JPH11145872A - AM receiver - Google Patents

Abstract

(57) [Problem] To enable an AM receiver to reliably listen to a broadcast from a weak electric field to a strong electric field. SOLUTION: A carrier signal forming circuit 22 for forming a quadrature carrier signal SQD from an AM intermediate frequency signal SAM is provided. An adder 21 is provided for adding the AM intermediate frequency signal SAM and the quadrature carrier signal SQD. A limiter 23 for making the amplitude of the output signal SWA of the addition circuit 21 constant, and a phase detection circuit 24 for performing phase detection of the output signal of the limiter 23 are provided. A correction circuit 25 for correcting distortion of the output signal Sθ of the phase detection circuit 24 is provided. An AM detection circuit 32 for AM detection of the AM intermediate frequency signal SAM is provided. A switch circuit 41 for selectively extracting an output signal of the correction circuit 25 and an output signal of the AM detection circuit 32, and a formation circuit 52 for forming a control signal S52 according to a reception level of the AM intermediate frequency signal SAM are provided. The switch circuit 41 is controlled by the control signal S52.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM receiver.

[0002]

2. Description of the Related Art Medium-wave broadcasting and short-wave broadcasting are generally performed by AM waves. However, in an ordinary AM receiver, an audio signal is demodulated by detecting a peak value of a received broadcast wave signal itself. ing. Therefore, AGC
Reception level (input signal level) enough to operate
Is large, the demodulated audio signal has a prescribed level, and the broadcast can be stably heard.

[0003]

However, in the above-mentioned AM receiver, when the reception level is so small that the AGC does not operate, the level of the demodulated audio signal decreases and the noise increases. Therefore, it becomes difficult to listen to the broadcast.

[0004] The present invention is to solve such a problem.

[0005]

Therefore, according to the present invention, a mixer circuit for converting the frequency of a received AM signal into an AM intermediate frequency signal and a signal having the same frequency and the same phase from the AM intermediate frequency signal are obtained. A carrier signal forming circuit for forming a quadrature carrier signal shifted by °, an adding circuit for adding the AM intermediate frequency signal and the orthogonal carrier signal, a limiter for making the amplitude of an output signal of the adding circuit constant, A phase detection circuit for performing phase detection of an output signal of the limiter, a correction circuit for correcting distortion of an output signal of the phase detection circuit, an AM detection circuit for performing AM detection on the AM intermediate frequency signal, and the correction circuit And a switch circuit for selectively extracting the output signal of the AM detection circuit and supplying the output signal to a subsequent circuit, and a control circuit according to the reception level of the AM signal. And a forming circuit for forming a control signal, wherein the correcting circuit forms a signal for correcting the distortion from the output signal of the limiter itself, and corrects the distortion. When the reception level of the AM signal is below a predetermined value, the output signal of the correction circuit is output. When the reception level of the AM signal is above the predetermined value, the output signal of the AM detection circuit is output. Is output as an AM receiver. Therefore, when the reception level is low, an audio signal obtained by phase detection and distortion correction is output, and when the reception level is high, an audio signal obtained by AM detection is output.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Considering an AM signal in a general AM broadcast, the AM signal is a double-sideband signal of all carrier waves. Therefore, if the AM signal is a signal SAM, the AM signal SAM is , SAM = (1 + k · G) cosωctk: modulation degree G: G = g (t). An information signal, that is, an audio signal ωc: can be expressed as a carrier frequency (angular frequency).

If a carrier signal whose frequency is equal to that of the AM signal SAM and whose phase is shifted by 90 °, that is, a quadrature carrier signal is a signal SQD, the quadrature carrier signal SQD is given by: SQD = sin ωct Can be expressed as

When the orthogonal carrier signal SQD is added to the AM signal SAM, a signal SWA resulting from the addition is given by: SWA = SAM + SQD = (1 + kG) cos ωct + sin ωct = a · cos (ωct−θ) ·・ ・ (1) a = {(1 + kG) ** 2 + 1 ** 2｝ ** 0.5 θ = arctan ｛1 / (1 + kG) Ｇ (2) (α ** β is α to the power of β. The same applies hereinafter. That is, when the orthogonal carrier signal SQD is added to the AM signal SAM,
The phase θ of the addition signal SWA is a function of the information signal G as shown by the equation (2).

