JPH05252125A - Am stereo receiver - Google Patents

Abstract

PURPOSE:To automatically eliminate a noise appearing on sub signal detection output due to the fluctuation of a local oscillation signal and to improve the S/N of the detection output by providing specific multiplier and feedback circuit. CONSTITUTION:The low frequency component of a tuning bias (T.B.) outputted from a PLL 41 is eliminated by passing a high-pass filter (HPF) 45. Therefore, the output of the HPF 45 goes to the AC component of the T.B., i.e., only a noise component, and it is inputted to a gain varying means 10 and a multiplier 11. The noise component Vnt of the T.B. is multiplied by G times by the gain varying means for gain G, and it goes to GVn, then, it is inputted to a comparison part 47. Vn' in which GVnt that is a feedback signal is subtracted from an (L-R) signal that is a reference signal can be obtained at the comparison part 47, and it is multiplied by Vnt at the multiplier 11, and DC output Vc is taken out via an LPF 12, and it is amplified by an amplifier A13, and it approaches to an origin(Vc=0, gain G=1) gradually at every circulation on a feedback loop.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM stereo receiver, and more particularly, it receives a signal after frequency conversion by a local oscillator using a PLL (Phase Locked Loop) / VCO (Voltage Controlled Oscillator) system. It relates to an AM stereo receiver.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram schematically showing a circuit configuration of a conventional AM stereo receiver. Reference numeral 50 denotes an antenna, which is connected to the high frequency amplifier circuit 49. Reference numeral 43 denotes a mixing section, and the input side of the mixing section 43 has a high frequency amplifier circuit 49 and a VCO (voltage controlled oscillator circuit).
42, and the output side is connected to an AM stereo detection circuit 44.

The VCO 42 is connected to a PLL (Phase Locked Loop) 41. The PLL 41 is connected to the VCO 42 and also to an HPF (high pass filter) 45. The HPF 45 is connected to the level adjusting circuit 46, and the level adjusting circuit 46 is connected to the negative input side of the comparing section 47.

The AM stereo detection circuit 44 has two output lines.
The main signal detection output line 44a is directly connected to the matrix 48, the sub signal detection output line 44b is connected to the plus input side of the comparison unit 47, and the output side of the comparison unit 47 is connected to the matrix 48. There is. The matrix 48 also has two output lines, and the L signal output line 48a,
Each of the R signal output lines 48b is connected to a low frequency amplifier circuit (not shown).

The AM stereo receiver circuit configured as described above operates as follows. When the broadcast radio wave is received by the antenna 50, the broadcast radio wave is sent to the high frequency amplification circuit 49 and amplified, and then output from the high frequency amplification circuit 49 to the mixing unit 43 while corresponding to the frequency of the broadcast radio wave. The PLL 41 to the VCO 42 to generate a local oscillation frequency. B. Is output, and the T. B. The local oscillation frequency (signal) generated by the VCO 42 is sent to the mixer 43. Then, the local oscillation frequency (signal) and the output signal RF from the high frequency amplifier circuit 49 are mixed in the mixing section 43, and the RF is the intermediate frequency signal IF.
Is input to the AM stereo detection circuit 44.

By the detection in the AM stereo detection circuit 44, the IF amplitude modulation signal, that is, the (L + R) signal, and the phase modulation signal, that is, the (LR) signal are extracted. The (L + R) signal is directly output to the matrix 48 via the main signal detection output line 44a, while the (L-R) signal is first output to the comparison unit 47 via the sub signal detection output line 44b. After being compared with the output from the level adjusting circuit 46, which will be described later, in the section 47, it is output to the matrix 48. The (L + R) signal and the (LR) signal input from the AM stereo detection circuit 44 to the matrix 48 are the L signal and the R signal in the matrix 48.
Signal and separated. And the separated L signal,
The R signal is an L signal output line 48a and an R signal output line 48b.
After being sent to a low-frequency amplifier circuit (not shown) via each of the antennas and subjected to low-frequency amplification, they are output from a speaker (not shown).

On the other hand, the T.S. B.
Is output to the VCO 42 and also to the HPF 45, low frequency components are removed by the HPF 45, and output to the level adjusting circuit 46 including a variable resistor or the like. The level adjustment circuit 46 has adjusted the level of the T.S. B. Is output to the comparison unit 47 and compared with the (LR) signal output from the AM stereo detection circuit 44 described above.

