JPH0237725B2 - KAHENGENSUIKI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable attenuator using diodes.

An example of the use of such an attenuator can be found in the AGC circuit of a receiver. By the way, the receiver is
It is required to receive signals without distortion, from weak signals of a few microvolts from distant broadcasting stations to large signals of 1V or more from nearby broadcasting stations, but if an excessive signal enters, it will adversely affect the local oscillation frequency and reduce the output. Causes distortion. For this reason, it is necessary to supply a signal of a constant size to the mixing stage, and when a strong radio wave is received, the gain of the high-frequency amplification stage is automatically lowered depending on the strength, and the input to the mixing circuit is reduced. An AGC circuit is required to keep it constant. AGC circuits are generally
It consists of a detection circuit that rectifies the output of the intermediate frequency stage, and a controlled circuit whose gain is controlled by the detection circuit. There are two ways to apply AGC: forward AGC, which performs gain control by increasing bias current, and reverse AGC, which performs gain control by decreasing bias current.

In order to perform the above-mentioned forward AGC, there has conventionally been a system in which a current is caused to flow through a diode 1 inserted between the signal path and the ground, as shown in FIG. 1a. It is well known that this AGC has better strong input characteristics such as intermodulation than those using other AGC elements such as transistors. However, in this case, the impedance of the signal path is low, so the following
There are drawbacks such as changes in BPF characteristics.

In addition, as shown in Figure 1b, diode 1 is inserted in series in the signal path to perform reverse AGC, but it is well known that in this case, strong input can cause interference such as intermodulation. It is.

In addition to the above, there is also a device in which diodes 1a to 1c are connected in a π-shape as shown in Figure 1c, but in order to maintain a constant impedance, the diodes inserted between the signal path and the ground must be completely connected. Since short pay is not possible, there are drawbacks such as the inability to obtain good strong input characteristics, as in the case described above.

In addition, in FIGS. 1a to 1c, 2 is a signal source, and 3 is a signal source.
is the signal source impedance.

The present invention has been made in view of the above points, and
The purpose is to provide a variable attenuator that can change the amount of signal attenuation while keeping the impedance seen from the output end constant.

The variable attenuator created for this purpose connects a diode between the signal path and ground at the input end of a quarter-wavelength transmission line or an equivalent LC circuit, and connects a diode between the signal path and ground at the output end. A diode and a resistance equivalent to the characteristic impedance of the transmission line or LC circuit are connected in series, and the amount of signal attenuation is changed by controlling the current flowing through both of the diodes.

The present invention will be described in detail below with reference to embodiments shown in FIG. 2 and subsequent figures.

FIG. 2 is a circuit diagram showing an embodiment of a variable attenuator according to the present invention. A signal source 2 having an impedance 3 of _Z0 is connected to an input terminal 4 of the variable attenuator. The signal is sent out from the output terminal 5 after being attenuated by a predetermined amount.

The attenuator has a quarter wavelength (λ/4) transmission line 10 with a characteristic impedance of Z ₀ . The input/output ends of the transmission line 10 are connected to the input/output terminals 4 and 5 via coupling/DC blocking capacitors _C1 and _C2 , respectively. A diode _D1 is also connected to the input end of the transmission line 10 via a coupling and direct current blocking capacitor _C3 between the signal path and ground. and,
At the output end of the transmission line 10, a diode D ₂ and a resistor R ₁ equivalent to the characteristic impedance Z ₀ of the transmission line 10 are connected in series between the signal path and ground.

Cathode of diode D ₁ above and diode D ₂
The anode of the diode D1 is connected to the signal path, and the control voltage terminal 6 is connected to the connection point between the anode of the diode _D1 and the capacitor _C3 via a high frequency blocking resistor _R2 . Therefore, by applying voltage to terminal 6, diode D ₁ , transmission line 10,
Current will now flow through diode _D2 and resistor _R1 . By changing the voltage applied to terminal 6, the current flowing through diodes _D1 and _D2 is controlled.

This current control changes the conductivity of the diodes D ₁ and D ₂ to control their high frequency resistance, thereby changing the magnitude of the signal on the signal path that is dropped to ground through the diodes D ₁ and D ₂ .
That is, a variable attenuator that can change the amount of attenuation of the signal transmitted from the signal source 2 is obtained.

In the illustrated example, by increasing the control voltage, the high frequency resistance of the diodes D ₁ and D ₂ becomes smaller, and the attenuation degree of the length attenuator becomes larger.