Therefore, the sum signal SWA is supplied to the limiter to make the signal cos (ωct-θ) having a constant amplitude, and if the signal cos (ωct-θ) is subjected to phase detection, the information signal G is obtained.
Will be obtained.

However, in this case, since the phase θ and the information signal G are expressed by the relationship of the equation (2) and are not linear,
The phase detection output is not the signal G itself. That is, the phase detection output includes distortion. Therefore,
It is necessary to correct the distortion.

Then, in order to expand equation (2) into a polynomial,
Approximately S = 1 / (1 + Y) (3) = 1-Y + Y ** 2-Y ** 3 + Y ** 4-Y ** 5 arctanX = X- (X ** 3) / 3 + (X ** 5) / 5 ... (4) In order to satisfy the condition of | X | ≦ 1, X = m · xm <1.

Then, the equation (2) is obtained from the following equations (3) and (4): θ = arctan {m / (1 + kG)} = {1-kG + (kG) ** 2- ( k × G) ** 3+...｝ × m − (1/3) [｛1−k · G + (k · G) ** 2- (k · G) ** 3 +...｝ × m ] ** 3+ (1/5) [｛1−k · G + (k · G) ** 2- (k · G) ** 3+...｝ × m] ** 5, but k When rearranging the terms in which G is higher than fourth order, θ = (1/15) (15m−5m ** 3 + 3m ** 5) + (− m + m ** 3-m ** 5) × k · G + (M−2m ** 3 + 3m ** 5) × (k · G) ** 2 + {− m + (10/3) m ** 3-7m ** 5} × (k · G) ** 3・ ・ (5)

In the equation (5), the first term indicates a DC component, the second term indicates a target information signal G, and the third and fourth terms indicate second- and third-order distortion components. .

When the degree of modulation is small, higher-order distortion components are not so noticeable, but when the degree of modulation is large, higher-order distortion components become noticeable.

For this reason, it is considered to cancel the higher-order distortion component. That is, it is now assumed that m = 0.5, and this value is substituted into Expression (5). Further, at this time, the DC component term (first term) is removed, and the coefficient of the target information signal G term (second term) is corrected to be 1. Then, equation (5) becomes: θ = −k · G + 0.844 (k · G) ** 2−0.514 (k · G) ** 3 (6)

Therefore, equation (6) is squared and cubed to obtain θ ** 2 = (kG) ** 2-1.688 (kG) ** 3 (7) θ ** 3 = − (k · G) ** 3 (8) However, in equations (7) and (8), terms of fourth order and higher are ignored.

.Theta.x = .theta.-0.844.theta. ** 2 + 0.905.theta. ** 3 (9) and substituting equations (6) to (8) into equation (9), 0.844 θ ** 2 + 0.905 θ ** 3 = -kG + 0.844 (kG) ** 2- 0.514 (kG) ** 3 -0.844 ｛(kG) ** 2-1.688 (k・ G) ** 3｝ +0.905 {− (k · G) ** 3} = −k · G (10) That is, by performing the signal processing of Expression (9), an information signal G containing no original distortion component can be obtained, as shown in Expression (10).

However, in the above method, it is assumed that the signal represented by the equation (2) is obtained as theoretically. When the obtained signal deviates from the equation (2), the equation (9) is used. Therefore, even if the distortion component is canceled, it becomes insufficient or overcompensated. Therefore, high-performance limiters and phase detection circuits are required to obtain the signal represented by the equation (2) as theoretically.

Although the distortion component is canceled in accordance with the equation (9), the equation (6) is an approximate equation. Therefore, when the reception level of the AM signal SAM is large, Equation (9) does not hold, and the distortion component remains or increases.

However, when listening to an AM broadcast in a weak electric field in which AGC does not work, noise is often more anxious than distortion.

Therefore, in the present invention, the AM signal S
When the reception level of AM is low, an audio signal is extracted by the above method, and when the reception level of AM signal SAM is high, the audio signal is extracted by envelope detection.

An example will be described below with reference to FIG.

