Broad Band Directional Signal Generator

1. Field of the Invention

The present invention generally relates to signal generators, and more particularly, to a broad band directional signal generator capable of generating signals on a transmission line or the like which, controllably, propagate in one direction on the line to the exclusion of the other direction or in both directions.

2. Description of the Prior Art

Numerous narrow band directional couplers exist and are typically employed to split or mix signals in a single-signal path. Directional couplers generally employ reactive circuit elements and, as a result, their frequency response is severely limited. Since directional signal generators employ directional couplers, it follows that directional signal generators have in the past been narrow band and sometimes complex devices.

It is therefore an object of the present invention to provide a broad band directional signal generator employing a conventional attenuator composed of linear, nonreactive circuit elements and whose frequency response is flat and flat to DC.

It is another object of the invention to provide a broad band directional signal generator of uniquely simple construction and adapted for use in analog and digital communication paths where two-way information flow exists.

These and other objects of the invention are achieved by providing a directional signal generator incorporating a conventional attenuator network which operates in conjunction with other circuitry. The attenuator network may be a .pi. or 0, T or H network and may be either symmetrical or unsymmetrical. The attenuator network is composed only of linear, nonreactive circuit elements and is readily adapted to incorporation in coaxial, microstrip and stripline structures, among others. Single ended or push-pull amplifiers are connected to the nodes of the attenuator in such a manner that a signal will controllably propagate in one direction on the line to the exclusion of the other, or in both directions.

The specific nature of the invention, as well as other aspects, uses and advantages thereof will clearly appear from the following description and from the accompanying drawings, in which:

FIGS. 1A and 1B are circuit diagrams of conventional, symmetrical .pi. and 0 attenuator networks, respectively, which are used to illustrate the principles of the invention;

FIG. 2A is a circuit diagram of one specific embodiment of a broad band directional signal generator according to the invention employing the symmetrical .pi. attenuator of FIG. 1A and two pairs of voltage to current converting amplifiers;

FIG. 2B is a circuit diagram of another specific embodiment of the invention employing a modified symmetrical .pi. attenuator network and a pair of push-pull voltage to current converting amplifiers;

FIG. 2C illustrates an alternative embodiment of the invention employing a modified symmetrical 0 attenuator network and two pairs of voltage to current converting amplifiers;

FIGS. 3A and 3B are circuit drawings of conventional, symmetrical T and H attenuator networks, respectively, which are used to illustrate the principles of the invention;

FIG. 4A illustrates a specific embodiment of the invention employing a modified symmetrical T attenuator network and a pair of push-pull amplifiers; and

FIG. 4B illustrates an alternative embodiment of the invention employing a modified symmetrical H attenuator network and two pairs of amplifiers.

For purposes of illustration only and in the interest of simplicity and clarity, the invention is illustratively described with reference to symmetrical .pi., 0, T, or H attenuator networks as shown in FIGS. 1A, 1B, 3A, and 3B. In any case, it will be appreciated by those skilled in the art from the following description that the principles of the invention are equally applicable to .pi., 0, T, or H networks which are not symmetrical. Referring first to FIGS. 1A and 1B, the series resistance R.sub.3 and the shunt resistances R.sub.1 are calcualted from the following equations: ##EQU1## where Z.sub.0 is the characteristic impedance of the transmission line, and K is the current or voltage ratio from the input to the output of the attenuator network hereinafter referred to as the attenuation factor. For the purposes of the following description, it is assumed that Z.sub.0 = 50 ohms and K = E.sub.i /E.sub.o = 2 (.apprxeq. 6dB) for which R.sub.1 = 150 ohms and R.sub.3 = 37.5 ohms. As will be seen, the magnitude of the attenuation provided by these attenuators is probably larger than that which would ordinarily be used in most applications; however, these relative values are useful to provide an illustrative numerical example.

Using the symmetrical .pi. attenuator network shown in FIG. 1A, a broad band directional signal generator can be constructed as shown in FIG. 2A. The attenuator may be considered a three terminal device having one terminal grounded. The node adjacent the left-hand terminal is labeled L.sub.o, while the node adjacent the right-hand terminal is labeled R.sub.o. The input nodes R.sub.I and L.sub.I are coupled to the nodes L.sub.o and R.sub.o of the attenuator network by isolation means such as voltage to current converting amplifiers. For the purpose of this description, it is assumed that the voltage to current converting amplifiers are perfect, that is, their input impedances are infinite, their output impedances are infinite, their transconductances are constant from DC upwards in frequency, and their delays are zero from DC upwards in frequency. Each input node R.sub.I and L.sub.I is connected to a pair of current to voltage converting amplifiers, the transconductances g.sub.1 and g.sub.2 of which are computed from the following equations: ##EQU2## assuming that an output voltage at nodes R.sub.o and L.sub.o equal to the input voltage at nodes R.sub.I and L.sub.I, respectively, is desired.

