JPH0213498B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a main signal detection device for an AM stereo receiver, particularly for AM-PM or AM-FM systems.
The present invention relates to a main signal detection device in an AM stereo receiver that receives AM stereo broadcast signals.

The AM stereo system includes a system in which a carrier wave is angularly modulated (PM or FM) using a difference signal between left and right channel signals, that is, a sub signal, and then amplitude modulated using a sum signal of both channel signals, that is, a main signal. In such a receiver, the main signal is detected by an AM detector that extracts the amplitude fluctuation component of the carrier signal, and after removing the amplitude fluctuation component of the carrier signal by a limiter, the main signal is detected by a PM or FM detector. Detection of the sub-signals is performed, and the main and sub-signals are matrixed and separated and derived as left and right channel signals.

In AM receivers, RF (high frequency) amplifiers and IF
Generally, a so-called selective amplification stage such as a (intermediate frequency) amplifier is provided with an AGC (automatic gain control) function to obtain a substantially constant detection output against fluctuations in electric field strength. However, it is difficult to make the AM detection output level constant with respect to the reception input level, and as shown in FIG. 2A, the main signal detection output level becomes sloped with respect to the reception input level. On the other hand, in sub-signal detection, the IF signal is removed by a limiter to remove amplitude fluctuation components and limited to a certain level, and then PM or FM
Since the detection is performed, the sub-signal detection output level is approximately constant with respect to the reception input level as shown in FIG. 2A. Therefore, when the main and sub-detection outputs are mixed in a matrix circuit to obtain left and right channel signals, the degree of separation, that is, the separation, is as shown by the dashed line in Figure 2A, when both detection outputs are at the same level. The values other than those that reach the maximum value are undesirable because they decrease and change as the receiving input level changes.

SUMMARY OF THE INVENTION An object of the present invention is to provide a main signal detection device capable of extracting a main signal detection output that can always obtain a substantially maximum and substantially constant separation characteristic even if the receiving input level changes.

The main signal detection device of the present invention has an AGC function and detects RF of an AM stereo reception signal of a desired frequency.
and an amplifying means for selectively amplifying the IF signal,
It is intended for a main signal detection device of an AM stereo receiver configured to detect main and sub signals from the output of the amplification means, and its feature is that the output of the selective amplification means is input. A voltage-controlled variable gain amplifier is provided, and the output of the variable gain amplifier is AM-detected and used as the main signal detection output, and a control voltage corresponding to the detection output level is generated and the control voltage is varied by this control voltage. The purpose of this invention is to control the gain of the gain amplifier.

The present invention will be explained below using the drawings.

FIG. 1 is a block diagram of an example of an AM stereo receiver including a main signal detector according to an embodiment of the present invention. This example shows an AM stereo receiver that receives AM-PM broadcast signals and uses a PLL (phase locked loop) circuit as a PM detector, but it is not limited to the PLL circuit detection method. PM detection method may also be used.
Of course, it can also be applied to FM receivers.

The AM-PMed AM stereo signal is applied to a mixer 2 via an RF amplifier 1, mixed with a local oscillator signal from a local oscillator 3, and frequency-converted. An IF amplifier 4 is provided that selectively extracts and amplifies only a specified IF signal from the output of the mixer 2.
This IF signal is used for main signal and sub signal detection. Also, the output of IF amplifier 4 is AGC
It is applied to the voltage generating circuit 5 to control the gains of the RF amplifier and IF amplifier 4 according to the received input level, and is equipped with an AGC function.

The main signal detector 6 receives the IF signal as input.
A VCA (voltage controlled variable gain amplifier) 61, a multiplier 62 as a synchronous detector that performs AM detection on this output, and a control system that generates a control voltage according to the detected output level and supplies it as the gain control voltage of the VCA. The other input of the multiplier 62 is applied with the output of the limiter 7 for removing the amplitude modulation part of the output of the IF amplifier 4, that is, a rectangular signal synchronized with the carrier wave of the IF signal. has been done.

The PLL circuit 8, which is a sub-signal detection circuit, has a well-known configuration, and includes a 90° phase comparator 82 that compares the frequency and phase of the limiter output and the output of a VCO (voltage controlled oscillator) 81, and this comparison output. A loop filter 83 that sets the DC level according to the phase difference.
and a DC amplifier 84 that amplifies this DC level, and the output of this DC amplifier becomes a control signal for the VCO 81.

Here, the AM-PM stereo signal e _c is expressed by the following equation.

e _c = {1+k ₁ (L+R)}cos{ωct+k ₂ (L-R)+A
cospt}...(1) Here, L and R are left and right channel signals, and _k1 is
AM modulation degree, k ₂ is PM modulation degree, ω _c is the angular frequency of the carrier, P is the angular frequency of the pilot signal (2πf)
and A indicates the amplitude of the pilot signal.
Therefore, the output _e'c of the limiter 7 is given by the following equation.

e _c ′=cos {ωct+k ₂ (L-R)+Acospt}...(2) Then, if the oscillation signal of VOC81 is sinωct, the output of the 90° phase comparator 82 will have the output of both input signals.
With the 90° phase difference as a reference, a signal corresponding to the phase difference, that is, a signal k ₂ (LR)+Acospt is obtained. Since this output indicates a sub-signal, the output of the comparator 82 is applied as a sub-signal detection output to the matrix circuit 9 together with the main signal detection output.

