JPS5888911A - Variable attenuator - Google Patents

Abstract

PURPOSE:To attain the variable attenuation while keeping a constant impedance, by controlling a current flowed to a series circuit consisting of diodes and resistors provided at input and output terminals of a 1/4 wavelength transmission line. CONSTITUTION:An output signal from a signal source 2 is transmitted to a 1/4 wavelength transmission line 10 via a coupling capacitor C1 and outputted from a terminal 5 via a coupling capacitor C2. In applying a DC control voltage to a terminal 6, a current flows to diodes D1, D2. The high frequency resistance of the diodes is changed depending on the current value. Thus, the attenuation of a signal transmitted from the signal source 2 can be changed. The resistance value of resistors R1, R2 connected in series with the diodes D1, D2 is made equal to the characteristic impedance of the line 10, allowing to make the impedance viewed from the terminal 5 constant.

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 MGO circuit of a receiver. By the way, a receiver is required to receive without distortion from a weak signal of a few microvolts from a distant nine broadcasting station to a large signal reaching 1v or more from a nearby broadcasting station.
If an excessive signal enters, it will adversely affect the local oscillation frequency and cause distortion in the output.

It is necessary to supply a signal of as constant a size as possible to this rounding mixing stage.When a strong radio wave is received, the gain of the high frequency amplification stage is automatically lowered depending on the strength of the signal, and the signal is input to the mixing circuit. A Mu Go circuit is required to keep the value constant. The Go circuit generally includes a detection circuit that rectifies the output of the intermediate frequency stage, and a controlled circuit whose gain is controlled.

There are two ways to apply the modulus Go: forward dome GC, which performs gain control by increasing the bias current, and reverse dome GO, which performs gain control by decreasing the bias current.

In order to perform the above How Dome Go, conventionally, Figure 181 (2)
) As shown in Figure 1, there is a type that is inserted between the signal path and the ground to allow current to flow through the diode 1. This mu Go is
It is well known that the intermodulation effect and strong input characteristics are better than those using other transistor-based Go elements. However, in this case, since the impedance of the signal path becomes low, there is a disadvantage that the characteristics of the following BPF change.

9. As shown in Figure 1 (to), a diode is inserted in series in the signal path to perform reversal GO, but in this case, strong input causes intermodulation interference. This is also well known.

In addition to the above, as shown in FIG.
There are books that connect ~10 in a π-shape, but if you try to maintain a constant impedance, you cannot completely shorten the diode inserted between the signal path and the ground, so as in the case above, it is necessary to There are drawbacks such as the inability to obtain input characteristics.

In addition, in FIG. 1 (1) to (0), 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 an object thereof 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 LO 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 LO circuit are connected in series, and the current flowing through both of the diodes is controlled to change the amount of signal attenuation.

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 zo is connected to an input terminal 4 of the variable attenuator.
The signal from the oscilloscope is transmitted from the erosive force terminal 5, which is attenuated by a predetermined amount.

The attenuator has a quarter wavelength (λ/4) transmission line 10 with a characteristic impedance of zo.

The input/output lamp of this transmission line 1G is connected to the input/output terminal 4 via coupling/DC blocking capacitors C1 and C2, respectively.
and 5, respectively.

A diode D1 is also connected to the input end of the transmission line 100 via a coupling and direct current blocking capacitor C between the signal path and the ground. Then, the transmission line 10
There is also a diode D between the signal path and ground at the output end of
and a resistor R1 equivalent to the characteristic impedance zo of the transmission line 10 are connected in series.

The cathode of the diode D and the anode of the diode DI are connected to the signal path, and the diode D1
Anode and capacitor of 0. A control voltage terminal 6 is connected to the connection point through a high frequency blocking resistor R3. At this point, applying a voltage to terminal 6 causes current to flow through diode DI, transmission 11A10° diode D, and resistor R1. By changing the voltage applied to terminal 6, the current flowing through diode D is controlled.

This current control causes diodes D and D! Since its conductivity is changed to control its high frequency resistance, the magnitude of the signal on the signal path that is dropped to ground through the diodes D and D is changed. 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 example shown in the figure, by increasing the control voltage, the high frequency resistance of the diodes D and D decreases, and the attenuation degree of the length attenuator increases.

Now, the diodes DI and D when the control voltages are applied are
! When the high frequency resistance of R is R, the impedance 2- seen from the output terminal 5 side is rounded, where the transmission m1le is λ/4, RZ.

RZ0 R+Z.

1 + □ R+Z.

Then, the impedance becomes constant KZo.

In the above-described embodiment, a λ/4 transmission line 1G is used, but an LO circuit having a characteristic impedance of 2° and equivalent to the transmission line 10 may be used instead. Examples of some of them include LPF types configured as visi+ and τ types, HPF types configured as π-fold and τ types, and gyrator types.

The above-mentioned circuits are typical examples, but all the circuits shown below have an impedance inversion effect.
As shown in Figure 3, R, m, and
When R- is connected, any circuit network that can be expressed as "Rlm Ram -" may be used.

FIG. 4 shows that the variable attenuator 22 according to the invention described above is connected between the receiver antenna 20 and the IIP amplifier 21,
An example of a MuGO circuit configured with a variable attenuator is shown in which the current flowing through the diode is controlled by the MuGo control signal of the receiver. In this way, when a MuGO circuit is configured with 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 the diodes work as forward beam GO elements for strong input, the strong input characteristics are good. This improves the modulation distortion rate during strong input and reduces intermodulation, as well as
There is no frequency shift of the input BPF of the RF amplifier, there is no change in the operation Q, and there is no reduction in the interference wave removal ability. Therefore, if the above 100 circuits are used in a vehicle-mounted receiver that runs in a strong electric field, a preferable receiver that is free from interference such as interference and sound distortion can be obtained.

As described above, the present invention connects a diode between a signal path and ground at the input end of a quarter wavelength transmission line or an equivalent I/C circuit, and connects a diode between the signal path and ground at the output end. The diode and the characteristic impedance of the transmission line or LO circuit are connected in series, and the current flowing through both diodes is controlled to change the amount of signal attenuation. The impedance seen does not change due to an increase or decrease in attenuation and can always be kept constant, which is an extremely effective effect in practical terms.

[Brief explanation of the drawing]

Figures 1 to 0 are circuit diagrams showing examples of conventional variable attenuators, Figure 2 is a circuit diagram showing an embodiment of the variable attenuator according to the present invention, and Figure 3 is a quarter A circuit diagram showing a generalized circuit equivalent to a wavelength transmission line, and FIG. 4 are circuit diagrams showing an application example of the variable attenuator according to the present invention. 10...Transmission Line D1+DI...Diode R1......Resistance (a) (b) (
c) translation#t&

Claims (1)

[Claims] A quarter wavelength transmission line or a similar LO circuit,
a diode connected between a signal path and ground at the input end of the transmission line or LC circuit, and a diode connected between the signal path and ground at the output end of the transmission line or LO circuit; 1. A variable attenuator comprising a series circuit of a zero circuit characteristic impedance and a low-cost resistor, and controlling the current flowing through both of the diodes to change the amount of signal attenuation.