The operation of the circuit shown in FIG. 2A is best visualized by a numerical example. Assume that the input nodes R.sub.I and L.sub.I are driven by sources which deliver either 0 volts or +1.5 volts. For these inputs, nodal voltages at nodes R.sub.I, L.sub.I, R.sub.o, and L.sub.o, and nodal current into nodes L.sub.o and R.sub.o are tabulated in Table I. In Table I, the convention has been adopted that currents flowing into the node are positive and currents flowing out of the node are negative. Thus, for example, for 1.5v at node R.sub.I and 0v at node L.sub.I, a current of 40ma flows out of node L.sub.o to the amplifier with a transconductance of g.sub.2, a current of 80ma flows into node R.sub.o from the amplifier with a transconductance of g.sub.1, and a current of 40ma flows into node L.sub.o through the resistor R.sub.3, assuming 50.OMEGA. for the characteristic impedance, Z.sub.0.

Since all components of the circuits of FIG. 2A are linear and nonreactive and superposition applies, it is obvious that:

1. A signal equal in magnitude and polarity to any signal at input node R.sub.I appears at node R.sub.o.

2. No signal appears at node L.sub.o for any signal at node R.sub.o.

3. A signal equal in magnitude and polarity to any signal at node L.sub.I appears at node L.sub.o.

4. No signal appears at node R.sub.o for any signal at node L.sub.o.

The circuit shown in FIG. 2A is thus a broad band directional signal generator circuit whose frequency response is flat and flat to DC.

As indicated previously, the value of K = 2 assumed in the above description, is probably impractically large. Magnitudes of R.sub.1, R.sub.3, g.sub.1, and g.sub.2 for other magnitudes of K at Z.sub.0 = 50 ohms are shown in Table II.

The bandwidth of the circuit of FIG. 2A will be limited by the parasitic reactances associated with R.sub.1 and R.sub.3 and the amplifier bandwidth. In most cases the latter limitation will dominate.

Other circuits which are functionally equivalent to the circuit shown in FIG. 2A will readily suggest themselves to those skilled in the art. For example, the circuit of FIG. 2A can be simplified by modifying the .pi. attenuator network as shown in FIG. 2B. This circuit requires only two amplifiers with push-pull outputs and uses the same .pi. attenuator network shown in FIG. 1A except that the shunt impedances R.sub.1 are divided into two series impedances R.sub.1A and R.sub.1B the values of which are computed as follows:

While the particular examples illustrated thus far employ symmetrical .pi. attenuator networks, it will be immediately obvious to those skilled in the art that the principles of these examples are equally applicable to a broad band directional signal generator employing a symmetrical 0 attenuator network as illustrated in FIG. 2C. In this circuit, the 0 attenuator network may be viewed as the 90 attenuator network of FIG. 2B imaged about the ground, which may be either virtual or physical, indicated by the dotted line. Thus, nodes of opposite polarity with reference to ground permit the use of amplifiers having equal and positive transconductances thereby simplifying circuit design. Obviously, in a practical circuit, three of the series impedances in each shunt arm of the attentuator may be replaced by a single impedance having a value ##EQU4## It follows that the operation of the circuit of FIG. 2C is identical with that of FIGS. 2A and 2B.

Next, referring to FIGS. 3A and 3B which show symmetrical T and H attenuator networks, the shunt resistance R.sub.3 and the series resistances R.sub.1 are calculated from the following equations: ##EQU5## By comparing equations (9) and (10) with equations (1) and (2), the analogy to the .pi. and 0 attenuator networks shown in FIGS. 1A and 1B is immediately obvious. Thus, the circuits shown in FIGS. 4A and 4B may be considered equivalents to the circuits shown in FIGS. 2B and 2C, respectively. Clearly, a similar circuit for FIG. 2A is readily apparent from a comparison of the foregoing figures. In FIGS. 4A and 4B, the circuit values are calculated from the following equations: ##EQU6## if a circuit similar to FIG. 2A is desired,

The circuits described have applications in analog and digital communications paths where two-way information flow exists and where a multiplicity of ports may share the same path. The use of these circuits can reduce the number of signal wires required between devices which communicate with each other. While the specific embodiments disclosed employed symmetrical .pi., 0, T, and H attenuator networks, it will be obvious to those skilled in the art that the teachings of the invention are equally applicable to .pi., 0, T, or H attenuator networks which are not symmetrical. In addition, it will also be obvious that, while the specific embodiments disclosed employ voltage to current converting amplifiers or current sources, the teachings of the invention can be equally implemented with equivalent voltage sources. It will therefore be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.