiving board 14 through a variable attenuator 13 for setting a level corresponding to a specified input electric field, with a desired carrier signal-to-noise ratio.

The intermediate frequency received signal from the receiving panel 14 is transferred to four-phase P through an on/off switch 15 for changing the drawing phase.
The signal is supplied to the SK phase demodulation board 16.

Bit errors in the demodulated pulse signal are detected by the error detection circuit 17, and an analog signal voltage corresponding to the number of errors is output.

This is given to the cathode ray tube oscilloscope 18 as a vertical deflection signal.

In this invention, the threshold bias value voltage of the demodulator 16 is changed into, for example, a sawtooth wave.

For this purpose, a sawtooth wave generation circuit 19 is provided, which generates the sawtooth wave voltage shown in FIG. The threshold bias voltage of is changed to a sawtooth waveform.

The sawtooth voltage from circuit 19 is supplied to a switch drive circuit 22, which provides a drive pulse at each rising point of the sawtooth voltage, as shown in FIG. be done.

This instantaneous interruption causes the retraction phase of the demodulator 16 to change.

Specifically, when demodulating a polyphase differential phase modulation wave, a reference local (reference phase) must be created.

As a circuit for reproducing this reference local, in the case of four-phase differential modulation, there is, for example, FIG. 1 of the above-mentioned document.

In this circuit, when there is no input modulated wave, the reference local source VCO oscillates in a free-run state, and if there is an input modulated wave near its center frequency, the output signal of the phase comparator is immediately It becomes a reference local locked to the output signal Vc(t) phase of the Loop Filter.

This output signal Vc(t) is the carrier wave eRM obtained by inversely modulating the input modulated wave with the demodulated output and the output e of the VCO.

Since the phase is compared with (1) and the frequency is limited, the phase of the output of the inverse modulator is 0.3 when the input frequency variable wave is input π 3 . It is uncertain whether the state will be complete, iπ, or π.

Therefore, four different pull-in phases can be taken due to instantaneous interruption of the input modulated wave.

Further, the sawtooth wave voltage is applied to the horizontal axis of the off-scilloscope 18.

Therefore, when the threshold voltage is controlled by one period of the sawtooth wave, a curve indicating the number of bit errors corresponding to each threshold voltage is drawn on the cathode ray tube oscilloscope 18, and at the beginning of each sawtooth wave period. As the pull-in phase of the demodulator 16 changes, a curve representing the bit error with respect to the threshold bias voltage corresponding to this is drawn on the off-scilloscope 18 for each sawtooth wave period.

Furthermore, by changing the center value of the obring of the threshold bias, four error threshold bias characteristic curves are obtained for each center value and corresponding to each of the four phase entrainment states, for example, as shown in FIG. As shown in A, B, and C, four characteristic curves are obtained, each corresponding to the obring center values Vc1, VC2, and VC3.

Therefore, by observing the state of these curves drawn on the off-scilloscope 18, the optimum threshold bias, ie c3 in FIG. 4, can be easily determined.

Note that by changing the period of the sawtooth wave voltage, the range of obstructing the threshold bias can be changed, and by obtaining the pulse that drives the switch 15 from the sawtooth wave voltage, this pulse period can also be changed automatically. Changes in conjunction with.

Instead of a sawtooth voltage, a periodically repeating staircase voltage may be used as shown in FIG. 3C.

In order to change the center value of the obring, the knob of the variable resistor for threshold adjustment provided on the demodulation board 16 may be manually controlled.

In order to set the bias threshold very strictly, it is sufficient to plot the bit error versus control signal characteristics in a state where the bit error rate is very good.

To achieve this, the amount of attenuation of the attenuator 13 should be reduced, and the demodulation board 1 should be
Measurement is performed by increasing the C/N value of the input signal No. 6.

In such a state, for example, a signal subjected to four-phase phase modulation by a pulse signal of 200 Mb/sec will be in a state where it receives an error rate of about LX/10-8, and the error detection circuit 17
Approximately two error pulses can be output per second.

Therefore, it is not possible to repeatedly measure at a relatively high speed with a sawtooth wave as described above.

Therefore, the observation period for controlling the threshold bias is set to be long, for example, each step of the staircase wave shown in FIG. 3C is selected to be 1 second, so that one curve is obtained in 10 seconds.

At each step of this staircase wave, each time one error is detected from the error detection circuit 17, the digital output terminal 17
One pulse from b is counted by the counter 24.

This counter is set by a pulse shown in Figure 3 which is synchronized with the rise of each step of the staircase wave controlling the threshold bias voltage.

Therefore, the number of errors in one step of the threshold bias voltage that changes in a staircase waveform is counted, and the counted value is converted to an analog length by the DA converter circuit 25, and the analog length is converted to an analog length by the level adjuster 26. After setting the level to an appropriate level, switch 2 to switch to the CRT off-scilloscope 18.
7 to the analog output terminal 17 of the error detection circuit 17.
supplied in place of the output of b.

In this way, the optimal threshold can be dynamically set even when the error rate is very good.

In FIG. 2, the circuits specifically provided in this invention are 15, 19, 22, 25, 26, and 27, and the others are conventionally used.

As explained above, the present invention can completely grasp the variation in the optimum threshold value due to the pull-in phase, and then automatically set the overall optimum bias voltage under various conditions.

This allows for complete equipment adjustment, reduced man-hours for equipment adjustment, and application during on-site maintenance.In the past, error characteristics were automatically displayed in this way, especially when the number of phases is large. This is very convenient compared to plotting the data one by one.

[Brief explanation of drawings]

Fig. 1 is an error rate characteristic curve diagram for threshold bias voltage, Fig. 2 is a block diagram showing an example of an optimal threshold setting device according to this invention, and Fig. 3 is a diagram for explaining the operation of this invention device. The waveform diagram and FIG. 4 are error characteristic curves observed with this invented device. 14: Receiving board, 15: Switch, 16: Demodulation board, 17:
Error detection circuit, 18-inch cathode ray tube oscilloscope, 1
9: Control signal generation circuit, 21: Rainbow threshold bias voltage control circuit, 22: Switch drive circuit.

Claims (1)

[Scope of utility model registration request] A circuit that periodically changes the threshold bias of an identification circuit of a polyphase phase demodulator using a control signal, and a circuit that momentarily interrupts the polyphase phase modulation signal to the demodulator every repetition period of the control signal, a circuit that changes the pull-in phase of the demodulator; a circuit that detects errors in the demodulated signal from the demodulator;
An optimal threshold setting device for a polyphase phase demodulator, comprising a display unit that displays the detected error output in response to a change in the control signal.