JPH0377457A - Demodulator - Google Patents

Abstract

PURPOSE:To decrease quantization noise and to prevent a dynamic range of an A/D converter from being exceeded by using a variable amplifier so as to control a gain in response to the quantity of noise. CONSTITUTION:A QPSK signal is branched into two system of signals at a branching device 1 and they are subject to synchronization detection by a recovered carriers having a phase difference of 90 deg. at a phase detector 2. The signal subject to synchronization detection is filtered by an analog LPF 3, amplified by an amplifier 11, A/D-converted by an A/D converter 5, filtered by a digital filter 6 and inputted to a viterbi decoder 8 through a demodulation arithmetic section 7. The demodulation arithmetic section 7 calculates magnitude of noise from an output data of the digital filter 6 to control a variable amplifier 11 so as to attain optimum gain in response to the quantity of noise. When noise is small, quantization noise is decreased and when noise is loud, the input signal voltage is prevented from being in excess of the dynamic range of the A/D converter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a demodulator used in satellite communications, etc., and particularly to a demodulator with improved noise characteristics.

[Conventional technology]

Fig. 3 is a block diagram of a conventional QPSK demodulator. 3 is an analog LPF that smoothes the output signal of the phase detector 2, 4 is an amplifier that amplifies the output signal of the analog LPF 3, and 5 is an A/D converter for the output of the amplifier 4. 6 is a digital filter that filters the output data of the A/D converter 5, and 7 is an A/D converter that performs carrier regeneration and chroma signal processing using the output data of the digital filter 6.
8 is a Viterbi decoder that Viterbi decodes the output data of the demodulation calculation unit 7; 9 is a voltage control whose oscillation frequency is controlled by the vcxoimt pressure output from the demodulation calculation unit 7; Oscillator (Voltage
Controlled Crystal 0scil
lator (hereinafter referred to as VCXO) 10 is 39 V
This is a 90° demultiplexer that demultiplexes the reproduced carrier output from the CX O9 into two signals having a phase difference of 90'.

Next, the operation will be explained.

The input QPSK signal is divided into Pch and Q by the splitter 1.
The signal is split into two systems called channels, and the phase detector 2
The signals are synchronously detected using reproduced carriers with a phase difference of 90°. These synchronously detected signals are each filtered by an analog LPF 3, amplified by an amplifier 4, and then each A/D converter 5 receives an analog signal.
/D converted. Next, the A/D converted signal is filtered by a digital filter 6, and then passed through a demodulation calculation section 7 and inputted to a Viterbi decoder 8, where it is Viterbi decoded and data is reproduced.

On the other hand, the demodulation calculation unit 7 performs carrier recovery, clock recovery, etc. based on the output data of the digital filter 6, and the phase of CX09 is controlled by the vcxo control voltage output from the demodulation calculation unit 7. The reproduced carrier is supplied to a 90° demultiplexer 10. The 90° demultiplexer 1
0, the reproduced carriers are two reproduced carriers having a phase difference of 90°, and are supplied to the phase detector 2, respectively.

By the way, the gain of the amplifier 4 is the same as that of the A/D converter 5.
It is determined by taking into consideration the dynamic range of and the amount of human noise. That is, if the quantization noise of the A/D converter 5 is to be made as small as possible, the A/D converter 5
The gain of the amplifier 4 may be determined so that the input signal level of the amplifier 4 is the same as its dynamic range. However, if noise is added to the input signal, the input signal level of the A/D converter 5 will exceed its dynamic range due to the noise component. It is necessary to avoid exceeding the dynamic range of the D converter 5.

[Problem to be solved by the invention]

Since the conventional demodulator is configured as described above, it is necessary to keep the input signal level of the A/D converter low even when the noise is small, which causes the problem of increased quantization noise. Ta.

This invention was made to solve the above-mentioned problems.When there is little noise, the quantization noise is made as small as possible, and when there is a lot of noise, the input signal voltage is It is an object of the present invention to obtain a demodulator that can prevent the dynamic range of a converter from being exceeded.

[Means to solve the problem]

A demodulation device according to the present invention uses a variable amplifier whose gain can be changed as an amplifier used in the device.

