JPH0139006Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a receiver with electronic tuning of amplitude modulation.

Conventional AM electronically tuned receivers have been considered disadvantageous in terms of practical sensitivity (defined as an input level (dBμ) at which S/N = 20dB) compared to μ-tuned receivers. This is because in the AM electronically tuned receiver, the antenna is a capacitive antenna, so when configuring the antenna tuning circuit, the capacitance change ratio becomes considerably large.
This is due to the fact that the input to the high-frequency amplification section is out of tune because a variable capacitance diode cannot be used.

Therefore, with the idea of improving the S/N ratio, an AM receiver with a modified antenna tuning circuit has been proposed, for example, in Japanese Patent Publication No. 14012/1983. The receiver uses a tapped coil and a variable capacitance diode for tuning to improve the S/N ratio. Even with this method, it is difficult to satisfy the capacitance change width of the diode that keeps the S/N ratio constant. Furthermore, this method not only has the disadvantage that the input level to the high frequency amplification section increases, which deteriorates the strong input characteristics, but also causes an increase in costs.

An object of the present invention is to provide an AM electronically tuned receiver that does not use a variable capacitance diode and has a high S/N ratio even though the input side of the high frequency amplifier section is asynchronous.

In order to achieve the above purpose, AM according to the present invention
The electronically tuned receiver includes a low-pass filter connected to the output of the detection circuit, a loop amplifier connected to the low-pass filter, and the output of the local oscillator.
Equipped with an AM modulator that performs AM modulation, the gain of the loop amplifier is reduced by the voltage obtained by rectifying and smoothing the signal from the intermediate frequency amplifier, and when the input signal from the antenna is strong, the gain is reduced, and the AM
The gist of the present invention is to suppress the modulation degree of the modulator and increase the gain when the input signal from the antenna is weak, thereby increasing the modulation degree of the AM modulator.

Hereinafter, the present invention will be explained in more detail using examples with reference to the drawings, but these are merely illustrative and it is understood that various modifications and improvements may be made without going beyond the scope of the present invention. Of course.

FIG. 1 is a block diagram showing the configuration of an AM electronically tuned receiver according to the present invention, in which a high frequency amplifier 1 amplifies an amplitude modulated signal input from an antenna ANT and inputs it to a mixer 2. This amplifier 1 has such a gain that the noise figure of the mixer 2 at the next stage can be ignored. The mixer 2 mixes the amplified output signal and the signal from the local oscillator 10 through the high frequency amplifier 1, and converts the received frequency to an intermediate frequency. An intermediate frequency amplifier 3 amplifies the intermediate frequency component to a level necessary for detection. The detection circuit 4 converts the amplitude modulated wave obtained in this way into an audio signal, and the output is outputted after unnecessary components are removed by a low-pass filter 5.
Sent from OUT to the audio stage.

On the other hand, the output of the detection circuit 4 is input to a loop amplifier 7 through another low-pass filter 6. The loop amplifier 7 is an audio amplifier for amplifying the detected audio signal, inputting it to the AM modulator 11, and amplitude-modulating the output of the local oscillator 10 with the feedback audio signal. On the other hand, a level detector 8 detects the intermediate frequency signal and converts it into a DC voltage corresponding to the level. Based on the DC voltage obtained by the level detector 8, the gain controller 9
controls the gain of the loop amplifier 7 so that it is lowered when the input level is high and raised when the input level is low. The AM modulator 11 modulates the output of the local oscillator 10 with the feedback audio signal, inputs it to the mixer 2, and mixes it with an amplitude-modulated high-frequency signal there. The AGC amplifier 12 controls the gain of the intermediate frequency amplifier 3 depending on the output level of the detector 4, and the level detector 13 detects high frequency signals and serves as a wideband AGC.

It will be explained below that the S/N ratio is improved by feeding back the detected audio signal to the local oscillator and amplifying and modulating the local oscillation frequency.

