JPS6017174B2 - variable attenuation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvement of attenuation dynamic characteristics in a variable attenuation circuit using diodes.

A conventional high variable attenuation circuit using this type of diode is composed of diodes DI to ○4 as shown in Figure 1, and a control voltage is applied from the control input terminal to vary the conductance of the diode, and the RI of the input circuit and Attenuation was obtained by the ratio of the conductances of D1 and D2 and the ratio of the conductances of R2 and D3 and D4.

A variable attenuator using this type of circuit is simple and can provide a wide dynamic attenuation characteristic, but if the signal frequency used is high, the junction capacitance at cutoff of the diodes DI to D4 cannot be ignored, and the resistor R1 , R2 are increased to some extent to obtain a high attenuation amount, the insertion loss increases due to the stray capacitance caused by the circuit configuration, and the dynamic range of the attenuation characteristic with respect to the control voltage is disadvantageously deteriorated. The present invention has been made to eliminate these drawbacks, and provides a variable attenuation circuit that has a simple circuit configuration and wide dynamic characteristics.

An embodiment of the present invention shown in FIG. 2 will be described below.

In Figure 2, R1 and R2 are resistors that determine the dynamic characteristics of attenuation, C1 and C2 are bypass capacitors,
DI to D4 are diodes whose conductance is varied by control voltage, TR is an amplifier transistor that exchanges impedance between the output and the variable attenuator, and L1 and L2 are tuning coils. As shown in Figure 3, the constant K type low-pass filter consisting of the junction capacitance of LI and D1,02 has a pass characteristic at the center frequency used (curve 1 in Figure 3), and the diodes ○1, D2
When the conductance reaches the maximum due to an increase in the control voltage, the junction capacitance of diodes D1 and D2 increases, so
Near the center frequency used, the constant K type filter's blocking characteristic (third
The value of the tuning coil LI is selected to give curve 2) in the figure. Similarly, the tuning coil L2 has diodes D3 and D.
It is designed to perform the above operations in combination with 4. In the variable attenuation circuit configured in this way, when the control voltage of the control input is low, the conductance of the diodes 01 to D4 is the minimum, and the input signal is applied to the transistor TR via L1, RI and L2, R2, and the passing attenuation is minimized. The losses caused by the junction capacitance of the diode and the stray capacitance of the circuit, which are problems at this time, are improved by adding tuning coils L1 and L2 to form a constant K type filter. Furthermore, when the control voltage of the control input is high, the conductance of the diodes DI to D4 becomes maximum, and the ratio of the resistors R1, R2 and the conductor Z-tance of the diodes D1, D2 and D3, D4 becomes the maximum, obtaining the maximum passing attenuation of the signal. become. Therefore, the tuning coils 1 and L2 form a low-pass filter by using the junction capacitance when the conductance of the diode is at its minimum, and the values of the resistors R1 and R2 are increased to increase the variable width of the attenuator. This function reduces the effect of stray capacitance that occurs when the diode is switched on, and lowers the insertion loss of the attenuation circuit.As a result, resistors R1 and R2 can be selected to be large, and at the same time, L1 , L2, it is possible to comprehensively obtain wide attenuation dynamic characteristics of the attenuator by utilizing changes in the attenuation characteristics of the low-pass filter of the constant-K color wave filter.

As described above, the variable attenuator according to the present invention can achieve wide attenuation dynamic characteristics even in a high frequency range with a small and simple circuit configuration, and has disadvantages such as using conventional circuits of this type in multiple stages. It has the effect of improving.

Brief Description of the Drawings Figure 1 is a circuit diagram of a variable attenuator using a slave diode, and Figure 2 is a variable attenuator according to the present invention. FIG. 3 is a diagram showing the attenuation characteristics of the variable attenuator shown in FIG. 2.

In the figure, R1 and R2 are resistors, C1 and C2 are bypass capacitors, DI to 4 are diodes, and L1 and L2 are tuning coils. Note that the same reference numerals in the figures indicate the same or corresponding parts. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 A high resistance is connected in series to the signal transmission circuit, a diode is connected between the circuit and the reference potential, and a forward control current is passed through the diode to vary the conductance of the diode, thereby achieving a predetermined value. In a variable attenuation circuit designed to obtain an amount of attenuation, a tuning coil is connected in parallel with the high resistance, and the inductance value of the tuning coil is made up of the junction capacitance of the tuning coil and the diode when the conductance of the diode is at its minimum. The variable attenuation is characterized in that the low-pass filter has a pass characteristic at the center frequency used, and is set so that the low-pass filter has a blocking characteristic at the center frequency when the conductance of the diode is at its maximum. vessel.