Low Cost Directional Couplers
A new series of low cost, high quality directional couplers is featured
Ever expanding applications of RF and microwaves for wireless and cable applications have revived the development efforts of components at these frequencies. There is a continuing demand to reduce size and cost, and to improve performance and quality at the same time. To satisfy these goals, a new series of directional couplers has been introduced to meet the demands of the market. These couplers measure 0.15" × 0.15" × 0.15" and are designed for assembly using automated pick and place machines.
WHAT CONSTITUTES A DIRECTIONAL COUPLER
The block diagram of a directional coupler is shown in Figure 1. It consists of a ferrite transformer, a resistor and a capacitor. The ferrite transformer is wound on a balun core and works as a four-port device. By proper selection of R and C, a three-port directional coupler is realized.
Couplers working in the 5 to 2000 MHz frequency range and having multiple-decade bandwidths are traditionally constructed using ferrite cores. This provides the requisite bandwidth in a small volume. Traditional ferrite-based couplers are housed in a package measuring 0.25" × 0.31" × 0.20". The company has introduced a series of couplers1,2 that require an external component on the user's motherboard. This reduces the cost, but the size of the device is only slightly reduced to 0.19" × 0.25". For higher frequency applications, an additional capacitor is required, and it increases the size. In the new approach, the resistor and capacitor are integrated into a multi-layer board using Blue Cell™ technology. This shrinks the overall area of the coupler to 0.15" × 0.15" and the height to less than 0.15".
The transformer is mounted on the top of the board, and all connections from the transformer to the base are made by welding. This procedure helps to ensure the preciseness of the assembly, with resulting high performance repeatability, and prevents any disconnection during reflow at the user's site. All connections from top to bottom metal layers are accomplished using filled vias. The transformer used in this series of couplers employs a patented technique3 to obtain flat coupling over the band.
As an example, the model DBTC-13-5-75 (shown in the photograph) is a coupler designed for 75 ‡ applications using this technology. The unit operates over the 5 to 1000 MHz frequency range with a nominal coupling of 13.2 ±0.5 dB and a flatness of ±0.6 dB. The model TCD-13-4-75 is a coupler that requires external components. A comparison is made between these two couplers in the subsequent data plots to show the performance advantages of each. Figure 2 shows the insertion loss of the couplers. The insertion loss is similar in both cases and is typically 1 dB. Figure 3 shows the directivity vs. frequency, which is typically 20 dB over the band. Note the improvement over the TCD-13-4-75 device. Figure 4 shows the coupling vs. frequency, which is typically 13.3 dB to 1000 MHz for the new coupler. Figure 5 shows return loss vs. frequency at all three ports, which is typically 20 dB (1.22 SWR) for input and output. Although the coupled-port return loss is lower than that of the TCD model, for most applications this return loss performance is not important. Table 1 lists the DBTC-13-5-75 directional coupler's complete electrical specifications along with specifications for nine additional couplers in both 50 and 75 Ω impedances.
A series of miniature couplers has been developed using Blue Cell™ technology and ferrites to realize their extremely small size. This feature will save much needed PCB area for users. These units utilize a patented technique to realize a very flat coupling response and are suitable for automated pick and place. All units are supplied in an AT790 case style and are priced at $1.99 each in 25 piece quantities.
1. "Do-it Yourself Low Cost Directional Couplers," Microwave Product Digest, September 2000.
2. "Do-it Yourself Couplers," Application notes (Directional Couplers) at http://www.minicircuits.com/appnote/application.htm.
3. US Patent 6140887
Brooklyn, NY (718) 934-4500.
Circle No. 302