[0008]

As described above, the (LR) signal output from the AM stereo detection circuit 44 is the audio signal included in the phase-modulated RF and the IF, so that the VCO 42 is used. When a fluctuation (jitter) occurs in the local oscillation signal (frequency) output from the AM, the fluctuation directly appears in the (LR) signal, and the AM
The SN ratio of the detection output in the stereo detection circuit 44 deteriorates. Therefore, it is necessary to take measures to prevent the noise generated in the (LR) signal due to the fluctuation of the local oscillation signal and prevent the deterioration of the SN ratio.

By the way, the fluctuation generated in the local oscillation signal output from the VCO 42 is a T.V. output from the PLL 41 to the VCO 42 as a control voltage. B. Since it is also the AC component (level fluctuation) of the T.T., if attention is paid to this point, the T. B. A possible method is to subtract the AC component of. In fact, in the conventional AM stereo receiver shown in FIG.
The T.I. output from L41. B. Is input to the level adjustment circuit 46 via the HPF 45, and the level adjustment circuit 46 causes the T. B. The level of the AC component is adjusted, and the comparison section 47 outputs it from the AM stereo detection circuit 44 (L-
The noise appearing in the (LR) signal is canceled by the fluctuation as compared with the (R) signal.

However, the conventional noise removing method as described above has the following problems. T. B. The relationship between the level of the AC component and the level of the noise component appearing in the (LR) signal is T. B. And the local oscillation frequency f, depending on the characteristics of the VCO 42. FIG. 4 shows the characteristics of the VCO 42. In FIG. 4, the vertical axis represents T.I. B. And a local oscillation frequency f on the horizontal axis, showing a schematic relationship between the two. As is clear from the figure, T. B. The relationship between and is non-linear. Thus, for example, T.
B. Assuming that the AC component (level fluctuation) is constant, the slope of the graph changes depending on the value of the local oscillation frequency f. B. The magnitude of the AC component reflected in the local oscillation signal f varies. In other words, since the graph is non-linear, the T.S. B. It means that the AC component of is not always converted into the fluctuation of the local oscillation frequency f at a constant ratio. As a result, the noise level appearing in the (LR) signal naturally changes depending on the value of the local oscillation frequency f. Therefore, if the level adjusting circuit 46 makes the T. B. If the level of the AC component is adjusted to cancel the noise appearing in the (LR) signal, the level adjusting circuit 46 can smoothly adjust the level of the AC component over the entire reception band. You must have the means to do it.

However, the level adjusting method of the conventional level adjusting circuit 46 is a manual adjusting method using the volume knob, and in the graph of FIG. 4, the noise level is adjusted only at each point on the graph. I couldn't. That is, the noise level must be manually adjusted each time the slope of the graph in FIG. 4 changes, and the adjustment method is not a method that corresponds to the change in frequency.

The present invention has been made in view of the above problems, and it is possible to automatically remove the noise appearing in the (LR) signal due to the fluctuation of the local oscillation signal, and to improve the SN ratio of the detection output. It is an object of the present invention to provide an AM stereo receiver capable of performing the above.

[0013]

In order to achieve the above object, an AM stereo receiver according to the present invention is an AM stereo receiver in which a frequency is converted by a local oscillator to receive the sub signal detection output and the local oscillator. T. B. A multiplier with an output is provided, and the multiplier has a feedback circuit connected to the sub-signal detection output side via an LPF and a gain varying means.

[0014]

When the two signals Acos θ and Bcos ψ are multiplied, Acos θ × Bcos ψ = (A · B / 2) {cos (θ
−ψ) + cos (θ + ψ)} (1) is output. Here, if θ = ψ, the right side of the equation (1) is (A ·
B / 2) (1 + cos2θ). By the way, it is a ^{cos2θ = 1-2sin 2 θ ‥‥‥ (2} ).
Therefore, if θ = ψ, the multiplication result of the two signals from the equations (1) and (2) is A · B (1-sin ² θ) (3).

As is clear from the equation (3), θ =
If ψ, that is, the frequency components of the two signals are the same, by inputting the multiplication result to the low-pass filter LPF, it is possible to obtain some DC value (A / B in the expression (3)).

In the AM stereo receiver according to the present invention, the T. B. Noise and sub-signal detection output, that is, (L-
R) noise included in the signal has the same frequency component,
Since there is a strong correlation, the T. If the noise of B and the noise included in the (LR) signal are multiplied and passed through the LPF, some DC value is output from the LPF as described above. This DC value is input to the gain varying means, and the gain of the gain varying means is determined. In the gain varying means, the T.S.
B. The noise level of is adjusted. The T.V. whose level has been adjusted by the gain varying means. B. Noise is fed back to the sub-signal detection output side and compared with the (LR) signal. The deviation obtained as a result of the comparison and the T.T. B. The original noise is multiplied by a multiplier, and some DC value is taken by the LPF from the output obtained as a result of the multiplication. Then, this DC value is input again to the gain varying means, and the above feedback process is repeated. This feedback process is repeated until the DC value output from the LPF becomes zero.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of an AM stereo receiver according to the present invention will be described below with reference to the drawings. FIG. 1 shows an example A
FIG. 3 is a block diagram schematically showing a circuit configuration of an M stereo receiver. It should be noted that constituent parts having the same functions as those of the conventional example are designated by the same reference numerals.

