Voltage Controlled Hybrid Attenuator

Abstract

This disclosure relates to attenuators and, particularly, to controllable, onstant-impedance attenuators. More particularly, this disclosure describes a voltage or current-controlled absorptive attenuator, using a four terminal hybrid circuit, wherein the amount of power passed from the input terminal to the output terminal can be varied from zero to maximum by controlling the amount of power diverted to the other two, quadrature terminals of the hybrid network.

Description

BACKGROUND OF THE INVENTION

Attenuators are very well known and the most common are, probably, the L and the T pad. These can be made variable, usually by a mechanical control of ganged resistances. Electrically-controlled variable impedances can also be used in L and T pads, but they are difficult to adapt because relatively-complex circuitry is required to connect the variable resistances -- and the means for controlling the variable resistances -- at the various points in the attenuator pad where they would be necessary. Electrically-controlled, L and T pad attenuators would be more difficult to balance, and would be less likely to maintain a constant impedance throughout the entire range of attenuation. This is critical since a correct balance of the impedances over the entire range is necessary to maintain the absorptive characteristic that is desirable, rather than a reflective characteristic that is undesirable, in an attenuator.

Hybrid networks, particularly of the 90.degree. or quadrature type, lend themselves to attenuation networks because of their unusual characteristic of dividing the entire input power between the two, quadrature, absorptive terminal impedances when they have the correct, characteristic-impedance termination. When the quadrature terminal impedances are decreased or increased from this characteristic-impedance termination, the input power is reflected to the output terminal. Electrically-controllable, variable-impedance impedance devices have been substituted for the quadrature, absorptive, terminal impedances and, when the variable impedances are adjusted to the correct characteristic impedance, the input power will be divided between them and the output will be a minimum.

However, the minimum-output control point will be critical in this system and will require a precise voltage on the voltage-controlled variable resistance. It also applies the full input load -- in addition to the control voltage load -- between the voltage-variable impedances. This would require heavy duty or specially-designed units to absorb all of the input power. Overloads -- which are always possible -- could even destroy the units.

This system also presents two distinct modes of operation; one where the output power is increased by increasing the control voltage, and another where the output power is increased by decreasing the control voltage, as will be illustrated in a graph and discussed later. The control is perfectly satisfactory in either mode, but as the control voltage reaches the minimum output, and the direction of control of the output power is suddenly reversed, it could reverse the direction or sensing of any automatic controls. In the case of automatic operation in a differential feedback system, for example, where hybrid attenuators such as this would have great potential use, the sudden reversing of sensing and control could destroy the equipment by operating in a positive feedback mode.

It is therefore an object of this invention to provide a voltage-controlled, constant-impedance, hybrid attenuator having a monotonic, attenuation vs. control-voltage characteristic from zero to maximum attenuation.

It is a further object of this invention to provide a voltage or current-controlled hybrid attenuation network that applies a minimum of the input power to the voltage-controlled elements.

SUMMARY OF THE INVENTION

These and other objects are accomplished by connecting a hybrid network between the output of a given source and the input of a utilization circuit. Resistive loads of the characteristic impedance of the system are connected to the other two, quadrature, hybrid terminals. A voltage-controlled, variable-impedance element is connected in parallel with each one of the resistive loads so that, at maximum value of the variable impedances, the full input load is divided between the quadrature terminals and there is no power reflected to the output terminal. As the impedance of the voltage-controlled, variable-impedance elements is decreased, the output power is increased until the full input power is reflected to the output. This provides a simple, linear, highly-stable, and easily-controllable voltage attenuator, having no negative resistance characteristics. The control voltage has only one direction and the power dissipated in the diodes is minimal since most of the power is transferred to the output circuit when the impedance of the voltage-controlled elements is low enough to draw any power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic presentation of the improved controllable, constant-impedance attentuator.

FIG. 2 is a typical strip line hybrid network which may be employed by FIG. 1.

FIG. 3 is the prior art representation of an attentuator's transmission gain vs. control voltage.

FIG. 4 is the improved characteristics due to the employment of the instant invention.

DETAILED DESCRIPTION

Referring now more particularly to FIG. 1, a hybrid network 10 has an input terminal 12, an output terminal 14, and two quadrature terminals 13 and 15. These two quadrature terminals have the correct, characteristic-impedance terminating resistors 26 and 28, and the output terminal has a load impedance 27. The quadrature terminals also have the variable-impedance networks including a capacitor 31 connected in series with a voltage-controlled, variable-resistance diode 33, in parallel with the terminating resistor 26, and the capacitor 32 connected in series with the voltage-controlled, variable-resistance diode 34 in parallel with the terminating resistor 28. A decoupling choke 35 connects the junction of the capacitor 31 and the voltage-controlled diode to the control voltage point 37 and a decoupling choke 36 connects the junction of the capacitor 32 and the voltage-controlled diode to the control voltage point 37.

In operation, with the control voltage at point 37 set to provide the maximum impedance of the diodes 33 and 34, the combined terminating impedance of the resistors 26 and 28 and their variable impedance networks are substantially that of the value of the resistors themselves, and are set to the characteristic-impedance of the hybrid network. This is, of course, the characteristic impedance of the source applied to the input terminal and the characteristic impedance of the output load. In this case, the input power is divided equally between the quadrature terminating impedances at the terminals 13 and 15 and there is no voltage or power from the input reflected to the output terminal 14 or applied to the output load 27.

