JPH0127303Y2 - - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to improvements in front-end circuits.

Generally, the front end circuit of a tuner is
As shown in FIG. 1, after a signal from an antenna input circuit 1 is selectively amplified by a high frequency amplifier circuit 2, a mixing circuit 3 such as a dual gate/MOS
One gate terminal of type field effect transistor _Q1
At the same time, the local oscillation output from the local oscillation circuit 4 is input to _{the other gate terminal G 2 ,} and the mixed output of both signals is passed through the intermediate frequency resonant circuit 5 to take out the intermediate frequency signal (10.7MHz). , has a configuration in which this intermediate frequency signal is input to an intermediate frequency amplification circuit 6.

In such a configuration, one of the causes of distortion occurring in the intermediate frequency amplification section is that the group delay characteristic of the intermediate frequency filter is not flat, but in particular, the group delay characteristic of the front end intermediate frequency resonant circuit 5 causes distortion. This may cause problems.

That is, as shown in FIG. 2, the group delay characteristic of the intermediate frequency resonant circuit 5 becomes more monophonic as the Q _L of the coil L becomes larger, and the distortion increases. Also,
The smaller the Q _L of the coil L, the flatter the characteristics and the less distortion. Considering this phenomenon in relation to the electric field strength, when the electric field strength is small, the group delay characteristic becomes monophonic, and when the electric field strength is sufficiently large, the transistor Q ₁ is saturated and the Q _L of the coil L is This is equivalent to dumping, and the group delay characteristic becomes almost flat (Figure 3).

In this way, the group delay characteristics of the intermediate frequency resonant circuit 5 are as follows: When the Q _L of the coil L is small, or when the electric field strength is sufficiently large, it is equivalent to the transistor Q ₁ being saturated and the Q _L of the coil L being dumped. However, on the other hand, the gain of the intermediate frequency resonant circuit 5 becomes smaller and the sensitivity becomes worse.

Therefore, conventionally, as shown in Fig. 1, a damping resistor R is connected in parallel with the intermediate frequency resonant circuit 5 to damp the Q _L of the coil L to balance sensitivity and distortion. was. Therefore, the sensitivity is insufficient when the electric field strength is low, and even when the electric field strength is sufficiently large and high sensitivity is not required, there is a drawback that sufficient distortion cannot be reduced.

The present invention was developed in view of these points, and when the electric field strength is large enough and high sensitivity is not required, the group delay of the intermediate frequency resonant circuit can be reduced by actively damping the Q of the coil of the intermediate frequency resonant circuit. It is characterized in that the characteristics are made sufficiently flat to significantly reduce distortion, and high sensitivity is maintained when the electric field strength is small.

An embodiment of FIG. 4 will be explained below. In the figure, the same parts as those in the conventional example shown in FIG.

In the figure, the output end of the intermediate frequency resonant circuit 5 is connected to a damping resistor 7, and a switching circuit 8, for example, a transistor Q2, is connected in series to the damping resistor ₇ .
The intermediate frequency signal taken out from a predetermined position of the intermediate frequency amplifier circuit 6 is grounded through the diode D.
After rectifying the voltage, it is smoothed and exchanged into a DC voltage, and this DC voltage is input to the control terminal (base terminal) of the transistor _Q2 as a field strength detection voltage.

The operation of the above configuration will be explained.

(1) When the electric field strength is low Since the level of the intermediate frequency signal is low and the field strength detection voltage is small, the transistor _Q2 is kept in a cut-off state, and therefore high sensitivity is maintained. In other words, unlike the conventional example, the Q _L of the coil L is not damped by the resistor R in order to balance sensitivity and distortion, so the group delay characteristic is monophonic as shown in Figure 5. There is no improvement in terms of distortion, but high-sensitivity reception is possible.

(2) When the electric field strength is sufficiently large When the level of the intermediate frequency signal is large and the field strength detection voltage becomes large enough to make the transistor Q ₂ conductive, the transistor Q ₂ becomes conductive, and the output terminal of the intermediate frequency resonant circuit 5 becomes conductive. is grounded via the damping resistor 7. Then, the Q _L of the coil L is sufficiently damped and becomes small, so
The group delay characteristic of the intermediate frequency resonant circuit 5 becomes sufficiently flat as shown in FIG. 5, and distortion is significantly reduced.

In this case, the electric field strength is sufficiently large, so high sensitivity is not required.

As described above, in the present invention, when the electric field strength is sufficiently large, the Q of the coil of the intermediate frequency resonant circuit 5 is actively damped, so when the electric field strength is small, the sensitivity is high, and when the electric field strength is sufficiently large, the sensitivity is high. has the advantage of significantly reduced distortion and good reception.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of a conventional front-end circuit, FIGS. 2 and 3 are explanatory diagrams, and FIG. 4 is a diagram showing the configuration of the front-end circuit of the present invention.
FIG. 5 is an explanatory diagram of the same. 1...Antenna input circuit, 2...High frequency amplification circuit, 3...Mixing circuit, 4...Local oscillation circuit, 5...
...Intermediate frequency resonant circuit, 7...damping resistor,
8...Switching circuit.

Claims (1)

[Scope of utility model registration request] After selectively amplifying the signal from the antenna input circuit 1 by the high frequency amplifier circuit 2, the signal is inputted to the mixing circuit 3 together with the local oscillation output from the local oscillation circuit 4,
In a configuration in which a mixed output output from the mixing circuit 3 is input to an intermediate frequency resonant circuit 5 and an intermediate frequency signal is taken out from the intermediate frequency resonant circuit, the output end of the intermediate frequency resonant circuit 5 is connected to a damping resistor. 7. Grounding via a switching circuit 8 connected in series to the resistor, and controlling opening/closing of the switching circuit 8 by the electric field strength detection voltage;
The front panel is characterized in that when the field strength detection voltage exceeds a predetermined level, the switching circuit 8 is brought into conduction to control the group delay characteristic of the intermediate frequency resonant circuit 5 to be flat. end circuit.