JPH028445Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to an AM receiver for vehicles, etc., and is designed to improve the S/N ratio.

[Prior Art] A capacitive whip antenna is used as an antenna in a conventional vehicle-mounted AM receiver. Therefore, in order to cancel this antenna capacitance, tuning is performed using an inductance L to improve the S/N ratio. However, with the electronic tuning type that is often used in car-mounted receivers these days, it is difficult to tune with inductance L in the relatively wide AM broadcast frequency range of 530 to 1620 KHz. Therefore, the received input signal suffers loss due to the antenna capacity and S/
N deteriorates.

Therefore, as another means to deal with this and improve the S/N, a modulation feedback loop is provided that feeds back the audio output signal component to the local oscillator in the frequency conversion circuit and applies AM modulation to the oscillation signal of the local oscillator. Some systems mix the AM modulated oscillation signal from the local oscillator with the broadcast reception signal to apparently increase the degree of modulation of the broadcast reception signal, thereby improving the S/N ratio. However, with such conventional modulation feedback means, if the input received signal level is sufficiently strong,
On the contrary, there was a problem in that the S/N ratio deteriorated.

[Purpose of the invention] This invention was made by paying attention to such conventional problems, and solves the above problems by opening the modulation feedback loop when the level of the broadcast reception signal is higher than a predetermined level. The purpose is to

[Example] This invention will be explained below based on the drawings. FIG. 1 is a diagram showing an embodiment of this invention.

First, to explain the configuration, in the figure, ANT is the antenna, RF is the high frequency amplifier, MIX and OSC are the mixer and local oscillator in the frequency conversion circuit, respectively, IF/AMP is the intermediate frequency amplifier, DET is the detector, and U ₁ is the signal level. Rectifier circuit, HC is a high cut (high frequency component cut) circuit, signal level rectifier circuit
_U1 rectifies the intermediate frequency output component and uses it as a driving voltage for an S meter (not shown). Further, the rectified voltage (S meter voltage) from the signal level rectifier circuit _U1 is also led to the high cut circuit HC. When the level of the broadcast reception signal is weak and the S meter voltage is below a predetermined level, the high cut circuit HC is activated and high frequency components are cut off.
AGC is a wideband AGC circuit, and Q is a transistor for wideband AGC. In addition, a low-pass filter LPF and a buffer amplifier AMP are connected to a line branched from the audio signal output line derived from the detector DET.
is interposed to form a modulation feedback loop L, which is connected to the local oscillator OSC. A low-pass filter LPF is provided to cut out noise components and return only the audio output component, and a buffer amplifier AMP is provided to sufficiently amplify the audio output component to the level required for modulation.

Further, in this invention, a normally closed switch means SW is interposed in the modulation feedback loop L, and the following control circuit for controlling this normally closed switch means is further provided. That is, a comparator (voltage comparison circuit) U ₂ is provided, and this comparator U ₂
The S meter voltage from the signal level rectifier circuit U ₁ is led to one input terminal of the , and the reference voltage set by the reference voltage generator U ₃ and the variable resistor VR is led to the other input terminal. . The output terminal of the comparator _U2 is connected to normally closed switch means SW via a Schmitt circuit _U4 . Comparator U ₂
outputs an "H" level voltage when the S meter voltage is below the reference voltage. The normally closed switch means SW is
It turns ON at this "H" level voltage, that is, at the normal received signal level. In this invention, "normally closed" means that it is always ON at the normal received signal level. The Schmitt circuit _U4 is arranged to provide the switch means _SW12 with hysteresis characteristics. This hysteresis characteristic is explained with reference to Figure 2. In the figure, S ₀ is the reference voltage, and S ₁ is the switch means.
Voltage at which SW turns OFF, S ₂ is the switch means
At the voltage when the SW turns ON, the S meter voltage is now
As the voltage gradually decreases from S ₁ , it passes the reference voltage S ₀ .
When you reach the S ₃ level, you will not be able to switch for the first time.
SW turns ON. That is, the ON/OFF operation can have a certain time constant. Since the level of AM broadcast waves fluctuates due to the fading phenomenon (variations in electric field strength), the S meter voltage also fluctuates accordingly. Therefore, by providing the above-mentioned hysteresis characteristic means, frequent ON/OFF operations of the switch means SW due to fluctuations in the S meter voltage can be prevented.

Next, the effect will be explained. When the input level of the received signal is at the normal reception level and the S meter voltage is less than the reference level, the output from comparator _U2 is "H" level and the normally closed switch means SW is maintained in the ON state, The modulation feedback loop L operates properly. As a result, the AM modulation degree of the broadcast reception signal apparently increases, and the S/N ratio is improved (this will be described later). If the input level is modulated and feedback is applied during such broadcast reception, the S/
When the S meter voltage increases to an excessive level that causes deterioration of N and the S meter voltage exceeds a predetermined level, the output of the comparator _U2 changes to the "L" level and the normally closed switch means SW turns OFF. As a result, the modulation feedback loop L becomes inactive, thereby preventing deterioration of the S/N ratio.

Furthermore, when the input level of the broadcast reception signal is at a weak signal level, the high-cut circuit HC operates to cut out high-frequency components, which tends to lower the overall audio output level and cause a change in volume. However, in this invention, modulation feedback is applied when the signal level is weak, so the audio output level is raised during high-cut operation to prevent changes in volume.

Here, the reason why the S/N is improved by modulation feedback will be explained using an equation.

Input signal wave Us(t)=(1+Mscosωt)cos(st+ _φs ) Local oscillation signal wave Uh(t)=(1+Mpcosωt)cos(ht+φ) When mixed, Us(t)・Uh(t)=b[1+MsMp/ 2+(Ms+Mp)cosωt+MsMp/2
cos2ωt] × {cos[(h+s)t+φ _h +φ _s ]+cos[(s
−h)t+φ _{h −φs} ]} Here, (h+s)t, 2ω are the low-pass filter LPF
It will be removed.

Therefore, Us(t)・Uh(t) = b[1+MsMp/2][1+(Ms+Mp)/1+(Ms
+Mp)/2·cosωt] ×cos[(s−h)t+φh−φs] It can be seen from the above equation that the modulation degree is increasing. As a result, the S/N ratio improves.

[Effect of the invention] According to this invention, the input level of the received signal is
The modulation feedback loop is opened when the signal increases to an excessive level where applying modulation feedback would actually degrade the S/N.
It is possible to improve the S/N at a normal received signal level, and at the same time, there is an effect that the S/N does not deteriorate even if the input level increases.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an AM receiver according to this invention, and FIG. 2 is a characteristic diagram for explaining the operation of the same embodiment. AMP...Buffer amplifier, HC...High cut circuit, IF/AMP...Intermediate frequency amplifier, L...
Modulation feedback loop, LPF...low-pass filter, MIX
...mixer, OSC ...local oscillator, U ₁ ...signal level rectifier circuit, U ₂ ...comparator, U ₃ ...
Reference voltage generator, U ₄ ...Schmitt circuit.

Claims (1)

[Scope of utility model registration request] It is equipped with a modulation feedback loop that feeds back the audio output signal component to the local oscillator in the frequency conversion circuit and applies AM modulation to the oscillation signal of the local oscillator, and mixes the AM modulated oscillation signal from the local oscillator with the received signal. In the AM receiver which apparently increases the degree of modulation of the received signal, a normally closed switch means is interposed in the modulation feedback loop, and the modulation feedback loop is configured to provide a normally closed switch means to control the intermediate frequency output component when the received signal level is equal to or higher than a predetermined level. An AM receiver characterized in that the normally closed switch means is turned off by an output control signal.