JPS61141228A - Fm demodulator - Google Patents

Abstract

PURPOSE:To apply proper demodulation without any variance in the center frequency by sampling and holding only the mean DC level of an output data period of an FM demodulator and using the result as an AFC voltage to recover the DC component of the FM demodulation output. CONSTITUTION:A detection output 12 of the FM detection circuit 11 is led to a data identification circuit 20 and a switch circuit 21, and an output reproducing data 22 of the circuit 20 is fed to a timing signal generating circuit 23. The circuit 23 generates a timing pulse 24 corresponding to the data period to turn on the circuit 21 for the data period only, a detection output 12 is led to an LPF13 and the mean DC level 25 of the data period is held. The level 25 is fed back negatively to a tuner circuit 2 as an AFC voltage 7 via a differential amplifier 15. Further, the level 25 is used also as a reference bias of the circuit 20, the threshold value for data identification is optimized and the reproduction of accurate data is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an FM demodulator for demodulating a modulated signal in which audio data, video signals, etc. are transmitted in a time-division manner.

[Technical background of the invention and its problems]

Time division transmission is one of the most popular signal transmission formats applied to satellite broadcasting. Figure 3 shows a type of time-division transmission, in which audio data,
The FM modulated Y signal and (C-Y) signal are configured in a time division manner. The audio data at this time is baseband digital data.

FIG. 4 shows the configuration of a conventional FM demodulator. A modulated signal 1 is supplied to a tuner circuit 2, and a modulated signal 3 corresponding to an arbitrary channel is selectively output.

This tuning is carried out by the tuning signal 8 obtained by superimposing both the tuning voltage 5 output from the tuning voltage generation circuit 4 and the AFC voltage 7 output from the AFC circuit 6 in the AFC superimposition circuit 17, and is generated within the tuner circuit 2. This is done by making the oscillation frequency of the local oscillator 9 variable. The selected modulated signal 3 is gain-adjusted by the AGC amplifier 10, and then F
The signal is guided to the M detection circuit 11. A detection output 12 demodulated by the FM detection circuit 11 is guided to the AFC circuit 6.

The AFC circuit 6 uses the detected output pressure as a low-frequency F wave generator (L P F
) 13 to obtain an average voltage 14 of the video component, and this voltage 14 is supplied to a differential amplifier 15.

A differential amplifier 15 compares a predetermined reference bias 16 and the average voltage 14, and outputs the AFC voltage 7 according to the difference. As described above, this AFC voltage 7 is superimposed with the tuning voltage 5 in the KAFC superimposition circuit 17, and is negatively fed back to the local oscillator 9 in the tuner circuit 2. As a result, the center frequency of the modulated signal output from the tuner circuit 2 is stabilized, and proper demodulation is performed.

However, when trying to demodulate the time-division modulated signal shown in Figure 3 using the conventional FM demodulator shown in Figure 4, the center frequency varies depending on the video signal, so There was a problem in that the components also fluctuated, making it impossible to properly demodulate audio data.

[Purpose of the invention]

The present invention has been made in view of the above problems, and provides an FM demodulator that eliminates fluctuations in the center frequency and can properly demodulate audio data, like demodulating a time-division modulated signal. The purpose is to

[Summary of the invention]

The FM demodulator of the present invention can sample and hold only the average DC level of the data period of the output of the FM demodulator and use that voltage as an AFC voltage, and can reproduce the DC component of the FM demodulated output. Therefore, proper demodulation operation can be performed without fluctuation of the center frequency. [Embodiment of the Invention] An embodiment of the FM demodulation circuit of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an FM demodulator according to an embodiment of the present invention. The same parts as those of the FM demodulator shown in FIG. 4 are given the same numbers, and their explanation will be omitted. Detection output 1 output from FM detection circuit 1
The signal is guided to a two-bit data identification circuit (data slicer) 20 and a switch circuit 21. The reproduced data n outputted from the data slicer is supplied to a timing signal generation circuit B. The timing generation circuit 3 generates a timing pulse corresponding to the data period, thereby controlling the opening and closing of the switch circuit 21. Whether or not the detection output 12 is supplied to the low-pass P wave generator 13 is controlled. FIG. 2 shows a timing chart of the timing pulses, in which the switch 21 is ON only during the data period, and the detection output 12 is
is guided to the low-pass P-wave device 13, and the low-pass F-wave device 13 is charged with the average DC level 5 during the data period. The switch 21 is OFF except for the above data period, and the detection output is 1.
2 is not supplied to the low frequency door transducer 13. During this period, the discharge path of the low-pass p-wave generator 13 is quickly cut off, and the average DC level of the data period described above is maintained. The average DC level δ thus obtained is negatively fed back to the tuner circuit 2 as an AFC voltage 7 via the differential amplifier 15. The average DC level 5 is also used as a reference bias for the data slicer (a), thereby optimizing the threshold for data identification and reducing the error rate in data identification.

〔Effect of the invention〕

As described above, according to the FM demodulator of the present invention, by reproducing the DC level of the data period, the center frequency of the data can be held constant, and the average level of the video signal (appropriate audio demodulation independent of That is, it is possible to prevent deterioration of the data eye opening ratio due to changes in the video signal, and accurate data reproduction can be performed.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an FM demodulator according to an embodiment of the present invention;
FIG. 2 is a time chart of timing pulses, FIG. 3 is a schematic diagram of a time-division modulation signal, and FIG. 4 is a configuration diagram of a conventional FM demodulator. l...Modulation signal, 2...Tuner circuit, 4...
・Chaining voltage generation circuit, 5... Chaining voltage, 6... AFC circuit, 7.
...AFC voltage, 8...Tuning signal, 10...AGC
Amplifier, 11...FM detection circuit, 12...Detection output, 13...Low frequency door filter. l5...Differential amplifier, 16...Reference bias, 2
1... Switch circuit, B... Timing signal generation circuit. Agent: Patent attorney Noriyuki Chika (and 1 other person) Figure 1 IIZ

Claims (1)

[Claims] A modulation signal in which data including audio information and an FM-modulated video signal are time-divisionally transmitted is input, and a desired modulation in the modulation signal is performed using a tuning signal obtained by superimposing a tuning voltage and an AFC voltage. A tuner circuit that selects a signal, an AGC amplifier that adjusts the gain of the selected modulated signal, an FM detection circuit that demodulates the gain-adjusted modulated signal, a low-pass filter, and this low-pass filter. and a differential amplifier that compares the output of the FM detector with a predetermined reference bias and outputs the AFC voltage. An FM demodulator characterized in that only the demodulated output is supplied to a low-pass filter of the AFC circuit.