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TC721 Attenuator: Attenuation Control

Technical Overviews

The TC721 is a voltage variable attenuator that operates from DC to 50 GHz. This application note highlights the advantages of the TC721 topology versus those of more traditional attenuators and explains the function of the DC reference circuit located on the TC721 chip. An example of a driver circuit for the attenuator is presented. Advantages and disadvantages of the given driver circuit are also discussed.

2.0 Topology

A schematic drawing of the TC721 attenuator is shown in Figure 1. Traditional GaAs FET attenuators are based upon the standard resistive T–configuration using FETs as the series and shunt resistors. This topology is practical for low–frequency applications; however, at microwave frequencies, the parasitic and solid state capacitances of the FETs degrade the performance of T–attenuators.

The distributed topology of the TC721 increases the operating bandwidth of the device by com­pensating for the FET capacitances. Instead of using one shunt FET, the TC721 has four shunt FETs separated by high–impedance, inductive transmission lines. At minimum attenuation, the shunt FETs are essentially capacitive and combine with the inductive lines to form a lumped–element, 50– ohm transmission line. This reduces the high–frequency roll–off of the device in its minimum attenuation state. At maximum attenuation, the high– impedance lines combine with the low resistance shunt FETs to form series–L, shunt–R, low–pass filters. This four–section filter rejects high–frequency signals that leak through the series FET capacitors. The net result is an attenuator with a dynamic range of 38 dB at 26.5 GHz. (Note: Due to the low–pass filter structure inherent in the distributed design, the dynamic range is greater at high frequencies than it is as low frequencies.)

A. Operation

For most applications the TC721 can be driven by two comple­mentary negative voltage ramps placed on V1 and V2. Some applications have additional requirements such as: single control voltage, improved VSWR, temperature compensation, and improved voltage versus attenuation linearity. The DC reference circuit on the TC721 attenuator chip can be used to satisfy these requirements.

As shown in Figure 1, the DC reference circuit is a non–distributed “T” attenuator designed to operate in a 500 Ω system and to track the control voltage versus attenuation characteristics of the RF attenuator. The driver that utilizes the DC reference circuit is shown in Figure 2.

Op amp 1 insures that the attenuator maintains a good input and output match to 50 Ω, while op amp 2 improves the linearity over that attainable when using only voltage ramps.

If optimum VSWR is all that is required, op amp 2 is eliminated while leaving R L connected to DCOUT on the TC721, and a negative control voltage is applied directly to V2. As shown, a voltage reference is fed to both terminals of op amp 1 through 500 resistors. (The voltage reference, VREF, shown in Figure 2, is a positive voltage and will be discussed later.) For a posi­tive reference the inverting terminal of op amp 1 is grounded through RREF (which is ideally 500 Ω), while the noninverting terminal is grounded through the DC reference circuit by connecting it to the DCIN port on the TC721. The reference circuit termination, RL, is connected to the DCOUT port and is also ideally 500 Ω.

At equilibrium, the voltages at nodes A and B must be equal which implies that the input impedance to the DC reference circuit is equal to RREF. When V2 is changed to a more nega­tive voltage, the voltage at node A becomes greater than that of node B. This voltage difference causes the output voltage of op amp 1 to move toward its positive rail until equilibrium is once again established. When V2 is changed to a less negative value the voltage at node A becomes less than that of node B and the output voltage of op amp 1 will swing toward its negative rail until equilibrium is established. If the reference circuit precisely tracks the RF circuit, the voltage output of op amp 1 at equilibrium insures that the RF circuit is matched to 50 Ω when the reference circuit is matched to 500 Ω.

C. Components

The OP–270, manufactured by Precision Monolithics, was used in the control circuit that produced the results shown in Figure 3; however, any low noise, low offset voltage op amp should produce similar results. Suggested supply voltages for

the OP–270 are 3.6 Volts on the positive supply and –6 Volts on the negative supply. These low supply voltages keep the positive and negative rails of the OP–270 relatively close to the absolute maximum voltage levels of the control lines on the TC721 while providing enough power to the OP–270 for it to perform satisfactorily as an operational amplifier. If larger supply voltages are unavoidable, voltage clamps are needed between the output of the operational amplifiers and the V1 and V2 control line inputs to prohibit large control voltages from damaging the TC721.

D. Circuit limitations

The driver circuit has some limitations and a few potential trouble spots. One of the potential trouble spots is a tendency to oscillate if not properly compensated. The capacitor C1 (shown in Figure 2) was used as a lead compensation net­work. A value of 220 pF was used for the OP–270. If other op amps are used, this value may need to be adjusted.

One of the limitations of the circuit is switching speed. The op amps were the limiting factor in the breadboard circuit that was built at MWTC. Faster op amps are available, but care must be taken in order to prevent the circuit from oscillating.

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