JPS6252497B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM stereo receiver equipped with a distortion removing means for removing distortion from a stereo demodulated output.

(Prior Art) Distortion of the FM demodulated output signal in a conventional FM stereo receiver occurs due to the frequency-to-amplitude characteristics and frequency-to-phase characteristics of the bandpass filter in the intermediate frequency stage.

Cosine wave 1/2cos pt (0<p≦2π×15kHz), -1
/2 cos pt is stereo-modulated as a left channel signal and a right channel signal, respectively, and the modulation signal S _M is S _M = cospt cos ω _c t = 1/2 cos (ω _c +p) t + 1/2 cos (ω _c − p )t. In the above equation, ω _c is the angular frequency of the subcarrier signal, and the pilot signal is omitted because the problem is harmonic distortion of cospt.

On the other hand, when the broadcast of the modulated signal S _M is received by an FM tuner, the FM demodulated signal S _D is calculated as follows: S _D = 1/2cos, considering only the largest distortion component caused by the influence of the bandpass filter in the intermediate frequency stage. It can be expressed as (ω _c +p)t +1/2cos(ω _c −p)t +kcos(ω _c +3p)t +kcos(ω _c −3p)t. Here, k is a constant determined by the characteristics exhibited by the bandpass filter of the intermediate frequency stage.

When the FM demodulated signal is multiplied by the subcarrier signal cosω _c t in the stereo demodulation circuit for stereo demodulation, the third term in the formula of the FM demodulated signal S _D is
The ω _c +3p and ω _c −3p components of the fourth term become 3p components, and there is a drawback that third harmonic distortion of the fundamental wave occurs in the stereo demodulated output signal.

(Object of the Invention) The present invention has been made in view of the above, and an object of the present invention is to provide an FM stereo receiver that removes distortion in a stereo demodulated signal caused by a bandpass filter in an intermediate frequency stage.

The present invention will be explained below with reference to Examples.

(Structure of the Invention) FIG. 1 is a block diagram showing the structure of an embodiment of the invention.

1 is an FM tuner, which supplies the output signal from the FM tuner 1, that is, the FM demodulated signal S _D output from the FM demodulation circuit, to a sub-signal double-sided signal extraction circuit 2 and a subtraction circuit 3, which is a coupling circuit. , the double sideband signal extraction circuit 2 extracts a double sideband signal based on the sub signal from the FM demodulated signal _SD . The double sideband signal extraction circuit 2 can be configured by, for example, a bandpass filter.

The double sideband signal S ₁ extracted by the double sideband signal extraction circuit 2 is supplied to the cube circuit 4 and raised to the third power, and the output signal S ₂ of the cube circuit 3 is supplied to the level adjustment circuit 5 and leveled. adjust. The output signal _S3 of the level adjustment circuit 5 is supplied to the subtraction circuit 3.
The output signal S ₃ of the level adjustment circuit 5 is subtracted from the FM demodulated signal S _D. The output signal _S4 of the subtraction circuit 3 is supplied to a stereo demodulation circuit 6 for stereo demodulation.
Note that 7 indicates an antenna.

Here, the cube circuit 4 includes, for example, as shown in FIG. 2, a multiplier circuit 3-1 that squares the input signal and a multiplier circuit 3-2 that multiplies the output signal of the multiplier circuit 3-1 and the input signal. It can be configured accordingly.

(Operation of the invention) The operation of the embodiment of the present invention constructed as described above is as follows.

The modulated signal S _M is as shown in the above equation, and the output signal S 1 of the double sideband signal extraction circuit 2 when the output signal of the FM tuner 1, that is, the FM demodulated signal S _D is supplied, is _{S 1 =} 1/2cos( _ωc +p)t+1/2cos( _ωc -p)t+kcos( _ωc +3p)t+kcos( _ωc- 3p)t. Here, the output signal _S1 of the double sideband signal extraction circuit 2 and the FM demodulated signal _SD are equal. For this reason, the double sideband signal extraction circuit 2 seems unnecessary, but in the above case, the main (L+R) signal component is a modulated signal that does not exist, and in reality, the main signal component also exists. is common. Therefore, a double sideband signal extraction circuit 2 is required to prevent the main (L+R) signal component from being present in the output signal _S1 .