In FIG. 1, an AM signal is received by an antenna 11, and the AM signal is supplied to an antenna tuning circuit 12 to extract an AM signal of a desired frequency.
This AM signal is supplied to the mixer circuit 1 through the high-frequency amplifier 13.
4, a local oscillation signal having a predetermined frequency is supplied from the local oscillation circuit 15 to the mixer circuit 14, and a target AM signal is frequency-converted into a predetermined intermediate frequency, for example, an intermediate frequency signal having a frequency of 450 kHz. This intermediate frequency signal is extracted through the intermediate frequency filter 16.

Thereafter, the intermediate frequency signal is processed as described above as the above-mentioned AM signal SAM. That is, the intermediate frequency signal SAM from the intermediate frequency circuit 16 is supplied to the adding circuit 21 and also supplied to the carrier signal forming circuit 22 to form a quadrature carrier signal SQD. This signal SQD is supplied to the adding circuit 21. You. Thus, the addition signal SWA represented by the equation (1) is extracted from the addition circuit 21.

The added signal SWA is supplied to the limiter amplifier 23 where it is amplified and converted into a signal cos (ωct-θ) having a constant amplitude. This signal is supplied to the phase detection circuit 24 ( The signal Sθ expressed by the equation (2) is extracted, and the signal Sθ is supplied to the correction circuit 25. An example of the correction circuit 25 will be described later. This is a circuit that performs the above-described distortion removal processing up to the item (9) and outputs the information signal G represented by the expression (10), that is, an audio signal. is there. The audio signal from the correction circuit 25 is supplied to the switch circuit 41.

Further, the intermediate frequency signal SAM from the intermediate frequency filter 16 is supplied to an AM detection circuit 32 of, for example, an envelope detection system through an amplifier 31 to extract an audio signal, and the audio signal is supplied to a switch circuit 41. .

A part of the detection output of the AM detection circuit 32 is supplied to an AGC voltage forming circuit 51 to form an AGC voltage, and this AGC voltage is supplied to the amplifiers 13 and 31 as a gain control voltage. Done.

Further, a part of the detection output of the AM detection circuit 32 is supplied to the switching voltage forming circuit 52, and becomes "L" when the reception level is lower than the predetermined level VTH, and when it is higher than the predetermined level VTH. A control voltage S52 that becomes “H” is formed. Then, the control voltage S
52 is supplied to the switch circuit 41 as its control signal,
When S52 = “L”, the switch circuit 41 is connected to the correction circuit 25 side as shown in the figure, and when S52 = “H”, the switch circuit 41 is connected to the AM detection circuit 32 side contrary to the figure.

As described above, the audio signal from the correction circuit 25 or the AM detection circuit 32 is selectively extracted from the switch circuit 41, and the extracted audio signal is supplied to the speaker 43 through the amplifier 42.

According to such a configuration, by setting the level VTH in advance, when the reception level of the AM signal SAM is low enough that AGC does not operate, S
52 = “L”, and the switch circuit 41 is connected to the state shown in FIG. Therefore, the audio signal from the correction circuit 25 is supplied to the speaker 43. That is,
When the reception level of the AM signal SAM is low, (1) to (1)
An audio signal is extracted by the method described with reference to equation (0).

Therefore, even if the level of the received AM signal SAM is low, the AM signal SAM is set to a constant high level by the limiter 23, so that it is possible to listen to the broadcast without lowering the volume. In addition, since the limiter 23 is used, amplitude noise is also suppressed. That is, even if the reception level is low, the broadcast can be heard in a state where the audio signal level is high and the noise is low.

On the other hand, if the reception level of the AM signal SAM is AGC
Is large enough to operate, S52 = "H", and the switch circuit 41 is connected in a state opposite to that shown in the figure. Therefore, the audio signal from the AM detection circuit 32 is supplied to the speaker 43. That is, when the reception level of the AM signal SAM is high, an audio signal is extracted by normal AM detection.

Therefore, when the level of the received AM signal SAM is large, the broadcast can be heard with little distortion and noise.

