JPH0546181U - Audio intermediate frequency conversion circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] When the output signal of the local oscillator circuit 1 is mixed with the audio intercarrier signal by the mixer circuit 2 and converted into a signal of a constant intermediate frequency, the audio intercarrier signal is transmitted to the local oscillator circuit. Provided is an audio intermediate frequency conversion circuit which prevents bypassing. [Structure] A varactor diode CD is provided between the local oscillation circuit 1 and the mixer circuit 2. This varactor diode
The value of the coupling capacitance C2 is optimally changed by changing the capacitance of the CD according to the input voice intercarrier signal. still,
The varactor diode CD has a plurality of band pass filters X1, X2, which are provided between the input terminals a and b and the mixer circuit 2.
The bias voltage of the switching diode D that selects X3 is applied.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an audio intermediate frequency (SIF) conversion circuit of a multi-reception system that always converts a variety of audio intercarrier signals that differ depending on the television standard system into a constant intermediate frequency, In particular, the present invention relates to an audio intermediate frequency conversion circuit that mixes with a specific oscillation frequency, frequency-converts it, extracts it as a single-frequency audio intercarrier signal, and performs detection.

[0002]

[Prior Art]

 In receiving circuits such as television receivers, the frequency of the audio intercarrier signal differs depending on the system. For example, it is 5.5 [MHz] in the CCIR / BG method, 6.0 [MHz] in the same I method, and 6.5 [MHz] in the same DK method.

A technique for demodulating a plurality of television standard methods having different audio intercarrier signals by a single audio intercarrier signal demodulation circuit is, for example, Japanese Utility Model Publication No. 60-31340 (H04N 5/46). ). When handling two types of voice intercarrier signals, this switch switches between the output of the local oscillator circuit and the voice intercarrier signal to be mixed, and the voice intercarrier signal not to be mixed, and passes through different paths. At the same time, the local oscillator circuit is turned on and off.

For example, FIG. 2 shows a conventional BG, I, and DK three-system audio intermediate frequency conversion circuit. First, during reception of the BG or DK system, if 5 [V] is supplied to the point c and 0 [V] to the point d of the switching means 3 from the input terminals a and b, respectively, the point e is 0 [V] and f. The point becomes 9 [V]. D is composed of multiple switching diodes, and uses two switching diodes D1 and D2 when receiving in the 3 system.

In the above case, the point g of the first switching diode D1 is 0 [V], and the point h of the second switching diode D2 is 9 [V]. Further, at points i and j, a voltage of 9 [V] is supplied from the power supply terminal + B through the resistor R3, and as a result, the first switching diode D1 is forward biased and the second switching diode D2 is , The reverse bias will be applied.

On the other hand, X1, X2, and X3 are band-pass filters of 5.5 [MHz], 6.5 [MHz], and 6.0 [MH z], respectively, and are caused by conduction of the first switching diode D1. The voice intercarrier signal passes through the band pass filters X1 and X2. Therefore, only the BG and DK voice inter-carrier signals (5.5 [MHz], 6.5 [MHz]) flow in the mixer circuit 2, and 500 [kHz] of the Colpitts type local oscillation circuit 1 is generated. The oscillation component is mixed with the mixer circuit 2 and output to the detection circuit as an 6.0 [MHz] inter-carrier signal. The capacitor C1 is a coupling capacitance between the local oscillation circuit 1 and the mixer circuit 2, and X4 is a band pass filter of 6.0 [MHz].

When receiving the I system, the second switching diode D2 is turned on and the first switching diode D1 is turned off by applying 0 [V] to the point a and 5 [V] to the point b contrary to the above. To As a result, the voice intercarrier signal passes through the band pass filter X3 and is input to the mixer circuit 2. Although not specifically shown, the local oscillation circuit 1 is stopped at the time of receiving the I method.

Further, from the terminal k, an intercarrier signal of 5.5 [MHz] for BG reception, 6.5 [MHz] for DK reception, and 6.0 [MHz] for I reception can be taken out as it is.

[0009]

[Problems to be solved by the device]

 As described above, simply switching the path to the mixer circuit 2 for each method and turning on / off the local oscillation circuit 1 causes the following inconveniences.

The capacitor C1 which is the coupling capacitance between the local oscillation circuit 1 and the mixer circuit 2 has a fixed value, and the voice intercarrier signal is bypassed to the local oscillation circuit 1 side only by turning off the local oscillation circuit 1. May result in voice deterioration.

[0011]

[Means for Solving the Problems]

 The present invention has been made in view of the above-mentioned problems, and by selectively operating local oscillating circuits for voice inter-carrier signals having different frequencies, the different voice inter-carrier signals are always kept at a constant intermediate frequency. In the audio intermediate frequency conversion circuit for outputting to the detection circuit as a signal of, the plurality of audio intercarrier signals input from the first and second input terminals a and b are connected in parallel to selectively pass the audio intercarrier signals. The band pass filters X1, X2, X3, a plurality of switching diodes D arranged in series with the band pass filters X1, X2, X3 and selecting the band pass filters X1, X2, X3, and the voice intercarrier. A switching means 3 for switching the switching diode D by generating a bias voltage according to a signal; and the band pass filter X A mixer circuit 2 to which outputs of 1, X2 and X3 are applied, a local oscillator circuit 1 connected to the mixer circuit 2 and selectively operated according to the frequency of the intercarrier signal, and the local oscillator circuit 1 And a varactor diode CD which is connected between the mixer circuit 2 and the bias voltage of the switching means 3 and whose coupling capacitance changes, and the coupling capacitance is increased when the local oscillation circuit 1 operates. An audio intermediate frequency conversion circuit, characterized in that the coupling capacitance is reduced when the local oscillation circuit 1 is not operating.