On the other hand, in the present invention, since the problem is a range in which the distortion level is extremely small with respect to the fundamental wave signal level, it can be considered that the constant |k|<<1.

Therefore, double sideband signal extraction circuit 2
Ignoring the _{third and} fourth terms in _the output signal S _{1 of} )t] ³ = 1/8cos ³ (ω _c +p) t + 3/8cos ² (ω _c +p) tcos (ω _c - p) t + 3/8cos (ω _c +p) tcos ² (ω _c - p) t +1 /8cos ³ (ω _c -p)t =9/32 [cos (ω _c +p) +cos (ω _c -p)t] +1/32 [cos (3ω _c +3p)t +cos (3ω _c -3p)t] +1/4 [cos (3ω _c +p)t +cos (3ω _c -p)t] +1/4 [cos (ω _c +3p)t + cos (ω _c -3p)t].

In the above output signal S ₂ , (3ω _c +p), (3ω _c
−p), (3ω _c +3p), (3ω _c −3p), (ω _c +p)
_{Since the components of _} It becomes t.

Here, if the input/output level ratio of the level adjustment circuit 5 is l=(=4k), the output signal _S3 of the level adjustment circuit 5 is _S3 = _lS2 =l/4cos( _ωc +3p)t+l/4cos (ω _c −3p)t.

Therefore, the output signal S ₄ of the subtraction circuit 3 is S ₄ =S _D −S ₃ =1/2cos(ω _c +p)t +1/2cos(ω _c −p)t +(k−l/4)cos( ω _c +3p)t + (k−l/4)cos(ω _c −3p)t.

Here, since l is adjusted to 4k, the (ω _c +3p) and (ω _c −3p) components in the output signal S ₄ of the subtraction circuit 3 are zero, and the output signal S ₄ of the subtraction circuit 3 is stereo When the signal is supplied to the demodulation circuit 6 and stereo demodulated by multiplication by the subcarrier cosω _c t in the stereo demodulation circuit 6, the third harmonic distortion component no longer exists in the stereo demodulated signal.

Furthermore, when the constant k in the FM demodulated signal S _D takes a negative value due to variations in the frequency vs. vibration sub-characteristics and frequency vs. phase characteristics of the bandpass filter in the intermediate frequency stage, the level ratio of the level adjustment circuit 5 The same effect as above can be obtained by setting l to (-l) or by using the subtraction circuit 3 as an addition circuit.

Furthermore, instead of the configuration of the subtraction circuit 3 and level adjustment circuit 5 shown in FIG. 1, they may be configured as shown in FIG. That is, the level adjustment circuit 51 has a gain of 1.
, a non-inverting amplifier 8 with a gain of 1, an inverting amplifier 9 with a gain of 1, and a variable resistor 10 that combines the output of the non-inverting amplifier 8 and the output of the inverting amplifier 9, and the subtracting circuit 3 as a coupling circuit is used as an adding circuit 31. . By doing so, the variable resistor 10 can be set to correspond to both positive and negative cases of the constant k. Note that since the non-inverting amplifier 8 simply functions as a buffer, it may be omitted in some cases.

Further, the double sideband signal extraction circuit 2 is not limited to the configuration of the embodiment shown in FIG. The same effect can be obtained even if the signal is modulated and supplied to the double sideband signal extraction circuit 2.

(Effects of the Invention) As explained above, according to the present invention, sub(L-
It is possible to eliminate third harmonic distortion when stereo demodulating an FM stereo signal modulated with the R) signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a block diagram showing an example of a cube circuit. FIG. 3 is a block diagram showing an example of a level adjustment circuit in an embodiment of the present invention. 1...FM tuner, 2...double sideband signal extraction circuit, 3...subtraction circuit, 4...cubic circuit, 5 and 51...level adjustment circuit, 6...stereo demodulation circuit, 31...addition circuit.

Claims (1)

[Claims] 1. A cube circuit that cubes a double sideband signal based on a sub-signal in a received signal, a level adjustment circuit that adjusts the output signal level of the cube circuit, and an output of the cube circuit that is level-adjusted with the received signal. 1. An FM stereo receiver comprising: a coupling circuit that adds or subtracts signals from the signal and supplies the output signal to a stereo demodulation circuit.