JPS632491B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a receiver using a phase lock loop (hereinafter referred to as PLL), which reliably removes unnecessary signals such as spurious signals.

In a receiver that shares the audio tuner circuit of a television receiver (hereinafter referred to as TV) and the tuner circuit of an FM receiver, and whose front end is configured using a PLL, the AGC circuit is activated only when operating as a TV tuner. , the AGC circuit is not activated when operating as an FM tuner.

However, when using such a receiver to receive FM broadcasts in areas with strong electric fields, the front end may be affected.
Since AGC is not applied, high frequency gain is high,
There is a problem in that unnecessary signals generated as spurious signals are received as they are.

The present invention provides a receiver that solves these conventional problems.

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

In Figure 1, 1 is an antenna, 2 is a front end including a PLL, 3 is an amplification/detection unit that has the function of amplifying the intermediate frequency output of the front end 2 and detecting the amplified output, and 4 is a block of the detected output. 5 is a low frequency output terminal, 6 is a part of the intermediate frequency output of the amplification/detection section 3, and applies AGC to the front end 2.
AGC control circuit, 7 is a control circuit composed of a microcomputer, etc., 8 is a sweep switch, and 9 is an A/D converter.

FIG. 2 shows a specific configuration of the AGC control circuit 6, in which 10 is an input terminal for the signal from the amplification/detection section 3, 11 is an output terminal for the AGC signal, 12 is an operational amplifier (hereinafter referred to as an operational amplifier), and 13 is an output terminal for the AGC signal. is a mutating pulse input terminal, Q ₁ is a buffer transistor, Q ₂ is a detection transistor, Q ₃
is AGC control transistor, C ₁ to _{C 4} are capacitors, R ₁ to _{R 10} are resistors, VR ₁ to _{VR 2} are volume,
D ₁ is a diode and D ₂ is a Zener diode.

In the above configuration, when receiving TV audio, a stepwise sweep pulse as shown in FIG. 1 is supplied from the output terminal a of the control circuit 7 to the front end 2 by operating the switch 8.
A desired broadcast signal is received by sequentially varying the oscillation frequency of the VCO. At this time, the intermediate frequency signal from the amplification/detection section 3 is applied to the input terminal 10 of the AGC control circuit 6, and the buffer transistor Q ₁ , the detection transistor Q ₂ , the operational amplifier 12 , the decoupling resistor R ₈ for ripple removal, and the capacitor C _Four ,
It is output as an AGC signal to the output terminal 11 via the resistor ^R9 , and the high frequency gain of the front end 2 is reduced by this AGC signal, so-called AGC is applied. At this time, if the signal level in the amplification/detection section 3 is below a predetermined level, the A/D converter 9 outputs a pulse as shown in FIG. 1, which is input to the terminal 6 of the control circuit 7. . Then, in the control circuit 7, the pulse width is expanded by keeping the rise time the same and delaying the fall time, and a wide muting pulse as shown in FIG. 1 is output from the terminal C of the control circuit 7, and this is mutated. In addition to circuit 4, noise during sweeping is removed. This muting pulse was also applied to the input terminal 13 of the AGC control circuit 6, and the transistor _Q3 was turned on only during the period when the muting pulse was applied, dividing the AGC output between the resistor _R9 and the volume _VR2 . lower to the level. Therefore
If it is assumed that the lower the AGC output is, the lower the high frequency gain of the front end 2 is, the AGC will be applied more deeply only during the period when the muting pulse exists, and reception of unnecessary signals can be reliably blocked.

On the other hand, during FM reception, the reception frequency was varied by the sweep pulse from the control circuit 7, as in the case of TV audio reception, but at this time, the reception frequency was varied by the sweep pulse from the control circuit 7.
The signal path applied to the input terminal 10 of the AGC control circuit 6 is cut off, and normal AGC no longer operates.
That is, at this time, the reference output of the operational amplifier 12 in the AGC control circuit 6 is directly connected to the output terminal 1.
1 and is fixedly supplied to the front end 2. Therefore, the high frequency gain remains fixed at a certain constant value. However, even in this case, as long as the signal level of the amplification/detection section 3 is below a predetermined level, the A/D converter 9 outputs a pulse.
A muting pulse is output from terminal C of the control circuit 7. Therefore, the low frequency signal line is cut off by the muting circuit 4 to eliminate unpleasant noise during sweeping, and at the same time, this muting pulse is also supplied to the terminal 13 of the AGC control circuit 6.
When the transistor Q ₃ is turned on, the AGC output appearing at the output terminal 11 drops from the reference output of the operational amplifier 12 to the level obtained by dividing it by the resistor R ₉ and the volume VR ₂ . As a result, AGC is instantaneously applied to the front end 2 only during the period when the muting pulse exists, and the high frequency gain is reduced. Therefore, even when receiving in a strong electric field area, it is possible to reliably prevent the problem of erroneously receiving unnecessary signals caused by spurious signals.

In addition, in the above embodiment, when receiving TV audio, the normal
AGC and AGC using muting pulses are applied at the same time, but since TV audio originally has wide channel spacing, there is no practical problem even if only normal AGC is applied. Also, although the AGC due to the mutating pulse is applied only momentarily, the front end 2 is connected to the PLL.
Since the device is constructed using a so-called frequency synthesizer method including the following, it is sufficient to achieve the desired purpose simply by instantaneously lowering the high frequency gain in synchronization with a pulse that sweeps the reception frequency.

As described above, the present invention uses a muting pulse to reduce the high frequency gain of the front end, so it is suitable for use in areas with strong electric fields.
Even when receiving FM broadcasts, it is possible to reliably solve the problem of erroneously receiving unnecessary signals caused by spurious signals.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
This figure is a circuit diagram of a specific configuration of the AGC control circuit shown in FIG. 1. 1...Antenna, 2...Front end, 3...
...Amplification/detection section, 4...Mutating circuit, 5...
...Output terminal, 6...AGC control circuit, 7...Control circuit, 8...Sweep switch, 9...A/D converter, 10...Input terminal, 11...AGC signal output terminal, 12... ...Operational amplifier, 13... Muting pulse input terminal.

Claims (1)

[Claims] 1 a front end having a phase lock loop; means for generating a muting pulse when the output signal of the front end is below a predetermined level; and means for reducing the high frequency gain during the presence of the receiver.