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The function of an attenuator is to reduce the amplitude level without substantially distorting the waveform of a microwave signal in the process. It contains a lossy element along the direction of the electromagnetic field vector, in order to dissipate the RF energy. This article describes a digital attenuator to adjust a signal to a desired amplitude level via an electronic command.
The theory of operation is to reduce the level of a known source of power by a predetermined amount expressed in decibels, which are obtained from the logarithm of the power ratio desired. The RF topology for this digital attenuator consists of a transmission line with diodes spaced optimally to cover the desired frequency range and digital-to-analog drive circuitry to control the variable analog attenuator.
The key component of the attenuator is the choice of diodes. It is an important design consideration. The diodes are mounted in a shunt configuration with and without Q spoiling networks. Diodes oriented in the same direction eliminate bi-directional currents for improved temperature compensation. Using chip diodes with a ribbon lead has its drawbacks. In a high frequency broadband application, the inductance of the ribbon lead adversely affects the attenuation and bandwidth. A beam-lead diode, installed using a proprietary technique, reduces the series inductance and provides a significant improvement in performance. To minimize the adverse effects from biasing, the network is buried as far as possible from the input and output ports.
In the RF design of a digital attenuator, the preferred transmission media is microstrip, which allows access to the circuitry while being measured. This provides for a more precise tuning capability. This technique yields an overall improvement in both optimization and performance. Even with the improvement in microwave performance, it is the control circuitry that provides the absolute accuracy for the overall device. It has to be capable of canceling the nonlinear effect of the PIN diodes and provides a monotonic linear control of the input slope characteristic. In order to achieve the absolute accuracy required for the attenuator, the digital section has a 64K resolution capability to control and compensate the performance. For any desired resolution, the optimal performance values for each digital attenuator are stored in the driver’s EEPROM, ready to be commanded from the external digital control input. This is accomplished by using a computer with I/O and IEEE controller cards, a G.T. Microwave proprietary program and a vector network analyzer. The computer’s I/O port sets the external control input for the desired digital attenuation, then ramps the driver’s 64K of resolution down the dynamic attenuation range, using another I/O port to an internal control input. While ramping the digital attenuator driver, the vector network analyzer measures the attenuation at each step and sends the data to the computer via the IEEE bus. When the optimal condition is determined, the computer programs the driver’s EEPROM for the external control input count.
The digital attenuator described is optimized over a 9:1 bandwidth, 2 to 18 GHz with 105 dB of dynamic attenuation range and 0.03 dB resolution, 12 bits of TTL compatible binary logic, and is capable of switching between any state within 350 ns. The insertion loss is 5.0 dB, the attenuation flatness is ±1.0 dB to ±8 percent of the set attenuation value and the VSWR is 2.2. The digital attenuator envelope is 3.0" × 2.0" × 0.75". Using the techniques described herein, the test data shown in Figure 1 illustrates the typical performance achieved.
This technology hosts a variety of products, which include, but are not limited to BPSK, QPSK and vector modulators, phase shifters and phase free attenuators. Models are offered with options that include digital control with up to 64K of resolution, linearized or any desired control input slope characteristic, narrowband optimized performance, temperature compensation, video filtering and sub-assembly integration.
New modulation techniques will demand a technology requiring a new generation of components. These components will need an improved performance at a lower cost. Industry can now welcome the arrival of ultra-broadband digital attenuators that will provide tomorrow’s capability, at the leading edge in performance and available today.
G.T. Microwave Inc.,
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