JPH02125509A - Attenuator - Google Patents

Abstract

PURPOSE:To expand the variable range of an AGC circuit equivalently and to improve the distortion of a receiver by providing a 1st switching element connected in cascade with a signal input terminal, an attenuation circuit and a 2nd switching element connected in parallel between the signal input terminal and an output terminal of the attenuation circuit. CONSTITUTION:A bias drive circuit 50 drives a 1st bias circuit 21 when a level of an inputted detection output exceeds a high decision level, the 1st bias circuit 21 is driven and even when the detection output level is lowered, the drive state is kept till the level goes to a low level or below. When the output level of the detection circuit 40 goes to a low level of the bias drive circuit 50 or below, the 1st switching element 11 is interrupted and a 2nd bias circuit 22 is driven simultaneously and the 2nd switching element 12 is conductive. Thus, it is equivalent to the expansion by the attenuation of the attenuator 30 for the variable range of the AGC and the distortion is improved remarkably.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an attenuator, and particularly to an attenuator suitable for a receiving device.

[Conventional technology]

When receiving antenna radio waves transmitted from broadcasting stations or other sources, the strength of the spatial electric field of the received radio waves generally varies significantly depending on the receiving location, topography, environment, etc. The signal-to-noise ratio of the received signal changes significantly depending on the presence or absence of a nearby noise source. In order to always receive antenna radio waves satisfactorily despite such significant changes in reception conditions, conventional techniques have included inserting a manually operable attenuator into the input side of the receiving device, and using automatic gain controls such as transistors. There were some that inserted an attenuator that took advantage of the AGC) characteristics.

FIG. 3 is a block diagram of a radio wave receiving section of a conventional receiving device.

A received radio wave inputted from the antenna 1 through the signal input terminal 2 passes through the capacitor C1 attenuator 31, and is inputted from the signal output terminal 3 to the amplifier or mixer of the receiving device.

FIG. 4 is a circuit diagram of a first example of a conventional attenuator.

This attenuator consists of fixed resistors Rr, R2 and variable resistor VR
It is a T-type circuit using a variable resistor VR, and is adjusted by manually operating a variable resistor VR.

FIG. 5 is a circuit diagram of a second example of a conventional attenuator.

This attenuator uses a transistor T to pass a part of the signal output to the signal output terminal 3 to the detection circuit 40 and the terminal VAGC1.
Feedback to the base of the transistor T via the resistor R3,
It performs automatic gain control.

[Problem to be solved by the invention]

However, when an attenuator whose attenuation amount is manually varied is used, there is a drawback that it cannot sufficiently follow significant changes in the spatial electric field strength of received radio waves. In addition, in the case of automatic gain control using an attenuator that utilizes the AGC characteristics of transistors, etc., the more you try to widen the range of the spatial electric field of the received radio wave from the weak electric field to the strong electric field, the more you There is a need for a receiving device with a wide gain control width and a front-end amplifier with a small NF and good linearity. An AGC that simultaneously satisfies these three contradictory requirements regarding gain control width, NF, and linearity.
Although it is not impossible to design and manufacture a receiving device with a functional attenuator, it is technically difficult and a major factor in increasing costs.

SUMMARY OF THE INVENTION An object of the present invention is to provide an attenuator suitable for use before the front end of a receiving device, that is, immediately after the antenna circuit.

[Means to solve the problem]

The attenuator of the present invention includes a signal input terminal, a first switching element and an attenuation circuit connected in cascade with the signal input terminal, and a first switching element and attenuation circuit connected in parallel between the signal input terminal and an output terminal of the attenuation circuit. a second switching element, a means for controlling the second switching element to a conductive state when the output of the attenuation circuit is smaller than a predetermined value, and a means for controlling the second switching element to a conductive state when the output of the attenuation circuit exceeds a predetermined value. and means for controlling the switching element to be in a conductive state.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of one embodiment of the present invention.

The attenuator of this embodiment includes a signal input terminal 2 and a first switching element 11 connected in cascade with the signal input terminal 2.
and an attenuation circuit 30, a second switching element 12 connected in parallel between the signal input terminal 2 and the output terminal of the attenuation circuit 30, and a second switching element 12 connected in parallel between the signal input terminal 2 and the output terminal of the attenuation circuit 30; The bias circuit 22, bias drive circuit 50, and detection circuit 40 serve as means for controlling the element 12 to be in a conductive state, and when the output of the attenuation circuit 30 exceeds a predetermined value, the first switching element is controlled to be in a conductive state. It is configured to include a bias circuit 21, a bias drive circuit 50, and a detection circuit 40 as means.