[Effect]

In this invention, since a variable amplifier is used instead of a conventional fixed amplifier, its gain can be controlled to be optimal according to the amount of noise.As a result, when the noise is small, quantization noise becomes small, and when the noise is large, it is possible to prevent the input signal voltage from exceeding the dynamic range of the A/D converter.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block configuration diagram of a demodulation device according to an embodiment of the present invention. In the figure, the same symbols as in FIG. It is a variable amplifier whose gain changes depending on the control signal.

Next, the operation will be explained.

The QPSK signal, which is the input signal, is converted to Pch by the splitter 1.
, Qch, and are synchronously detected by a phase detector 2 using regenerated carriers with a phase difference of 90''. This synchronously detected signal is filtered by an analog LPF 3, and then sent to an amplifier 4. After being amplified, the signal is A/D converted by an A/D converter 5.

After the A/D converted signal is filtered by a digital filter 6, it is inputted to a Viterbi decoder 8 through a demodulation calculation section 7, and the Viterbi decoded data is reproduced.

Here, the demodulation calculation section 7 calculates the magnitude of noise from the output data of the digital filter 6, outputs an amplifier control signal such that the gain of the variable amplifier 11 decreases as the noise increases, and controls the variable amplifier 11. At the same time,
Conversely, the gain is controlled to increase as the noise decreases.

In this way, in this embodiment, instead of the conventional fixed amplifier,
Since we used a variable amplifier that can change the gain, it is possible to control the gain to be optimal depending on the amount of noise, and as a result, when the noise is small, the quantization noise becomes small, and When the noise is large, it is possible to prevent the input signal voltage from exceeding the dynamic range of the A/D converter.

In the above embodiment, the gain of the variable amplifier 11 is controlled by the amplifier control signal from the demodulation calculation unit 7, but the control of the variable amplifier is not limited to this. In this case, as shown in FIG. 2, the Viterbi decoder 8 estimates the amount of noise by re-encoding the decoded data and comparing it with the pre-decoded data, and adjusts the variable amplifier accordingly. 11 gains are controlled.

Further, in the above embodiment, a variable amplifier is used as a means for adjusting the input signal level of the A/D converter 5, but the means for adjusting the input signal level is not limited to this.
For example, a fixed amplifier and a variable attenuator may be used, and in this case, the same effects as in the above embodiment can be achieved by controlling the amount of attenuation of the variable attenuator.

〔Effect of the invention〕

As described above, according to the demodulator according to the present invention, the conventional fixed amplifier is replaced with a variable amplifier and the gain is controlled according to the amount of noise. The effect is that the input signal level of the A/D converter can be kept from exceeding the dynamic range by making it small, and if the noise is small, the gain of the amplifier can be increased to reduce the quantization noise of the A/D converter. There is.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a demodulator according to an embodiment of the present invention;
FIG. 2 is a block diagram of a demodulator showing another embodiment of the present invention, and FIG. 3 is a block diagram of a conventional demodulator. In the figure, 1 is a branching filter, 2 is a phase detector, 3 is an analog LPF, 4 is an amplifier, 5 is an A/D converter, 6 is a digital filter, 7 is a demodulation calculation section, 8 is a Viterbi decoder, and 9 is a Viterbi decoder. A voltage control generator (■CX0), 10 is a 90@ branching filter, and 11 is a variable amplifier. Note that the same reference numerals in the figures indicate the same or equivalent parts. FNσlObeD

Claims (1)

[Claims] (1) A duplexer that divides the input signal into two, two phase detectors that synchronously detects the output signal of the duplexer, and two analog LPFs that smooth the output signals of the phase detectors, respectively. and two amplifiers that amplify the output signal of the analog LPF; a demodulation calculation unit that performs carrier recovery and clock recovery calculations from the filtered output signal of the amplifier after A/D conversion; and the demodulation calculation unit. a Viterbi decoder that Viterbi decodes the output data of the voltage-controlled oscillator; a voltage-controlled oscillator whose oscillation frequency is controlled by the voltage-controlled oscillator control signal from the demodulation calculation section; and a 90° splitter that divides the signals into two signals and supplies them to the two phase detectors. A demodulator characterized in that it is a variable amplifier that is variable depending on a signal.