JPS61222349A - Demodulator - Google Patents

Abstract

PURPOSE:To decrease the scale of a circuit by providing a frequency band shift circuit of a reception signal to the pre-stage of a demodulator to convert a ratio of a sampling frequency to a frequency of a demodulation carrier in the demodulator to a prescribed value. CONSTITUTION:A frequency shift circuit 10 consists of a tone input section giving a prescribed input to an output of an A/D converter 11 and a tone input having a frequency corresponding to a carrier frequency of the output of A/D conversion. A demodulation circuit 12 gives a prescribed carrier frequency to the A/D conversion output or a band-shift signal. Since the signal is subjected to band shift by the frequency band shift circuit 10 and the ratio of the carrier frequency to the sampling frequency is limited to a prescribed value, the arithmetic scale in a roll-off filter 13 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Overview A modem, ie, a v1m device, uses a roll-off filter to shape the waveform of a phase-modulated received signal. This roll-off filter has multiple stages of delay circuits and multipliers, and multiplies and calculates each output via a filter coefficient set according to the waveform. In this roll-off filter, if the ratio between the carrier frequency and the sampling frequency of the received signal is a predetermined value, an output characteristic having 0 points at equal intervals can be obtained. Although it is possible to reduce the circuit scale of
In general, reception in a demodulator includes high group (high frequency) reception and low group (low frequency) reception, and demodulation operations must be performed for each different frequency band, so ultimately the circuit size can be reduced. It was considered impossible. The present invention
By performing frequency band shift processing on the received signal, the frequency is varied, and the same waveform shaping processing can be performed on both high group signals and low group signals, reducing the amount of calculation for the roll-off filter. This will reduce the

(2) Field of Industrial Application The present invention relates to a demodulator in a modem receiving section of a data transmission system.

(3) Conventional technology In modems of conventional data transmission systems.

A roll-off filter (ROF) is used to waveform shape the position modulation data. This includes a plurality of stages of delay circuits connected in series, and the outputs of each stage are input to an adder via filter coefficients set according to the waveform and output. This output characteristic is a low-pass filter configured to have zero points at equal intervals and a cosine characteristic between the zero points. In a data transmission system, this waveform shaping is performed on the transmitting side,
There are cases where the receiving side shares the responsibility, and cases where the receiving side performs 100% of the work.

When demodulating on the receiving side, as shown in Figure 5, an analog-to-digital converter (hereinafter referred to as A) is installed before the automatic equalizer.
/D converter) 1.1111 circuit 2, and an O-ROF filter (hereinafter referred to as ROF) 3 are provided.

In this case, the I11 circuit 2 and the ROF 3 perform demodulation and waveform shaping by digital processing. For this reason, A/D
The converter 1 performs sampling at a sampling frequency that is at least twice the carrier frequency and performs digital conversion.

FIG. 6 is a detailed explanatory diagram of the demodulation circuit 2 and ROF 3 in FIG. 5. A constant carrier frequency ωC is given to the D/D conversion output, the output is divided into a real component and an imaginary component, and each output is rolled. Filter circuits 3A and 3B in off-filter 3
send to As mentioned above, the filter circuits 3A and 3B are
Delay circuits 4-1 to 4-n each having a plurality of cosine characteristics, and their respective outputs are multiplied by filter coefficients R1 to Rn according to the characteristics in multipliers 5-1 to 5-n. The added values are output as the ROF real component and imaginary component.

(4) Problems to be solved by the invention In such a demodulator, if the modem receiving section is implemented using digital signal processing, the values handled will be discrete values, so this can be used to solve the problem in the demodulation circuit. This allows the scale of the ROF circuit to be reduced. However, such miniaturization of the circuit scale is only possible when the ratio of the sampling frequency to the frequency of the demodulating carrier is 4=1, and when this relationship does not hold (for example, when the above ratio is 8=1) ), it was not possible to downsize the demodulation circuit and the ROF circuit by using the above method.

The present invention has been made by focusing on such conventional problems, and its purpose is to satisfy the condition that the ratio between the sampling frequency and the frequency of the demodulating carrier is constant when performing II modulation operation by a demodulating circuit. It is an object of the present invention to provide a demodulation method that can reduce the circuit scale of a demodulation circuit and an ROF even if the following is not satisfied.

(5) Means for Solving the Problems In order to achieve the object described in F, the present invention provides a method for waveform-shaping the phase-modulated received signal by passing it through a roll-off filter as shown in FIG. In a data transmission system, a frequency band shift circuit for the received signal is provided before the demodulator, and the sampling frequency of a3 and 11! are set in the demodulator. The gist of this is that the frequency ratio of the JJ carrier is converted to a constant value.

(6) Operation After the phase-modulated received signal is converted from analog to digital, the frequency band is shifted as necessary.

After this frequency band shift has been performed, the frequency of the carrier component of the signal has a constant ratio to the sampling frequency, that is, 1:4. Therefore, since zero sampling is performed twice during one cycle of the carrier frequency, there is no need to perform arithmetic processing on this zero sampling value, and the configuration of the roll-off filter is simplified accordingly.