Next, the operation of this embodiment will be explained.

When the operation of the received signal starts, the output level of the detection circuit 40 is °'0゛, and this output value is applied to the bias drive circuit 50.
Since it is below the low level of the judgment level, the bias drive circuit 5
0 drives the second bias circuit 22 and makes the second switching element 12 conductive, and the high frequency current from the antenna 1 is 1. The signal is inputted to the signal output terminal 3 through the second switching element 12 .

Next, an excessive input of a spatial electric field (for example, 120
dBμ/75Ω), the output level value of the detection circuit 40 rises, and when it finally reaches or exceeds the high judgment level, the drive of the second bias circuit 22 of the bias drive circuit 50 is stopped, At the same time, the first bias circuit 21
starts to drive, thus making the first switching element 11 conductive, so that the high frequency current from the antenna 1 passes through the first switching element 11, passes through the attenuator 30, and is input to the signal output terminal 3. At this time, the output level of the signal output terminal 3 decreases due to the attenuator 30, but the low level of the judgment level is set so that the output level of the detection circuit 40 does not fall below the low judgment level of the bias drive circuit 50. I'll keep it.

By the way, this bias drive circuit 50 drives the first bias circuit 21 when the level of the input detection output exceeds the high level of the judgment level, and thereafter, even if the detection output level decreases, it will not fall below the low level. This driving state is maintained until the Next, when the voltage falls below the low level, it has the function of switching to driving the second bias circuit 22, so that the path passing through the attenuator 30 described above is maintained.

However, as the electric field strength of the antenna input gradually decreases, the output level of the detection circuit 40 finally reaches the level of the bias drive circuit 5.
0, the driving of the first bias circuit 21 is stopped from the above, and therefore the first switching element 11 is cut off, and at the same time, the second bias circuit 22 is driven, and the second bias circuit 21 is turned off. The switching element 12 is made conductive.

Therefore, a path through which the high frequency current passes through the second switching element 12 is established. The above-described operation is repeated according to the electric field fluctuation of the antenna input.

FIG. 2 is a circuit diagram showing an example of a specific configuration of the circuit shown in FIG. 1.

The circuit configurations of the second bias circuits 21 and 22 are all the same, and are comprised of a signal current blocking choke coil 5.6 and a capacitor 4, as shown in the figure.

The bias drive circuit 50 is composed of a DC amplifier 5, a Schmitt trigger circuit 52, and an inverter 53.

As described above, by automating the operation of switching between the normal first path that does not pass through the attenuation circuit 30 and the second path that passes through the attenuation circuit 30, the A
Now that the GC circuit can be used at the very edge of its effective range, when viewed from the perspective of the entire receiving device, it is equivalent to widening the variable range of A and GC by the attenuation rate of the attenuator 30, which is a significant improvement. , - distortion can be improved.

In the above embodiments, a diode is used as the switching element, but the diode may be either a PN junction diode or a PIN diode, and a transistor or a relay may also be used instead of the diode.

〔Effect of the invention〕

As explained above, the present invention can be applied to an existing AG of a receiving device.
This has the effect that the variable range of the C circuit can be equivalently expanded, thereby improving the distortion of the receiving device, and that the variable range of AGC can be widened at low cost and with low distortion.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a detailed circuit diagram of the embodiment shown in FIG. 1, FIG. 3 is a block diagram of a radio wave receiving section of a conventional receiving device, and FIG. 4 and FIG. 5 are circuit diagrams of first and second examples of conventional attenuators, respectively. DESCRIPTION OF SYMBOLS 1... Antenna, 2... Signal input end, 3... Signal output end, 11.12... Switching element, 21.2
2...Bias circuit, 30...Attenuation circuit, 40...
Detection circuit, 50...bias drive circuit, 51...C
D amplifier, 2... Schmi I. 1~Riga circuit, 53... Invert.

Claims (1)

[Claims] a signal input terminal; a first switching element and an attenuation circuit connected in cascade with the signal input terminal; a second switching element connected in parallel between the signal input terminal and the output terminal of the attenuation circuit; means for controlling the second switching element to be conductive when the output of the attenuation circuit is smaller than a predetermined value; and means for controlling the first switching element to be conductive when the output of the attenuation circuit exceeds the predetermined value. and controlling means.