JPS62283721A - All band tuner - Google Patents

Abstract

PURPOSE:To relax deterioration of a noise factor (NF) of a system, which is caused by a loss of an AGC circuit, by providing a high frequenty amplifying circuit on the front stage. CONSTITUTION:A high frequency amplifying circuit 50 in which an input part and an output part have been separated as a high frequency is provided on the front stage, therefore, even if an input voltage standing wave ratio (VSWR) of an AGC circuit 60 of the rear stage is deteriorated due to an AGC, an input VSWR of the high frequency amplifying circuit 50 is not deteriorated, and accordingly, a reflected wave from a tuner is suppressed. Also, a loss of the AGC circuit 60 of the rear stage, and also, a loss of a filter 70 are not so much concerned in an NF of a system. Moreover, a fact for reducing a loss in the AGC circuit, and fact for increasing a gain suppression amount are each contrary item but the AGC circuit which can be obtain a large gain suppression amount by enlarging a little an allowable loss can be designed easily.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Field of Industrial Application] This invention provides an all-band tuner, that is, VHF, UH
The present invention relates to tuners for F and CATV broadcasting, and in particular to improvements around the high frequency amplification section.

[Conventional technology]

General VHF and UHF TV tuners are
As shown in the figure, VHF high frequency signals RFv and UH
Input terminals 10 to which high frequency signals RF of F are respectively inputted.
．． 20, a tuning circuit 11.21 interlocked with the tuning voltage and a high frequency amplification circuit 12.22 are provided, for example a mixer 13.23 is provided at the subsequent stage, and AGC is applied directly to the high frequency amplification circuit 12.22. It has a circuit configuration. ■^ is the AGC voltage.

This is because in a TV tuner, if there is a strong level interference wave other than the desired channel, the second and third order intermodulation distortion of those interference waves will fall into the receiving channel.
This is a measure against reception interference. That is,
This is because a tuning circuit 11.21 is provided immediately after the input terminal 10.20 to reduce the level of interference waves input to the high frequency amplifier circuit 12.22 and suppress the intermodulation distortion. Also,
In TV tuners, reflected waves from the antenna input terminal rarely cause interference to other receivers.
The specifications for human power VSWR (voltage standing wave ratio) are loose, so there is no problem even if AGC is applied directly to the high frequency amplifier circuit 12.22.

On the other hand, the CATV converter, as shown in Figure 3,
A variable attenuator 31 having AGC characteristics is provided immediately after the input terminal 30 to which the high frequency signal RFC of the ATV is input, and the circuit configuration is such that a tuning circuit is not used.

This is because in CATV broadcasting, the input levels of the high frequency signals RF are almost the same (for example, when the interference wave has a strong level, the desired signal also has a strong level).
This is because, with the circuit configuration as shown in the figure, interference waves having a strong level will not be input to the high frequency amplifier circuit 32, and intermodulation distortion will not become large enough to become a problem. Also, CAT
In the case of a V converter, the reflected wave from it causes interference to other joint receivers, so the specifications for input VSWR are strict, so AGC is not applied directly to the high frequency amplifier circuit 32, and the variable attenuator 31 does not deteriorate the manual VSWR. I try to apply AGO like this. Therefore A
Even if GC is applied, there is almost no interference with co-recipients.

[Problems to be solved by the invention] ``General TV broadcasting and CA
When configuring an all-band tuner that can receive both TV signals, in order to avoid deteriorating the input VSWR and intermodulation distortion characteristics described above, the configuration of the CATV converter shown in Figure 3 is close to the purpose, that is, the configuration of the CATV converter is AGC
It is conceivable to have a circuit configuration in which an attenuator with special characteristics is provided, followed by a broadband high-frequency amplifier circuit without a tuning circuit, but if such a circuit configuration is adopted, the loss of the first-stage attenuator will increase the NF (noise) of the system. index). Therefore, in order to improve NF, AGC
Reverse the order of the attenuator and high frequency amplifier circuit,
A high frequency amplifier circuit is installed immediately after the input terminal, and then an AGC
It is also possible to bring a special attenuator, but in that case the intermodulation distortion characteristics will deteriorate.

SUMMARY OF THE INVENTION An object of the present invention is to provide an all-band tuner that has low distortion and low noise characteristics, has good input VSWR over a wide band, and does not change the input VSWR even when AGC is applied.

[Means for solving problems]

The all-band tuner of the present invention includes a high-frequency amplification circuit using a bush-pull amplifier connected to an input terminal of a high-frequency signal, and an AGC connected to the rear stage of the high-frequency amplification circuit.
It is characterized by comprising a circuit.

[Effect]

In this all-band tuner, since a high frequency amplifier circuit is provided at the forefront stage, deterioration of the system NF due to loss in the AGC circuit is alleviated.