FIG. 2 shows an example of the carrier signal forming circuit 22. In this example, the circuit is constituted by a PLL. That is, the intermediate frequency signal SAM from the intermediate frequency filter 16 is supplied to the limiter 221 to be an alternating signal having a constant amplitude. This signal is supplied to the comparison circuit 222, and the VCO 224 is provided. The comparison output is supplied to the comparison circuit 222, and the comparison output is supplied to the VCO 224 through the low-pass filter 223 as a control voltage.

Therefore, in the steady state, the oscillation signal of the VCO 224 is a signal having the same frequency as that of the signal SAM and having a phase shifted by 90 °, so that this oscillation signal is nothing but the quadrature carrier signal SQD. That is, the orthogonal carrier signal SQD is formed.

FIG. 3 shows an example of the correction circuit 25.
That is, the signal Sθ from the detection circuit 24 is supplied to the DC cut circuit 251 to remove the DC component corresponding to the first term of the expression (5), thereby obtaining a signal represented by the expression (6). The signal is supplied to the circuit 252. Further, the signal from the DC cut circuit 251 is supplied to the square circuit 253 and the cubic circuit 2.
55 to form the squared and cubic signals represented by the equations (7) and (8). These signals are supplied to the level correction circuits 254 and 256 and correspond to the coefficients in the equation (9). After being corrected to the adjusted level, it is supplied to the addition circuit 252 and added. Therefore, the audio signal G represented by Expression (10) is extracted from the addition circuit 252.

FIG. 4 shows a measurement example of the reception level versus output level characteristic. The horizontal axis indicates the reception level (antenna input level), and the vertical axis indicates the output levels of the audio signal and the noise. The curve S20 and the curve N20 are the audio signal (including noise) obtained from the correction circuit 25 and the level characteristics of the noise, and the curve S30 and the curve N
Numeral 30 denotes an audio signal (including noise) obtained from the AM detection circuit 32 and a level characteristic of the noise.

As is apparent from the measurement results, the case of the above-described phase detection and distortion correction (curve S
In case 20 and curve N20), as the reception level decreases, the noise level increases, but the audio signal level does not decrease. Therefore, even if the reception level decreases, the broadcast can be reliably heard. However, the noise level in the case of phase detection and distortion correction (curve N20) does not improve much as shown by the curve N20 even when the reception level increases.

On the other hand, in the case of normal AM detection (curves A30 and N30), as the reception level increases, the noise level is greatly improved, but as the reception level decreases, the noise level increases. Not only that, but the audio signal level drops sharply, making it difficult to listen to the broadcast.

However, in the present invention, since the switch circuit 41 is switched according to the input level, if the reception level VTH at the switching point is set to, for example, about 65 dB as shown by a chain line, the reception level becomes small. Regardless of whether it's big or big, you can definitely listen to the broadcast. further,
When the reception level is large, distortion does not increase.

Although the processing contents of the correction circuit 25 are somewhat special, the squaring circuit 253 and the cubing circuit 254
Since the DC cutoff circuit 251 can be realized by using a capacitor or the like while being able to be realized by utilizing the non-linearity such as the forward characteristic of the transistor, the configuration itself is simple.

Further, once the received AM signal is
Even if the signal is converted into a B signal and subjected to signal processing called RZ-SSB, the signal can be made less susceptible to noise. In this case, a narrow band filter is required to convert the AM signal into an SSB signal. Or the need for a DSP, which makes the receiver expensive. AG
A C circuit is also required. In contrast, the above-described receiver does not require a narrow-band filter or a DSP, and does not require a special circuit or element. Therefore, the configuration of the entire receiver is simple and inexpensive.

In equation (9), the coefficient of the second term is-
Since the coefficient of the third term is close to 1 and the coefficient of the third term is close to 1, assuming that the coefficient of the second term is -1 and the third coefficient is 1, and calculates the distortion factor included in the audio signal G, It was 3% or less. Therefore, when allowing this degree of distortion,
Since equation (9) becomes θx = θ−θ ** 2 + θ ** 3, the correction circuit 25 can omit the level correction circuits 255 and 256, for example, as shown in FIG.

