PHYSICAL CONSTRAINTS FOR BROADBAND OPERATION
Most microwave hardware engineers have had the sobering experience of testing a new design and not having it look like the model. This is particularly true as more applications require wideband, high frequency modules. These broadband, high frequency modules usually have gain roll-off at the high end of the band from a combination of inductive and capacitive parasitics in the components and packages. These parasitic elements create gain roll-off and VSWR mismatch losses while interconnecting transmission lines and other passive components have loss characteristics that increase with frequency.
GAIN EQUALIZERS: THE HISTORICAL APPROACH
This issue is typically addressed with a gain equalizer. The loss characteristic of these devices is positive versus frequency, meaning they have more loss at lower frequencies. The overall module will have more loss, but more amplifier gain can compensate for this loss. The positive loss slope of the gain equalizer is designed to flatten the loss characteristic over the entire band. Historically, passive gain equalizers have utilized a stacked combination of surface-mount chip resistors and chip capacitors. The resulting parallel RC circuit is mounted in series across a gap in the microstrip line. Stacking the chips minimizes the circuit footprint, which helps minimize reflection losses.
Pre-assembled RC circuits work well with pick-and-place assembly and reflow methods. However, typical SMT chip termination materials make it difficult to pre-attach the resistor and capacitor chips without the connection reflowing and possibly disassembling during the board attachment process. The RC chip stack needs to be small, usually in 0201 or up to 0402 case sizes, to maintain good VSWR performance across electronic warfare (EW) bandwidths. Even with all these precautions, the actual performance of a stacked chip gain equalizer may not be as close to ideal performance as a designer would like.
A NEW BREED OF GAIN EQUALIZER
Knowles’ Dielectric Laboratories, Inc. (DLI) brand gain equalizers employ monolithic construction with precision thin film conductor and resistor films and proprietary high dielectric constant ceramics for superior RF performance repeatability. Integrated R and C values are realized to produce the desired gain slope. The AEQ series of equalizers provides solutions for applications ranging from 6 to over 40 GHz. The gain equalizers have terminations compatible with standard SMT or conductive epoxy attachment. This series targets EW applications and the gain equalizers come in a variety of sizes, as shown in Table 1. The physical dimensions of the equalizers are defined in Figure 1.
Figure 1 Gain equalizer physical layout.
Figure 2 Gain equalizer measured performance.
Figure 2 illustrates the typical measured performance of the series. The AEQ54xx-series operates from 50 MHz to 20 GHz, with slopes ranging from 1.0 to 3.5 dB. The other entries in Table 1 are typically a little bigger and these gain equalizers operate from 50 MHz to 34 GHz, with slopes ranging from 0.6 dB to 3.5 dB. The gain equalizers are intended to be used in small-signal applications. Thermal dissipation is a limitation because of the small size of the equalizer and the low thermal conductivity of printed circuit board materials. The dissipated power will be greater for lower frequency signals since the insertion loss is lower in higher frequency ranges.
CONCLUSION
If you have gain slope issues in microwave modules, you should consider gain equalizers. Knowles’ DLI brand gain equalizers are designed as a small, low-cost solution to gain slope challenges. These equalizer designs employ a monolithic construction with precision thin film conductors and resistor films with proprietary high dielectric constant ceramics for superior RF performance and repeatability. Components are well suited for use with pick-and-place equipment. They are available in tape-and-reel packaging for high volume applications.
Dielectric Laboratories, Inc.
A Knowles Precision Devices Brand
Cazenovia, N.Y.
www.knowlescapacitors.com/Products/Microwave-Products/Gain-Equalizers