JPH048679Y2 - - Google Patents

Description

[Detailed description of the invention] The invention uses an MPX circuit of an FM tuner that separates an FM detected stereo composite signal into L and R channels.
The present invention relates to a stereo demodulator that demodulates a sub-signal (LR) that is a DSB signal.

Generally, an FM receiver, as shown in Figure 1,
Front end section 1 and intermediate frequency band filter 2
, an intermediate frequency amplifier 3, an FM detector 4,
It consists of MPX circuit 5. The front end section 1 is
The bandpass filter 2 functions to replace the desired signal in the receiving frequency range with a certain intermediate frequency signal, and the bandpass filter 2 functions to eliminate interference caused by adjacent frequency signals.
The FM detector 4 works to demodulate the FM modulated composite signal, but this FM detector 4
In addition to the composite signal of the desired signal, interference signals that cannot be completely eliminated by the bandpass filter 2 are also demodulated.

The MPX circuit 5 detects the pilot signal (19KHz) included in the composite signal,
Create a subcarrier signal with twice the frequency,
It functions to separate the L and R channel signals included in the composite signal. This MPX circuit has conventionally been configured as shown in FIG. In Fig. 2, 6 is an input terminal into which an FM-detected composite signal is input, 7 is a switching circuit for sub-signal demodulation, 8 is a sub-carrier signal generation circuit, and 9 and 10 are matrix circuits. 8 creates a subcarrier signal consisting of a square wave as shown _{in FIG} . subcarrier signal
By inverting the polarity at the rising and falling edges of _S2 and demodulating it, a demodulated sub-signal as shown in _S3 in Figure 3 is created, and this demodulated sub-signal _S3 and the main signal in the composite signal ( L+R) are taken by matrix circuits 9 and 10, respectively.
Separate into L and R channels and output 1 each
1 and 12.

In this conventional circuit, a square wave signal _S2 is used as the subcarrier signal, and the demodulated subsignal _S3 is obtained by switching the subsignal _S1 in the composite signal using the signal _S2 using a so-called analog switch. Therefore, a, in Figure 3.
The sub-signal S ₃ becomes discontinuous at the points shown in b, c, etc., and the sub-signal S ₃ includes harmonics of the sub-carrier signal S ₂ . Therefore, a beat occurs between the interference signal existing in a frequency range higher than the frequency band occupied by the composite signal output from the FM detector 4 and the harmonics of the subcarrier signal, and this beat falls within the stereo demodulation band. There is a problem in that it is output as an interference signal.

An object of the present invention is to provide a stereo demodulation circuit in which such interference signals are not demodulated.

An embodiment of the present invention will be described below with reference to FIGS. 4 to 6. In Fig. 4, 8 generates a subcarrier signal _S2 consisting of a square wave as shown in Fig. 5 from the subsignal _S1 (see Fig. 5) in the composite signal after FM detection inputted from the input terminal 6. a subcarrier signal generation circuit, 13 is the subcarrier signal;
It is a filter (low-pass filter or bandpass filter) that inputs _S2 and extracts the fundamental wave component (that is, removes high-frequency components), and its output signal _S4 is a sine wave as shown in Figure 5. Become. 14 inputs the composite signal and the fundamental wave component _S4 of the subcarrier signal and outputs the subsignal _S1 and the fundamental wave component.
It is a linear multiplier that multiplies with S ₄ , and both signals
The output level changes linearly in response to changes in the levels of S ₁ and S ₄ , and the output signal S ₅ changes discontinuously as shown in Figure 5, as shown in S 3 in Figure ₃ . This becomes a sinusoidal signal without points a, b, c, . . . .
As in the conventional case, this output signal _S5 , that is, the demodulated sub signal, is matrixed with the main signal (L+R) in the composite signal by matrix circuits 15 and 16, so that it is output to each output terminal 1.
Separate output can be performed for each L and R channel from 1 and 12.

As shown in FIG. 6, the linear multiplier includes a squaring circuit 1 that squares one input (signal S ₁ ).
7, a squaring circuit 18 that squares the other input (signal S ₄ ), an adder 19 that adds the two inputs, and a squaring circuit 20 that squares the addition output (S ₁ +S ₄ ).
and from the output of the squaring circuit 20, the squaring circuit 1
Adder 21 that subtracts the outputs of 7 and 18 to obtain an output S ₅ proportional to S ₁ and S ₄ as shown in equation (1) below.
It is possible to think of something consisting of.

S ₅ = (S ₁ + S ₄ ) ² −S ² ₁ −S ² ₄ =2S ₁ S ₄ ...(1) If you use a linear multiplier like the one shown in Figure 6, each 2
As the multiplication circuits 17, 18, and 20, field effect transistors with a bias voltage V _GS as an input and a drain current _ID that changes with a square characteristic with respect to the bias voltage as an output can be used. The power property can be obtained.

As described above, the stereo demodulator of the present invention inputs the composite signal and the fundamental wave component of the subcarrier signal to a linear multiplier whose output level changes linearly with changes in both input levels. It is characterized by demodulating the sub-signals included in the signal, and since the demodulated sub-signals do not include discontinuous parts, they can be used in a frequency region higher than the frequency band occupied by the FM-detected composite signal. It is possible to purely demodulate only the sub-signal without demodulating the existing interference signal.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a general FM receiver, Fig. 2 is a circuit block diagram of a conventional stereo demodulator, Fig. 3 is a waveform diagram explaining the operation of Fig. 2, and Fig. 4 is a diagram of a conventional stereo demodulator. FIG. 5 is a waveform diagram showing its operation, and FIG. 6 is a circuit diagram showing an example of a linear multiplier used in the circuit of FIG. 4. 8...Subcarrier signal generation circuit, 13...Filter, 14...Linear multiplier, 15, 16...
matrix circuit.

Claims (1)

[Scope of utility model registration request] The FM-detected stereo composite signal and the fundamental frequency component of the subcarrier signal are input to a linear multiplier whose output level changes linearly with changes in both input levels to generate the sub signal contained in the stereo composite signal. A stereo demodulator characterized in that it is configured to demodulate.