As shown in FIG. 6, for example, the output of the manually operated switch 53 and the control voltage S52 from the control circuit 52 are supplied to the AND circuit 54, and the switch circuit 41 is operated by the AND output as described above. Can also be controlled. That is, by doing so, the switch 53
Is off, the control voltage S52 is directly supplied to the switch circuit 41 through the AND circuit 54, so that the audio signal from the correction circuit 25 and the audio signal from the AM detection circuit 32 are changed according to the reception level as described above. Are selectively extracted from the switch circuit 41.

However, when the switch 53 is on,
Regardless of the control voltage S52, the output of the AND circuit 54 becomes "L" and the switch circuit 41 is connected to the state shown in the figure. As a result, the audio signal from the correction circuit 25 is always taken out. In the case where pulse noise is generated even when the electric field is higher than the electric field, it is effective for the noise.

Further, when switching the switch circuit 41 in accordance with the reception level of the AM signal SAM, a hysteresis characteristic can be provided.

[0047]

According to the present invention, a broadcast can be stably listened to in a state where the audio signal level is large regardless of the reception level. Also, when the reception level is high, the distortion does not increase.

Further, no special circuit or element is required, and the overall configuration of the receiver is simple and inexpensive.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an example of the present invention.

FIG. 2 is a system diagram showing an example of a circuit that can be used in the present invention.

FIG. 3 is a system diagram showing an example of a circuit that can be used in the present invention.

FIG. 4 is a diagram showing an example of measurement results of characteristics of a receiver according to the present invention.

FIG. 5 is a system diagram showing another example of a circuit that can be used in the present invention.

FIG. 6 is a system diagram showing a part of another example of the present invention.

[Explanation of symbols]

12: Antenna tuning circuit, 14: Mixer circuit, 15: Local oscillation circuit, 16: Intermediate frequency filter, 22: Carrier signal forming circuit, 23: Limiter, 24: Phase detection circuit,
25 correction circuit, 32 AM detection circuit, 41 switch circuit, 43 speaker, 51 AGC voltage forming circuit, 5
2 ... Switch voltage forming circuit, 222 ... Phase comparison circuit,
224 VCO, 251 DC cutoff circuit, 253 2
Multiplying circuits, 254 and 256... Level correcting circuits, 255
... 3rd power circuit

Claims (5)

[Claims] 1. A mixer circuit for converting a frequency of a received AM signal into an AM intermediate frequency signal, and a carrier signal forming an orthogonal carrier signal having the same frequency and a phase shift of 90 ° from the AM intermediate frequency signal. A circuit, an addition circuit for adding the AM intermediate frequency signal and the quadrature carrier signal, a limiter for making the amplitude of an output signal of the addition circuit constant, and a phase detection circuit for performing phase detection of the output signal of the limiter A correction circuit for correcting distortion of an output signal of the phase detection circuit; an AM detection circuit for AM detection of the AM intermediate frequency signal; an output signal of the correction circuit; and an output signal of the AM detection circuit And a forming circuit for forming a control signal according to a reception level of the AM signal, The correction circuit forms a signal for correcting the distortion from the output signal itself of the limiter and corrects the distortion, and the switch circuit is controlled by the control signal,
The output signal of the correction circuit is output when the reception level of the AM signal is equal to or lower than a predetermined value, and the output signal of the AM detection circuit is output when the reception level of the AM signal is equal to or higher than the predetermined value. AM receiver. 2. The AM receiver according to claim 1, further comprising an AGC voltage forming circuit for forming an AGC voltage from a part of an output signal of the AM detecting circuit, wherein the AGC voltage is used to generate at least the AM intermediate frequency. An AM receiver in which AGC is performed on a signal. 3. The AM receiver according to claim 1, further comprising a manually operated switch, wherein an output signal of said correction circuit is output by said switch output regardless of said reception level. AM receiver to output from. 4. The AM receiver according to claim 1, wherein said correction circuit comprises: a DC cut circuit for removing a DC component of an output signal of said phase detection circuit; A squaring circuit for squaring the output signal of the circuit; a squaring circuit for squaring the output signal of the DC cut circuit; an output signal of the DC cut circuit; an output signal of the square circuit; An addition circuit for adding an output signal of the circuit at a predetermined ratio. 5. The AM receiver according to claim 1, 2, 3, or 4, wherein said forming circuit forms said control signal from an output signal of said AM detection circuit. AM receiver.