On the other hand, if a high frequency amplifier circuit without a tuning circuit is placed at the front stage, intermodulation distortion becomes a problem, but generally 2
It is known that distortions with large orders, such as the 3rd order and the 3rd order, become smaller. Therefore, when a push-pull amplifier is used as a high-frequency amplifier circuit, the second-order intermodulation distortion, which is the largest component of distortion, is reduced by cancellation, and the permissible human power level for intermodulation distortion is increased.

Furthermore, when a high frequency amplifier circuit is provided at the forefront stage, due to the buffer effect, even if the input VSWR of the AGC circuit at the subsequent stage changes due to AGC, the input VSWR at the input end does not change.

〔Example〕

FIG. 1 is a circuit diagram showing an all-band tuner according to an embodiment of the present invention. This tuner uses an all-band broadcast high frequency signal RF.

A high frequency amplification circuit 50 using a push-pull amplifier is provided immediately after the input @40 into which is input, and an AGC circuit 60 is provided at the subsequent stage.

In this example, the high frequency amplification circuit 50 has two FETs 53 and 54 connected in push-pull between wideband balanced unbalanced transformers 51 and 52 for input and output, thereby amplifying the high frequency signal RFo of all-band broadcasting. I try to do that. Note that this high frequency amplifier circuit 50 is not provided with a tuning circuit.

In this example, the AGC circuit 60 is an attenuator with AGC characteristics that includes two PIN diodes 61 and 62, and it adjusts the gain of the high-frequency signal from the high-frequency amplifier circuit 50 based on the voltage vA supplied from the outside. I'm trying to control it.

In this example, the output from the AGC circuit 60 is supplied to the mixer 80 via a band-switching fixed filter 70, where it is configured to be frequency-converted based on the local oscillation signal LO from the local oscillation circuit 90. has been done.

In the above-mentioned all-band tuner, the high-frequency amplifier circuit 50 in which the input section and the output section are separated in terms of high frequency is provided at the front stage, so that the input VSWR of the rear stage (7) AGC circuit 60 is higher than that of the AGC circuit. Even if the input VSWR of the high frequency amplifier circuit 50 deteriorates over time, the input VSWR of the high frequency amplifier circuit 50 does not deteriorate. Therefore, reflected waves from the tuner can be suppressed.

In addition, since the first stage is the high frequency amplifier circuit 50, the second stage A
The loss of the GC circuit 60 and further the loss of the filter 70 do not contribute much to the NF of the system. Moreover, reducing the loss and increasing the amount of gain suppression in the AGC circuit are contradictory items, but if this configuration is adopted,
Another advantage is that it is possible to easily design an AGC circuit that can suppress a large amount of gain by slightly expanding the allowable loss.

Furthermore, since the high frequency amplifier circuit 50 uses a push-pull amplifier, second-order intermodulation distortion and the like can be suppressed. Therefore, there is no need to provide a tuning circuit, and hence no need for band switching. In other words, one high frequency amplifier circuit 50
That's enough.

As the amplification element for the push-pull amplifier used in the high-frequency amplification circuit 50, it is preferable to use an amplification element that has as good a linearity as possible in its bare characteristics.This will reduce third-order distortion and further improve the tolerance of interference waves. You can increase the input level. For example, it is preferable to provide negative feedback by providing feedback circuits 55 and 56 as shown in this example, and by doing so, third-order distortion can be further suppressed. Moreover, FET53
．． 54, applying voltage feedback lowers the input impedance and makes matching easier, which also has the effect of improving the input VSWR.

Further, although the filter 70 after the AGd circuit 60 is not indispensable, in the VHF low channel and high channel, the second-order distortion of the low channel interference wave falls into the high channel reception channel, so the filter 70 is not necessary. It is preferable to provide a filter that can switch bands such as low-pass when receiving VHFK low channel, bandpass when receiving VHF high channel, and through at other times. Secondary distortion can be further suppressed.

Further, the push-pull amplifier used in the high-frequency amplifier circuit 50 is not necessarily limited to one using FET53,54 as in this example, and the AGC at the subsequent stage
The circuit 60 is not necessarily limited to using PIN diodes 61 and 62 as in this example.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain an all-band tuner that does not require a tuning filter, has low distortion and low noise characteristics, and does not deteriorate the input VSWR even when AGC is applied.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an all-band tuner according to an embodiment of the present invention. FIG. 2 is a block diagram showing relevant parts of a conventional television tuner. FIG. 3 is a block diagram showing relevant parts of a conventional CATV converter. 40... Input end, 50... High frequency amplifier circuit, 53.
54... FET, 60... AGC circuit, 61°6
2... PIN diode, 7o... filter,
80...Mixer, 90...Local oscillation circuit.

Claims (1)

[Claims] (1) An all-band tuner characterized by comprising a high-frequency amplification circuit using a push-pull amplifier connected to an input end of a high-frequency signal, and an AGC circuit connected to a subsequent stage of the high-frequency amplification circuit.