[0012]

[Action]

 According to the present invention, since the coupling capacitance C2 between the local oscillator circuit 1 and the mixer circuit 2 changes according to the method of the voice inter-carrier signal, the coupling capacitance C2 increases during the operation of the local oscillator circuit 1. As a result, the impedance is reduced, so that the mixing is performed optimally. When the local oscillation circuit 1 is not operating, the coupling capacitance C2 decreases and the impedance increases, so that the voice intercarrier signal is input to the mixer circuit 2 without bypassing to the local oscillation circuit 1 side.

[0013]

【Example】

 One embodiment of the present invention is shown in FIG. The same parts as those in FIG. 2 are denoted by the same reference numerals.

The feature of the present application is that a varactor diode CD is provided in series with the coupling capacitor C1 between the local oscillation circuit 1 and the mixer circuit 2, and the coupling capacitance C2 (the series of the capacitance of the capacitor C1 and the capacitance of the varactor diode CD is provided. The capacitance is variable and the bias voltage control of the varactor diode CD is based on the switching means 3 for generating the bias voltage of the switching diode D. The cathode side of the varactor diode CD is connected to the point e of the switching means 3 via the resistor R1, and the anode side thereof is connected to the point f via the resistor R2.

Similar to the above-mentioned conventional example, first, during reception of the BG system or the DK system, 5 [V] is supplied to the point c of the switching means 3 and 0 [V] is supplied to the point d, and the first switching diode is supplied. D1 is made conductive and a voice intercarrier signal is applied to the mixer circuit 2 through the band pass filters X1 and X2. Therefore, when receiving the BG method and the DK method, the varactor diode CD is forward-biased and becomes conductive, and the coupling capacitance C2 becomes the capacitance of the capacitor C1. Therefore, the coupling capacitance C1 becomes large and the impedance becomes small. The output of the local oscillation circuit 1 is input to the mixer circuit 2 for mixing, and is taken out as an intercarrier signal of 6.0 [MHz]. And 6. It is input to a detection circuit (not shown) through a 0 [MHz] band pass filter X4.

Contrary to the above, at the time of reception of the I system, by applying 0 [V] to the point c and 5 [V] to the point d, the signal is input to the mixer circuit 2 through the second switching diode D2 and the band pass filter X3. To be done. At this time, the varactor diode CD is reverse-biased, the capacitance increases, the coupling capacitance C2 decreases, and the impedance increases, so the audio intercarrier signal is hardly affected by the local oscillator circuit 1 and the mixer. It is input to the circuit 2. At this time, it is preferable to stop the local oscillator circuit 1 as well.

Further, it is preferable to select the value of the capacitor C1 so that the value of the coupling capacitance is optimum when receiving the BG and DK systems.

[0018]

[Effect of the device]

 Since the present invention is configured as described above, the output from the local oscillation circuit 1 becomes optimum during reception of the BG and DK systems, and the efficiency of frequency conversion in the mixer circuit 2 increases. At the time of reception by the I system, the coupling capacitance between the local oscillation circuit 1 and the mixer circuit 2 becomes small, and it becomes difficult for the audio intercarrier signal to bypass the local oscillation circuit 1 side. Therefore, it is possible to obtain characteristics that can be averaged by a plurality of methods, and it is particularly effective for the voice sensitivity (S / buzz).

Further, since the change of the coupling capacitance is also controlled by the switching means 3 which switches the signal path, a new control means is not required and the structure is very simple.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a main part of an audio intermediate frequency conversion circuit.

FIG. 2 is a circuit diagram of a main part of an audio intermediate frequency conversion circuit.

[Explanation of symbols]

 1 Local oscillation circuit 2 Mixer circuit 3 Switching means C1 Coupling capacitor CD Varactor diode C2 Coupling capacitance D1 First switching diode D2 Second switching diode X1 5.5 [MHz] band pass filter X2 6.5 [MHz] band pass filter X3 , X4 6.0 [MHz] bandpass filter

Claims (1)

[Scope of utility model registration request] 1. An audio intermediate circuit for outputting different audio intercarrier signals to a detection circuit as a signal of a constant intermediate frequency at all times by selectively operating local oscillating circuits of audio intercarrier signals having different frequencies according to the frequencies. In the frequency conversion circuit, a plurality of band-pass filters X1, X2, X3 connected in parallel, which selectively pass the voice intercarrier signals input from the first and second input terminals a and b, respectively, and the band. A plurality of switching diodes D arranged in series with the pass filters X1, X2, X3 and selecting the band pass filters X1, X2, X3; and the switching by generating a bias voltage according to the voice intercarrier signal. Switching means 3 for switching the diode D, and a mixer circuit to which the outputs of the band pass filters X1, X2, X3 are applied. 2, local oscillator circuit 1 for selectively operating in accordance with the frequency of the connected and the intercarrier signal to the mixer circuit 2
And a varactor diode CD which is connected between the local oscillation circuit 1 and the mixer circuit 2 and whose coupling capacitance changes according to the bias voltage of the switching means 3, and the coupling capacitance when the local oscillation circuit 1 is operating. Is increased and the coupling capacitance is decreased when the local oscillation circuit 1 is not operating.