As featured in the IMS Edition of Microwave Journal

 

Being able to change the signal power of an RF system is a fundamental need. There are many reasons why it is preferable to attenuate the signal close to the source. When developing and testing a new product, it is often useful to bring the input power up slowly to avoid damage and to ensure proper characterisations. When a vector network analyzer is calibrated, an absolute standard such as a TRL offset enables accurate results. When the transmission medium is waveguide, power levels may be controlled with a waveguide attenuator.

The industry standard defines an attenuation value as a ratio of the input power to the output power, expressed in decibels (dB). However, to fully specify an attenuator, the user must understand the specific requirements of the application. Some of the specifications will involve determining:

• Fixed or variable attenuation
• Maximum attenuation range and insertion loss
• Power level
• Frequency range
• Attenuation accuracy
• Phase change

These specific requirements are combined with requirements for cost, quantity, size, mass, environment, integration and control to determine the most appropriate waveguide attenuator solutions for a particular application.

The mix of waveguide solutions reflects the market segmentation. Automatic test and programmability have become important test attributes, driven by factors such as the growth in data traffic, more sophisticated testing equipment with closed-loop feedback, manufacturing advances, evolving digital control in IoT applications and advances in design tools such as finite element modeling analysis. These test and measurement applications are likely to use variable attenuators for the continuous attenuation that they can offer.

Flann Microwave believes that future market growth for products will come from ATE applications and competitive pricing. Flann offers standard rotary vane attenuators that deliver high accuracy over a full waveguide band. They have a flat attenuation profile versus frequency and have attenuation ranges from 0 dB up to 60 dB. This style of attenuator comes in various forms with both manual and programmable versions for ATE applications. Some examples of these attenuators are shown in Figure 1. 

Figure 1 ⇓

 

 

 

 

 

 

 

 

 

 

Figure 2 shows a plot of attenuation versus frequency for the 29625-03 attenuator shown as the first device in Figure 1. This device operates in the WR-06 waveguide frequency band with a flat attenuation response over this 110 to 170 GHz range. The Flann rotary vane attenuator family is capable of operating up to 500 GHz. While this family of attenuators offers excellent performance, some applications may require a smaller size with a device that is less costly and easier to manufacture. For these applications, Flann provides a basic variable attenuator. These designs are easy to manufacture, compact and low-cost devices. The trade-off with these devices is an attenuation range of 0 to 25 dB or 30 dB and 40 to 60 percent attenuation variation with frequency, along with a simple lookup table that varies for every frequency for calibration.

Figure 2 ⇓

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The performance of one of these basic attenuators is shown in Figure 3. This attenuator is shown in the middle of Figure 1. The attenuation flatness and accuracy show the performance trade-offs made to reduce size and cost. While this performance is not sufficient for ATE applications, these attenuators are good solutions to improve mismatches or in “level set” applications requiring only a single frequency with calibration lookup tables. The applications for this low-cost family are also limited by the method of changing attenuation. Historically, devices of this type have used a manual micrometer to adjust attenuation. Automating the attenuation process requires a compact actuator that would be of comparable size to the attenuator. The cost and size of these actuators have been prohibitive, but suitable actuators are becoming available and Flann is re-evaluating the design approach.

Figure 3 ⇓

 

 

 

 

 

 

 

 

 

 

 

 

Introducing a product suitable for high performance ATE applications meant minimizing the disadvantages while maintaining the advantages of the low-cost versions. To achieve this, Flann designed programmable/remote attenuation control from a PC using a USB interface with power. The attenuation range was increased to 50 dB and the attenuation flatness was improved to ±10 percent. All this was accomplished with an eye toward reduced size and moderate cost targets.

Flann’s new design uses an improved RF design and an absorbing material, coupled with a compact actuator. The design relies on a thorough understanding of EM waves, aided by FEM software analysis. Figure 4 shows dashed lines that represent modeled results along with the measured data represented by the solid lines. The attenuator can be seen to the far right of Figure 1.

Figure 4 ⇓

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Combining the RF design with an actuator having a 5 μm resolution enables the specifications shown in Table 1.

Table 1 ⇓

In addition to the performance, the design uses an element that is entirely retracted from the waveguide at the minimum attenuation value. This ensures that the insertion loss of the device is the only insertion loss of the waveguide as this will be an important advantage as frequencies go higher.