JPS6243573B2 - - Google Patents

Description

Detailed Description of the Invention The present invention provides an S/N ratio of a local oscillation circuit of an FM receiver.
Regarding the improvement of

Generally, when the local oscillation circuit of an FM receiver is constructed using a varicap, it is constructed as shown in block A in Fig. 1, and the capacitance of the varicap D is changed by applying a control DC voltage V to the terminal IN. This changes the oscillation frequency. However, if the control DC voltage V contains an AC component such as hum, the oscillation frequency will drift due to the AC component. For this purpose, a method is used to remove the alternating current component using a decoupling capacitor _C1 , but in a 9V system variable cap, the relationship between voltage and oscillation frequency is approximately 1MHz/
1V, or 1Hz/1μV, with 100% modulation
Since it is 75KHzdev, the noise when the S/N is 80dB is 7.5Hzdev, so the varicap has 7.5KHzdev.
If μV AC component is applied, the S/N will be 80dB.
You can't do more than that.

In the experiment, a decoupling capacitor was
Even if the value is 1000μV, it is difficult to remove the 7.5μV hum component, and a decoupling capacitor with a larger value inevitably has a large time constant.
This causes problems in response time and poor tuning feeling.

The present invention has been developed in view of this point, and is designed so that alternating current components such as hum superimposed on the control direct current voltage are applied to both ends of the varicap in substantially the same phase, so that the capacitance of the varicap does not change due to the alternating current components. The purpose of this is to improve the S/N ratio.

An embodiment of the present invention will be described below with reference to FIG.

Block A' represents an oscillation circuit, and a resistor _R3 is connected between one end of the variable cap D, that is, the anode, and the ground. This resistor _R3 is made sufficiently small with respect to the insulation resistance of the varicap D, so that the entire control DC voltage V is applied to the varicap D. For example, the insulation resistance of the varicap D is about 10 MΩ, whereas the resistor R ₃ is about 100 KΩ. One end of a coil _L2 , which determines the oscillation frequency between the varicap D and the varicap D, is connected to the other end of the varicap D, that is, the cathode. This coil L ₂ has an inductance of 0.1 μH, and if the variable cap D has a variable capacitance range of 59 to 26 pF, the local oscillation frequency is 65.3 to 79.3 MHZ (lower local of the FM receiver). frequency) is obtained. Also, the other end of this coil _L2 and the varicap D
A capacitor C ₃ of approximately 10 _μF is connected between the capacitor C 3 and the connection point with the resistor R 3, and a control DC voltage V is applied to the connection point between the capacitor C ³ and the coil L ₂ via the resistor R ₂ . is configured to do so.

Note that the capacitor C 2 connected between the connection point of the coil L ₂ and the capacitor C _{3 and} the ground functions as a decoupling device.

As described above, the control DC voltage V is applied to the varicap D via the resistor _R2 and the coil _L2 , and the varicap D is determined by this control DC voltage V.
The oscillation effect is performed by the capacitance of L2 and the inductance of the coil _L2 .

Here capacitor _C3 is 10μF as mentioned above
Since the coil L ₂ is selected to have a value of 0.1 μH, the impedance of the capacitor C ₃ will be reduced against the hum component coming from the commercial power supply at approximately 50 to 60 Hz.
Zc is Zc≒300Ω, impedance Zl of coil _L2 is
Since ZL≒1Ω, it exhibits a sufficiently low impedance, and the resistor R ₃ is set to 100KΩ, which is sufficiently large compared to the impedance of the capacitor C ₃ , AC such as hum superimposed on the DC control voltage V. The components are applied to both ends of the varicap D in phase. As a result, the potentials at both ends of the varicap are the same with respect to the AC component, that is, there is no potential difference, and therefore the varicap D changes its capacitance only with respect to the control DC voltage.

As described above, the oscillation frequency drift due to the AC component superimposed on the control DC voltage is eliminated, and high S/N reception becomes possible.

[Brief explanation of the drawing]

Figure 1 is a local oscillator circuit diagram of a conventional FM receiver.
FIG. 2 is a local oscillation circuit diagram of an embodiment of the present invention.

Claims (1)

[Claims] 1. In a local oscillation circuit of an FM receiver that applies a control DC voltage to a varicap to determine the oscillation frequency, the current of the control DC voltage connected between the varicap and one end of the varicap and the ground and applied to the varicap. A resistor, one end of which is connected to the other end of the varicap, determines the oscillation frequency between the resistor and the other end of the varicap, and a coil exhibiting low impedance to the commercial power frequency; One end of the coil is connected to the capacitor, the other end is connected to the connection point between the variable cap and the resistor, and the capacitor exhibits low impedance with respect to the commercial power frequency, and the control DC voltage is connected to the connection point between the coil and the capacitor. A local oscillation circuit for an FM receiver, characterized in that it applies .