Reference numeral 50 denotes an antenna. The antenna 50 is connected to a high frequency amplifier circuit 49, and the high frequency amplifier circuit 49 is connected to one input side of the mixing section 43. Mixing section 43
The other input side is connected to the VCO 42, and the output side of the mixing section 43 is connected to the AM stereo detection circuit 44.

The AM stereo detection circuit 44 has two output lines.
The main signal detection output line 44a is directly connected to the matrix 48, the sub signal detection output line 44b is connected to the plus input side of the comparison unit 47, and the output side of the comparison unit 47 is connected to the matrix 48. There is. The L signal output line 48 a of the matrix 48 is connected to the speaker 16 via the low frequency amplifier circuit 14, and the R signal output line 48 b is connected to the speaker 17 via the low frequency amplifier circuit 15.

The VCO 42 is connected to the PLL 41, and the PL
L41 is connected to the VCO 42 and also to the high pass filter (HPF) 45. The HPF 45 is connected to the multiplier 11 as well as to the gain varying means 10. The gain varying means 10 is connected to the negative input side of the comparing section 47, and the output side of the comparing section 47 is connected to the matrix 48 and also to the multiplier 11. The multiplier 11 is connected to a low pass filter (LPF) 12, the LPF 12 is connected to an amplifier A13, and the amplifier A13 is connected to the gain varying means 10. The HPF 45, the gain varying means 10, the comparing section 47,
The multiplier 11, the LPF 12, and the amplifier A13 form an automatic level adjusting circuit 20.

The AM stereo receiver configured as described above operates as follows. When the broadcast radio wave is received by the antenna 50, the broadcast radio wave is amplified by the high frequency amplifier circuit 49 and output to the mixing unit 43, while the PLL 41 outputs the T.S. B. Is output to the VCO 42 and the automatic level adjusting circuit 20, and the local oscillation signal (frequency) corresponding to the broadcast radio wave is output from the VCO 42 to the mixing unit 43. The broadcast radio wave is mixed with the local oscillation signal by the mixing unit 43 and becomes an intermediate frequency signal IF, which is input to the AM stereo detection circuit 44. In the AM stereo detection circuit 44, the I
When F is detected, the (L + R) signal that is the IF amplitude modulation signal is output to the matrix 48 through the main signal detection output line 44a, and the (L-R) signal that is the IF phase modulation signal. Is output to the plus input side of the comparing section 47 via the sub-signal detection output line 44b, is compared with the output from the automatic level adjusting circuit 20 in the comparing section 47, and is then output to the matrix 48. The (L + R) signal and the (LR) signal output to the matrix 48 are separated by the matrix 48 into an L signal and an R signal,
The signal is output to the low frequency amplifier circuits 14 and 15 and the speakers 16 and 17 via the signal output line 48a and the R signal output line 48b.

Next, the operation of the automatic level adjusting circuit 20 which is a characteristic part of the present invention will be described. The T.38 output from the PLL 41. B. Passes through the HPF 45 to remove low frequency components. Therefore, the output from the HPF 45 is T.50. B. Of the AC component, that is, only the noise component, which is input to the gain varying means 10 and the multiplier 11.
T. B. This noise component of V _nt is _defined as v _nt , the noise component included in the (LR) signal is v _n , the signal output from the comparison unit 47 is v _n ′, the (DC) output of the LPF 12 is V c, and the gain varying means 10 If the gain of G is G, then (LR)
The signal and v _n ′ are represented by the following equations.

(L−R) = L−R + Kv _nt ... (4) v _n ′ = L−R + Kv _nt −Gv _nt (5) Here, LR is the (LR) signal. Indicates the audio (modulated) signal contained, where K is the T.T. B. 5 is a coefficient calculated based on the slope of the graph shown in FIG. 4, showing the degree of correlation between the noise component of R and the noise component included in the (LR) signal.

T. B. The noise component v _nt is G times the gain varying means 10 of the gain G, comparing section 47 becomes Gv _nt
Entered in. On the other hand, since the (LR) signal similarly input to the comparison unit 47 has the content represented by the equation (4), the comparison unit 47 feeds back from the (LR) signal which is the reference value signal. subtracting Gv _nt a signal (5) is obtained. The v _n ′ shown in the equation (5) is input to the multiplier 11 and is multiplied by the output v _nt from the HPF 45. The result of multiplication by the multiplier 11 is LPF1.
2 and is taken out from the LPF 12 as a DC output Vc represented by the following equation (6).