The input signal is u _S (t)=V _S (1+m _O cospt) cosω _C t …(1) The modulated local oscillator output is u _O (t)=V _O (1+m _S cospt) cosω _O t …(2) shall be. Here, p is the angular frequency of the audio signal, m _O is the degree of modulation of the input signal, ω _O is the angular frequency of the carrier wave of the input signal, and m _S is the local amplitude modulated by the feedback audio signal, determined by the amount of feedback. The degree of modulation of the oscillator output signal, ω _O is the angular frequency of the carrier of the local oscillator output signal.

When the signals expressed by equations (1) and (2) are mixed by a mixer, the output u _M (t) is expressed by the following equation.

u _M (t) = K _M [u _S (t) + u _O (t)] ² = K _M [u _S ² (t) + 2u _S (t) u _O (t) + u _O ² (
t)]...(3) Here, K _M is a mixer constant. Since the terms represented by u _S ² (t) and u _O ² (t) are removed by the filter in the intermediate frequency amplifier, u _M (t) = 2K _M u _S (t) u _O (t) …(4) becomes. Substituting equations (1) and (2), we get the following.

u _M (t)=K _M V _S V _O (1+m _O cospt) (1+m _S
cospt)×{cos(ω _O −ω _C )t+cos(ω _O +ω _C )t}
...(5) Since the second term is removed by a filter, equation (5) can be written as follows.

u _M (t)=K _M V _S V _O {(1+m _O m _S /2)+m _O +m _S )cosp
t}×cos(ω _O −ω _C )t ＋K _M V _{S V} O m _O m _S ／2cos2ptcos(ω _O −ω _C )t…(6)
The second term in equation (6) is a carrier suppressed wave and is not detected. Therefore, u _M (t) is finally given by the following equation.

u _M (t)=K _M V _S V _O {(1+m _O m _S /2)+(m _O +m _S )co
spt}cos(ω _O −ω _C )t…(7) As can be seen from equation (7), the modulation degree is from m _O to (m _O +
m _S ). Increasing the depth of modulation is equivalent to increasing the level of the input signal, which means that the S/N ratio improves.

Further, this S/N improvement effect depends on the degree of modulation of the input high frequency signal. That is, when the degree of modulation is large (the input level is high), even if the degree of modulation of the local oscillator output signal increases, the S/N improvement effect is not so great and may even worsen. Therefore, it is necessary to set the degree of modulation of the local oscillator output signal according to the strength of the input high-frequency signal. Therefore, it is necessary to perform level detection of the intermediate frequency signal, convert the signal level to a DC voltage, and control the gain of the loop amplifier. That is, when the input signal has a large modulation depth or amplitude level, the gain of the loop amplifier is reduced to shallowly modulate the local oscillator output signal, and vice versa, the modulation of the local oscillator output signal is reduced. The gain of the loop amplifier is controlled according to the strength of the input signal to set the degree of modulation of the local oscillator output signal so as to improve the S/N ratio.

As explained above, according to the present invention, even though the input side of the high frequency amplifier is not tuned, without using a variable capacitance diode, by amplitude modulating the local oscillator output with a demodulated baseband signal, high frequency An AM electronically tuned receiver with a signal to noise ratio can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an AM electronic tuning receiver according to the present invention. 1... High frequency amplifier, 2... Mixer, 3... Intermediate frequency amplifier, 4... Detection circuit, 5, 6... Low pass filter, 7... Loop amplifier, 8, 13...
... Level detector, 9 ... Gain controller, 10 ... Local oscillator, 11 ... AM modulator, 12 ... AGC
Amplifier, ANT...Antenna, OUT...Output to the audio stage.

Claims (1)

[Scope of utility model registration request] A low-pass filter connected to the output of the detection circuit, a loop amplifier connected to the low-pass filter, and an AM that modulates the output of the local oscillator.
A modulator is provided, and the gain of the loop amplifier is reduced by the voltage obtained by rectifying and smoothing the signal from the intermediate frequency amplifier, and when the input signal from the antenna is strong, the gain is reduced, and the modulation of the AM modulator is performed. 1. An AM electronic tuning receiver characterized in that the AM electronic tuning receiver is controlled so as to suppress the degree of modulation of the AM modulator, increase the gain when the input signal from the antenna is weak, and increase the degree of modulation of the AM modulator.