The nonlinearity of the change in resistance of the diodes with respect to control voltage compensates, to some extent, for the nonlinearity of the change in overall terminating impedances with the change in the variable resistance. The control voltage applied to the point 37 may, of course, be varied to further compensate for nonlinearity or to change the pattern of change in the attenuation.

While the terminating impedances 26 and 28 are shown as fixed resistors, it is obvious that other terminating impedances, that are suitable to the input and output are adaptable to this circuitry and the frequencies involved can be used and they can also be made variable to provide a means for adjusting the balance between the two quadrature terminating impedances, as well as the values of the overall terminating impedances to provide complete attenuation of the input signal and zero output across the load 27.

Since the diodes are identical, connected in parallel, and controlled by the same voltage, the changes in resistance will be identical in each diode, the overall terminating impedance will always be equal, and the hybrid network will be balanced throughout the entire range of attenuation.

The voltage-controlled, variable-impedance elements shown here are PIN diodes. These are current controlled with the resistance of the diode decreasing as the current through the diode increases. Other elements with similar characteristics and impedance control over a suitable range can, of course, be used in place of these diodes.

FIG. 2 shows a typical hybrid network, in a simple form, as applicable to this device. This hybrid network is a one-sided stripline which has a substrate 41 of a high dielectric material, such as alumina, which is backed by a layer of conductive material, such as gold, not shown. The input and output terminals 42 and 44 as well as the quadrature terminals 43 and 45 are connected to the ends of the strips 49A and B. These are narrow, conductive strips deposited or etched on the substrate. They are of a precise length, width and distance apart to provide the desired characteristic impedance of the system and to provide the necessary 3 db coupling for this hybrid attenuation.

The quadrature terminating impedances 46 and 48 are shown connected to the terminals 43 and 45 as they are in FIG. 1 and the output load impedance 47 is shown connected to the output terminal 44.

While one, typical, hybrid network is shown here, it will be obvious that any of the numerable variations of hybrid networks, that are well known in the art, would be applicable here. Others may be more efficient or effective, but they are usually more complicated or convoluted to reduce the size of the substrate or the efficiency of the hybrid network.

FIG. 3 shows the curves of transmission gain with respect to control voltage in a typical, prior art, hybrid-network attenuator with voltage-controlled, variable-impedance elements in place of the terminal impedances 46 and 48. The ordinant 51 is the transmission gain, or the portion of the input power applied to the output, and the abscissa 52 is the control voltage.

It is seen that the transmission of power from the input to the output terminal goes from a maximum to a minimum along the curve 53 as the control voltage increases to bring the voltage-variable impedances to the characteristic impedance of the network. Then, as the control voltage continues to increase, the transmission of power reverses to go from a minimum back to a maximum along the curve 54. The negative impedance characteristic of the overall control of the prior art attenuator is seen between curves 53 and 54.

FIG. 4 shows the typical curve of the same characteristics of transmission gain with respect to control voltage as they appear in this improved attenuator. The ordinant 61 is the transmission gain, the abscissa 62 is the control voltage and the curve 63 shows the comparatively linear increase in the transmission of power from the input to the output as the control voltage is increased.

In a typical embodiment of this invention, as in FIG. 1, the input to 12 is from a 50 ohm source such as a signal amplifier, not shown; the output 27 is a 50 ohm load that may be another signal amplifier; the quadrature terminals 13 and 15 have terminating impedances 26 and 28 of 50 ohms each; the capacitors 31 and 32 are of 1,000 picrofarads each; the diodes 33 and 34 are of the MA-4700 type of Microwave Associates; the chokes 35 and 36 are of 0.01 microhenries each; and a control voltage of from 0.3 to 0.75 volts will vary the voltage-controlled, variable-resistance diodes from a maximum value of 1,000 ohms to a minimum value of 0.4 ohms. The overall quadrature terminating impedances will vary from the characteristic impedance value of 50 ohms to a minimum value of 1 ohm.

The typical hybrid network shown in FIG. 2 has a substrate of high-dielectric alumina of 25 mils thickness, backed by a gold plating of 0.3 mils. The substrate has a width of six-tenths of an inch and a length of about 4 inches. The stripline conductors 49A and 49B are of about 0.15 inches wide and 4 inches long and are spaced three-tenths of a mil apart for a typical 3 db coupling.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

Claims

What is claimed is:

1. A voltage-controlled hybrid attenuator comprising:

a hybrid network of a given characteristic impedance having an input terminal, an output terminal, a first quadrature terminal, and a second quadrature terminal;

a source of input signals of said given characteristic impedance connected to said input terminal;

an output load of said given characteristic impedance connected to said output terminal;

a first terminating impedance connected to said first quadrature terminal;

a second terminating impedance connected to said second quadrature terminal;

a first capacitor and a first, voltage-variable impedance connected in series across said first terminating impedance;

a second capacitor and a second, voltage-variable impedance connected in series across said second terminating impedance;

a source of control voltage;

a first inductive choke connected between said source of control voltage and the junction of said first capacitor and said first voltage-variable impedance;

a second inductive choke connected between said source of control voltage and the junction of said second capacitor and said second voltage-variable impedance;

the overall impedance across each of said terminating impedances being the value of said given characteristic impedance, when said voltage-variable impedances are at a maximum value and approaching zero as the voltage from said source of control voltage decreases said voltage-variable impedances to a minimum value, whereby said transmission gain, between said input and said output terminals, varies from a minimum to a maximum.

2. A voltage-controlled hybrid attenuator as in claim 1 wherein said voltage-variable impedances are voltage-variable resistance diodes.

3. A voltage-controlled hybrid attenuator as in claim 1 wherein said given characteristic impedance is 50 ohms.