Vc = (1/2) Vnt (K−G) (6) As can be seen from the equation (6), the value of Vc changes depending on the magnitude relationship between K and G. The meaning of the equation (6) will be described with reference to FIG. FIG. 2 is a graph schematically showing the characteristics of the gain varying means 10, with the vertical axis representing the gain G of the gain varying means 10,
The DC output Vc is taken on the horizontal axis.

When the multiplication result Vc is plus (K> G).
Since Vc is amplified by the amplifier A13 and its value becomes high, and this increased Vc is input to the gain varying means 10, the gain G of the gain varying means 10 becomes large. This makes G
Since v _nt increases, v _n ′ in the equation (5) decreases. Therefore, the DC output Vc extracted from the multiplication result of v _nt and v _n ′ also becomes small, and the Vc becomes small, so that the gain of the gain varying means 10 also becomes small. Since the relationship between the DC output Vc and the gain G of the gain varying means 10 is as shown in FIG. 2, if Vc is a positive value, each time the feedback loop of the automatic level adjusting circuit 20 makes one round. Vc and the gain G decrease, and gradually approach the origin (Vc = 0, gain G = 1).

When the multiplication result Vc is negative. Vc is amplified and lowered by the amplifier A13, and as a result, the gain G of the gain varying means 10 is reduced. This allows _GVnt
Becomes smaller, so that v _n ′ in the equation (5) becomes larger.
Therefore, the DC output Vc extracted from the multiplication result of v _nt and v _n ′ also becomes large, and as the Vc becomes large, the gain G of the gain varying means 10 also becomes large. That is, when Vc is negative, contrary to the case where it is positive, Vc and the gain G increase every time the feedback loop of the automatic level adjusting circuit 20 makes one round, and gradually approach the origin.

As described above, the automatic level adjusting circuit 2
When 0, DC extracted from the deviation of the comparison unit 47
Since the output Vc is controlled to be zero, (L-
It is possible to cancel the noise Kv _nt contained in R) signal.

As described above, the AM stereo receiver according to the embodiment has the automatic level adjusting circuit 20, so that an (LR) signal is generated due to the fluctuation of the local oscillation signal output from the VCO 42. The generated noise can be removed automatically, and the SN ratio of the detection output of the AM stereo detection circuit 44 can be improved over the entire reception band.

If the automatic level adjusting circuit 20 is applied to an AM stereo receiver of up-conversion system, a particularly excellent effect can be exhibited. That is, since the automatic level adjusting circuit 20 according to the embodiment can prevent the SN ratio of the detection output from being deteriorated due to noise from the local oscillator, a particularly excellent effect can be obtained in the up-conversion system including the multi-stage local oscillator. Can be demonstrated.

[0031]

As described in detail above, in the AM stereo receiver according to the present invention, in the AM stereo receiver in which the frequency is converted by the local oscillator and received, the sub signal detection output and the T of the local oscillator are transmitted. ． B. Since a multiplier with an output is provided, and the multiplier has a feedback circuit connected to the sub-signal detection output side via an LPF and a gain varying means, the local oscillation signal transmitted from the local oscillator is Noise generated in the sub-signal detection output due to fluctuations,
It can be canceled automatically by the feedback circuit, and the SN ratio of the detection output of the AM stereo receiver can be improved over the entire reception band.

[Brief description of drawings]

FIG. 1 is a block diagram schematically showing a circuit configuration of an AM stereo receiver according to an embodiment of the present invention.

FIG. 2 is a graph schematically showing the characteristics of the gain varying means.

FIG. 3 is a block diagram showing a circuit configuration of a conventional AM stereo receiver.

FIG. 4 is a graph schematically showing the characteristics of a VCO.

[Explanation of sign]

 10 Gain Variable Means 11 Multiplier 12 Low Pass Filter (LPF) 20 Automatic Level Adjustment Circuit (Feedback Circuit) 41 Phase Locked Loop (PLL) 42 Voltage Controlled Oscillation Circuit (VCO) 44b Sub Signal Detection Output Line

Claims (1)

[Claims] 1. In an AM stereo receiver that receives a signal after frequency conversion by a local oscillator, a sub-signal detection output and a tuning bias of the local oscillator (hereinafter referred to as "T.
B. And a feedback circuit connected to the sub-signal detection output side via a low-pass filter (hereinafter referred to as LPF) and gain varying means. AM stereo receiver.