In the main signal detection circuit 6, the AGC-controlled IF signal is further level-controlled in the VCA 61 according to the AM detection output level and outputted, so the gain control characteristics of the VCA 61 should be set appropriately. Therefore, as shown in FIG. 2B, it is possible to control the main signal detection output level to be approximately constant with respect to the reception input level.
It can be adjusted in the same way as the sub signal level. Therefore, as shown in the figure, the separation characteristic maintains its maximum value and does not change with respect to the received input level, which is advantageous.

FIG. 3 shows a specific circuit of an embodiment of the present invention, and parts equivalent to those in FIG. 1 are designated by the same reference numerals. The input IF signal is converted to DC by the level shifter 64.
The level is shifted and converted into current _I5 by transistor _Q1 and resistor _R1 . Transistors Q ₂ , Q ₃
and resistor R ₂ constitute VCA61,
The voltage between the bases of the transistors Q ₂ and Q ₃ controls the ratio of the current I ₆ flowing through the transistor Q ₂ to the current of the current source I ₅ , that is, the shunt ratio, and operates as a VCA. The amplification characteristics of this VCA are shown in Figure 4, where the base control voltage e _f of transistor Q ₃ is
and the reference base bias E ₁ of transistor Q ₂ ( _ef − E ₁ ) and the relationship between I ₆ /I ₅ .
That is, when e _f ≫E ₁ , I ₆ /I ₅ = 0, and when e _f ≪E ₁ , I ₆ /I ₅ = 1, and for the base potential difference between them, the shunt ratio I ₆ /I ₅ changes approximately linearly. Therefore, VCA characteristics can be obtained.

The output I ₆ of the VCA 61 becomes a current source for the transistors Q ₄ and Q ₅ in the differential amplifier configuration, and the output of the limiter 7 turns both transistors Q ₄ and Q ₅ on and off alternately, thereby increasing the current (I ₁ , I ₂ ) switching is performed. The transistors Q ₆ to _{Q 8} and the resistors R ₃ and R ₄ form an active load for the transistors Q ₄ and Q ₅ , and have a current mirror configuration with a mirror ratio of 1. Figure 5 A shows the waveform of the IF signal e _c , B,
C indicates the limiter output. Therefore, the transistor
It can be seen that the full-wave rectified output shown in figure D is obtained as the collector output of _Q4 . Therefore, these transistors Q ₄ to Q ₈ and resistors R ₃ and R ₄ constitute a multiplier type synchronous detection 62. This synchronous detection output _I3 is derived as a voltage from the emitter resistance of the emitter follower transistor _Q12 and becomes the main signal.

Transistors Q ₉ , Q ₁₀ , Q ₁₁ and resistors R ₅ , R ₆ ,
R ₁₀ forms a DC amplifier, and the transistor
The AC component of the collector current at _Q11 is the capacitor
It is removed by C ₁ and becomes the control voltage _ef of the VCA 61. Note that resistors R ₇ to _{R 9} and R ₁₂ represent base bias resistors of transistors Q ₁₀ and Q ₁₁ .

In this way, by using the multiplier method as an AM detector, a detection output with a low distortion rate can be obtained and the characteristics can be improved.Also, the limiter output for subchannel detection can be used as a switching signal for the multiplier. There is also the advantage that they can be used together.

In this way, in the invention, in addition to having the AGC function in the preceding selective amplification stage, the AGC function is also added to the main signal detection stage, so that the levels of the main signal and sub signal are equal to the input signal level. This has the advantage of being substantially the same regardless of the relationship, resulting in good separation.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of an AM stereo receiver using a main signal detection circuit according to an embodiment of the present invention, FIG. 2A is a diagram illustrating the characteristics of a conventional AM stereo receiver, and FIG. B is a diagram explaining the characteristics of the AM stereo receiver shown in Figure 1, Figure 3 is a circuit diagram of an embodiment of the present invention, Figure 4 is a characteristic diagram of the VCA, and Figure 5 explains the operating principle of the multiplier. FIG. Explanation of symbols of main parts, 1...RF amplifier, 4
...IF amplifier, 5...AGC voltage generator, 6...
Main signal detection circuit, 7...limiter, 8...sub signal detection circuit, 61...VCA, 62...multiplier, 63...control voltage generation circuit.

Claims (1)

[Claims] 1 Amplifying means for selectively receiving and amplifying a desired frequency signal of an AM stereo broadcast signal in which a carrier wave angle-modulated by a sub-signal is amplitude-modulated by a main signal, and having an AGC function; A main signal detection device in an AM stereo receiver configured to detect the main signal and the sub-signal from the output of the amplification means, the voltage-controlled variable gain amplification means receiving the output of the amplification means as an input. AM detection means comprising a multiplier to which the output of the voltage-controlled variable gain amplification means and the sub-signal whose amplitude is limited by a limiter are applied; 1. A main signal detection device for an AM stereo receiver, comprising: means for generating a control voltage for a controlled variable gain amplification means.