(7) Embodiment FIG. 2 shows a frequency band shift circuit 10, a demodulation circuit 12, and a demodulation device according to an embodiment of the present invention shown in FIG.
FIG. 2 is a detailed explanatory diagram of the ROF 13. In this embodiment, the frequency band shift circuit 10 is constituted by a tone input section that provides a predetermined input to the A/D conversion output.
A tone input at a frequency corresponding to the carrier frequency of the D-converted output is performed.

As explained based on FIGS. 4 and 5, the demodulation circuit 12 applies a constant carrier frequency ωC to the A/D conversion output or the band-shifted signal, and divides the received signal into a real component and an imaginary component. Divide into In this embodiment, the received signal is band-shifted by the frequency band shift circuit 10, and the ratio between the carrier frequency and the sampling frequency is regulated to a predetermined value, so that the scale of calculation in the ROF 13 can be reduced. Therefore, ROF13 has about half the delay circuit 14-1, 14-2 compared to the conventional ROF3.
,...9 multipliers 15-1.15-2. . . . has the circuit configuration of the calculation section 16.

The demodulating operation during reception by the demodulating device having such a configuration will be explained. Let us consider the case where this reception demodulation is performed by a V22bis modem.

The received signal is high group 2400) 1z, low group 1200) 1
Received at z. The sampling frequency for this received signal is 9600)1z. First, in order to demodulate a high group signal, the frequency ratio of the sampling frequency and demodulation carrier is 4:1 from the above frequency relationship, and as shown in Fig. 3 (b).
), the sampling point is 131°B2. ...
Since half of them are zero (0) sampling, arithmetic processing can be performed by RO[13, which is reduced in size to half of the above.

On the other hand, let's look at the case of performing low group reception on the 1200H2. In this case, if demodulation is performed in the same manner as the above-mentioned high group reception, the frequency ratio of the sampling frequency and demodulation carrier will be 8:1, and the received signal will be 8:1 in one period as shown in Fig. 3 It will be sampled at points. And since the zero sampling is not half of the total sampling, the performance ww4 model becomes large, and the above R
Arithmetic processing becomes difficult in OF13.

Therefore, the frequency band shift circuit 10 shifts the frequency band of the received signal. In this example, in the frequency band shift circuit 10, the received signal (1200
H2) is given a 1200 Hz tone input. As a result, the received signal becomes as shown in Fig. 4(a).
The one whose center frequency was initially 1200H2 is shown in the same figure (
As shown in b), there are two signals with center frequencies of OH2 and 2400 Hz.

Then, if the signal component with the center frequency OH2 is eliminated by performing certain processing on this signal, the center frequency will be shifted to 2400 Hz as a result. Since the signal of this 2400 LIZ is the same as the received signal in the case of the high group receiver described above, the same processing operation as the above high group receiver can be performed, and the calculation processing is performed by the ROF 13 which is reduced in size to half. .

In this way, the demodulation operation of the received signal can be performed by a small-scale ROF regardless of whether it is a high group reception or a low group reception. Note that in the present invention, in order to shift the received signal, it is necessary to provide an extra frequency band shift circuit.
The overall configuration of this valve device seems to be large-scale, but while a simple configuration can be used for this frequency band shift circuit, in general, the ROF has a conventional configuration of about 80 stages, but only half of the conventional configuration. Since it is possible to reduce the scale to a configuration of about 40 stages, the latter has a greater advantage in terms of reducing the scale of the entire device.

(8) As described in detail, according to the present invention, a frequency band shift circuit is provided before the demodulation circuit of the demodulation device, and the frequency band of the received signal is shifted as necessary for demodulation processing. , it becomes possible to adjust the ratio between the sampling frequency and the carrier frequency to a constant value, thereby making it possible to reduce the scale of calculation in the demodulator.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of a demodulation device of the present invention, FIG. 2 is a diagram showing the configuration of a demodulation device according to an embodiment of the present invention, and FIGS. 3(a) and (b) are the above-mentioned 11WA device. FIG. 4 is a diagram showing the operation of the frequency band shift circuit used in the demodulator, FIG. 5 is a diagram showing the basic configuration of the conventional demodulator, and FIG. The figure shows the internal structure of the conventional demodulator. 1.11...Analog-to-digital converter 2.12...
・Return 3.13... Roll-off filter 4.14... Delay circuit 5, 15... Multiplier 6
．． 16... Arithmetic unit 10... Frequency band shift circuit patent applicant Fujitsu Ltd. 82 Mr. BnobuO No. 35m Figure 4

Claims (1)

[Claims] In a data transmission system in which a phase-modulated received signal is input to a demodulation circuit, and the demodulated output is passed through a roll-off filter to shape the waveform and input to an equalizer, a frequency band shift circuit is provided before the demodulation circuit, and the reception A demodulation device characterized in that a ratio between a sampling frequency and a carrier frequency for demodulation becomes a constant value by performing predetermined multiplication processing on a signal.