Now, when a certain control voltage is applied, the diode
Assuming that the high frequency resistances of D ₁ and D ₂ are R, the variable attenuator shown in FIG. 2 can be represented by an equivalent circuit as shown in FIG. 3. In the equivalent circuit of Fig. 3, when the impedance Z _x of a λ/4 transmission line with characteristic impedance Z ₀ viewed from A-A' in the figure is calculated according to the definition of a distributed constant circuit, Z _x = Z ₀ ² /RZ ₀ =Z ₀ /R・Z ₀ /R+Z ₀ . Therefore, the impedance Z _put seen from the output terminal 5 side is the parallel connection of (R + Z ₀ ) to Zo in the above equation, so Z _put = Z _{x ·} (R + Z ₀ ) / Z _x (R + Z ₀ ) =Z ₀ ² /R・Z ₀ /R+Z ₀ (R+Z ₀ ) /Z ₀ ² /R・Z ₀
/R+Z ₀ +(R+Z ₀ ) =Z ₀ ² (R+Z ₀ )/Z ₀ ² +R・Z ₀ /R+Z ₀ (R+Z ₀
)=Z ₀ ² (R+Z ₀ )/Z ₀ ² +R・Z ₀ =Z ₀ (R+Z ₀ )/Z ₀ +R=Z ₀ , and the impedance is always Z ₀ , which is a constant impedance.

In the embodiment described above, the transmission line 10 of λ/4 is used, but instead of the transmission line 10 having a characteristic impedance of
An LC circuit with Z ₀ and equivalent to the transmission line 10 can also be used. To illustrate some of them using well-known circuits, as shown in FIG.
π-type and T-type as shown in Figure 4c and d, respectively.
HPF type each configured in the mold, Figure 4 e
Examples include gyrator-type ones configured as shown in h to h (in addition, +C and -C are positive capacitance and negative capacitance, and +L and -L are positive inductance and negative inductance).

The above is a typical circuit example, but in short, the F matrix is All the circuits represented by have an impedance inversion effect, and when R _io and R _put are connected to the input and output, respectively, as shown in Figure 5, the circuit network represented by R _io R _put = k ² . Bye.

FIG. 6 shows the variable attenuator 2 according to the present invention between the receiver antenna 20 and the RF amplifier 21.
2 is connected, the current flowing through the diode is controlled by the receiver's AGC control signal, and the AGC is controlled by the variable attenuator.
An example of a circuit configuration is shown. In this way, when an AGC circuit is configured using the variable attenuator of the present invention, the output impedance is always constant, so the signal can be attenuated without impairing the characteristics of the input BPF 23 of the RF amplifier 21. Moreover, since all diodes work as forward AGC elements for strong input, the strong input characteristics are good. As a result, the modulation distortion factor at the time of strong input is good, intermodulation is reduced, and there is no frequency deviation of the input BPF of the RF amplifier, no change in the operation Q, and no deterioration in the prevention wave removal ability. Therefore, if this AGC circuit is used in a vehicle-mounted receiver that runs in a strong electric field, it is possible to obtain a preferable receiver that is free from interference such as interference and sound distortion.

As described above, the present invention connects a diode between the signal path and ground at the input end of a quarter wavelength transmission line or an equivalent LC circuit, and connects a diode between the signal path and ground at the output end. The above transmission line or
A resistor equivalent to the characteristic impedance of the LC circuit is connected in series, and the current flowing through both of the diodes is controlled to change the amount of signal attenuation, so the impedance seen from the output end changes depending on the amount of attenuation. This has the effect of being able to maintain a constant value without changing depending on the conditions, which is extremely effective in practice.

[Brief explanation of drawings]

1A to 1C are circuit diagrams showing examples of conventional variable attenuators, FIG. 2 is a circuit diagram showing an embodiment of the variable attenuator according to the present invention, and FIG. 3 is a circuit diagram showing the variable attenuator of FIG. 2. Figure 4 is a circuit diagram illustrating an LC circuit equivalent to a quarter wavelength transmission line, Figure 5 is a generalized circuit diagram equivalent to a quarter wavelength transmission line. The circuit diagram and FIG. 6 are circuit diagrams showing an application example of the variable attenuator according to the present invention. 3... Impedance, 10... Transmission line, D ₁ ,
D ₂ ... diode, R ₁ ... resistance.

Claims (1)

[Claims] 1. A quarter-wavelength transmission line whose characteristic impedance is equivalent to the signal source impedance, or an LC circuit equivalent to the transmission line, and a signal path and ground at the input end of the transmission line or LC circuit. and a series circuit of the diode and a resistance equivalent to the characteristic impedance of the transmission line or LC circuit, connected between the signal path and ground at the output end of the transmission line or LC circuit. , A variable attenuator characterized in that the amount of signal attenuation is changed by controlling the current flowing through both of the diodes.