JPH0354461Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial application field The present invention provides a mixing circuit that can eliminate reception interference caused by non-operating band RF circuits to the mixing circuit in a multi-band receiving type CATV tuner that shares a mixing circuit. Regarding RF signal input method.

Conventional technology A CATV tuner 1 that receives VHF bands in three bands, low band, high band, and hyperband, conventionally has an RF circuit 2, as shown in FIG.
2', mixing circuit 3, 3' and local oscillation circuit 4, 4'
Independent tuner circuits A and B with a
(Refer to Publication No. 56841).

In order to make the CATV tuner 1 smaller and more compact, a configuration in which the mixing circuit 3 is shared as shown in FIG. 3 has been proposed.

This mixed circuit shared type CATV tuner 5 is
Output of RF circuit 2' for VHF low band and high band reception and RF for VHF hyperband reception
The output of the circuit 2 is inputted to one input terminal 3a of a single mixing circuit 3 via switches 6 and 6' which are selectively opened.

In this configuration, one RF circuit [e.g.
When making the VHF hyperband RF circuit 2] perform reception operation, close the switch 6 and connect the other RF circuit.
Circuit [e.g. VHF low band/high band RF
By opening switch 6' of circuit 2', only one RF circuit is connected to mixing circuit 3, and the other RF circuit is connected to mixing circuit 3.
The circuits are separated and only the output of one RF circuit is input to the mixing circuit 3.

Each RF circuit 2, 2' constitutes a tuning circuit by coils 7, 7... and variable capacitance diodes 8, 8..., and the RF circuit 2' for VHF low band and high band is connected in series. High band reception and low band reception are switched depending on whether or not the connecting point of the two coils is grounded by switch 9.

Problems to be Solved by the Invention In the conventional configuration shown in FIG.
6' cannot perform a complete blocking operation when viewed from a high frequency perspective.

That is, even if a corresponding switch is opened in order to isolate one RF circuit that is not in reception operation from the other circuit that is in operation, a capacitance C exists between both terminals of this switch from a high frequency perspective.

For this reason, unnecessary signals may be injected into the mixing circuit 3 from the RF circuit that does not perform reception operation, or this
The tuned circuit in the RF circuit becomes an absorption trap,
Interfere with reception by attenuating some of the output signal components of other operating RF circuits.

Means for Solving the Problems The present invention was made to solve the above conventional problems. In the receiving CATV tuner, the output end of the first RF circuit for receiving VHF high band and low band and the output end of the second RF circuit for receiving VHF hyperband,
They are connected to different input terminals of the differential amplifier provided on the input side of the single mixing circuit, and a grounding switch circuit is provided for each input terminal. When one RF circuit is in reception operation, the output of the other RF circuit is This CATV tuner circuit is characterized in that a grounding switch circuit of a receiving input terminal is short-circuited.

Effect According to the above means, one RF circuit is connected to each of two input terminals by using a differential amplifier, and two grounding switch circuits are provided corresponding to each RF circuit. Since the output terminal of the RF circuit that is not operated for reception is grounded by one side,
The RF circuit during non-receiving operation can be almost completely isolated from other circuits in terms of high frequency.

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

In the CATV tuner circuit 10 shown in FIG. 1, 11 is an RF circuit for hyperband, which is composed of a coil 12, variable capacitance diodes 13, 13, and the like. 14 is for low band and high band reception.
In the RF circuit, two coils 15 connected in series,
15, switch 1 that grounds the connection point of the variable capacitance diodes 16, 16 and the two coils 15, 15;
Consists of 7th magnitude. Reference numeral 18 denotes a mixing circuit, in which a differential amplifier 20 in which two transistors 19, 19 are connected in parallel is provided at the front stage, and two input terminals 21, 2 are connected.
2 is formed. 23 and 24 are this input terminal 2
A grounding switch circuit such as a diode switch is provided between each of 1 and 22 and the ground, and 25 is a local oscillation circuit.

In the above configuration, when the hyperband RF circuit 11 is operated for reception, one grounding switch circuit 23 provided at the input terminal 21 receiving this output is opened, and the other grounding switch circuit 24 is opened.
Close. Also low band and high band RF
When the circuit 14 is to perform a receiving operation, one of the grounding switch circuits 23 is closed and the other grounding switch circuit 24 is opened.

As a result, the output terminal of the RF circuit during non-receiving operation is grounded by the grounding switch circuit, and is almost completely isolated from other circuits in terms of high frequency, so that only the output of the RF circuit during reception operation is grounded. , differential amplifier 2
It is input to 0. Differential amplifier 20 has input terminal 2
Regardless of whether the RF output is input from 1 or 22,
At that time, the other input terminal is grounded, so
An amplified output corresponding to this RF output is supplied to the mixing circuit 18 to perform a predetermined mixing operation.

Effects of the invention According to the invention, in a multi-band reception type CATV tuner in which the circuit configuration is simplified by sharing the mixing circuit, the RF outputs of the two RF circuits are
The two input terminals of a differential amplifier equipped with a grounding switch circuit are connected to each other, and the input terminal corresponding to the RF circuit that is not in reception operation is selectively grounded, so that only the output of the RF circuit that is in reception operation is connected. Since the signal is input to the mixing circuit, the RF circuit in non-receiving operation can be almost completely isolated from other circuits in terms of high frequency. Therefore, the conventional mixed circuit shared type
Reception interference caused by capacitance between switch terminals, which was a problem with CATV tuners, can be eliminated.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the configuration of a CATV tuner circuit that is an embodiment of the present invention, Figure 2 is a block diagram showing the configuration of a conventional CATV tuner having two independent tuner circuits, and Figure 3 is a block diagram showing the configuration of a conventional CATV tuner circuit that has two independent tuner circuits. FIG. 2 is a schematic configuration diagram of a shared CATV tuner that receives multiple bands. 10... CATV tuner, 11... RF circuit for hyperband reception, 14... RF circuit for low band and high band reception, 18... Mixed circuit,
20... Differential amplifier, 21, 22... Two input terminals, 23, 24... Grounding switch circuit.

Claims (1)

[Scope of utility model registration request] In a CATV tuner that receives the VHF band in three bands: high band, low band, and hyperband, the output end of the first RF circuit that receives the high band and low band, and the output end of the second RF circuit that receives the hyperband. are connected to different input terminals of a differential amplifier provided on the input side of a single mixing circuit, and a grounding switch circuit is provided at each input terminal, so that when one RF circuit is in reception operation, the other RF circuit is A CATV characterized in that the input terminal that receives the output is grounded.
Chuna circuit.