Passives for RF design

(Passives for RF Design) Designers can choose from fully integrated systems or fully discrete solutions with their associated pros and cons. Both thin film and PMC are rapidly developing technologies, and a number of new devices will soon be emerging. The PMC process is ideal for integration of the passive content of the RF circuit to optimize PA or LNA performance for a given application.

Radio continues to be one of the most powerful technology drivers in microelectronics with many evolutionary changes taking place in the last few years. Consumer and mission-critical applications require solutions that are smaller, increase functionality and reliability, and improve signal clarity, while attaining lower power consumption.

To achieve this, almost all modern radios now use digital modulation schemes instead of standard analog. This also fits with the trend of higher frequencies to make use of the wider bandwidth required for these applications. The higher-frequency designs also drive to smaller physical layout because the shorter wavelengths can be accommodated in much smaller packages (i.e., the wavelength at 5.8 GHz is nearly 6.5 times smaller than at 900 MHz).

Although radio design continues to evolve with increasing complexity and need for security, the actual radio block diagram has remained virtually unchanged.

Figure 1 is a block diagram of a dual-conversion super-heterodyne transceiver, one of the most common radio design schemes used today. However, the direct-conversion (zero intermediate frequency) receiver is gaining momentum. The direct-conversion receiver has a single mixing stage that converts the received signal directly to baseband and reduces the cost considerably — at the expense of performance.

Figure 1. Basic super heterodyning transceiver.

The basic radio consists of several key components, and each has a specific job in the chain: antenna, switch, low-noise amplifier (LNA), filter, mixer, oscillator, modulator/demodulator. As the applications have evolved, so has the component technology that has enabled major downsizing of these elements to take place.

Early radio designs were necessarily based on discrete passive components designed for high-frequency performance. These use low-loss, high-frequency dielectrics (NP0 ceramic, porcelain or glass), but require precious metal electrode systems that can withstand the high firing temperatures associated with ceramic material processing and still provide good conductivity. These devices remain a suitable solution for high-power RF applications but are nearing the limit of downsizing for microcircuit applications.

LTCC technology

The next stage of evolution came with low-temperature co-fired ceramic (LTCC) technology. This technology enables integration of passive components within compact modules and can provide a platform for actives. This integrated technology does yield small size but requires careful design discipline to achieve repeatable characteristics. LTCC is essentially a “wet” or tape process technology: Material is screened in place, building up the device layers, while the lower temperature manufacturing process enables the use of more standard conductive metals such as copper for internal electrodes and contacts. Plated-through vias are used for interlayer interconnection, and the electrical conductors can be configured in plates (capacitors) or spirals (to make efficient inductors). Different low-temperature ceramic materials with a variety of characteristics are available, including high-capacitance density dielectrics, but these yield high-dielectric constant at the expense of tighter temperature and voltage coefficients that characterize the high-temperature ceramic types.

This type of lay-down process also has wider process tolerances (from the capability of the wet system to the variability of post-firing shrinkage) with interconnect line widths limited to ~150 microns. Table 1 gives comparative data for key characteristics. The technology is applicable to the passive and active modules, such as power amplifiers (PAs), RF switches, and RF front-ends by integrated capacitors, resistors, and inductors in a small area. The process results in a mechanically strong and compact structure but does limit the materials available for RF component design.

One challenge facing designers working with LTCC is the ability to characterize the RF properties of the complex internal structures that lack electrical models. Most embedded components, especially spiral inductors and parallel plate capacitors, suffer from significant parasitic coupling due to their large area and proximity to other structures or to ground planes.

It follows that LTCC modules are more suited to application-specific designs in high-volume platforms, as varying the design for different configurations becomes more problematic as their complexity increases. This technology also benefits from economies of scale — once upfront costs for custom design and characterization have been met, the process itself can support low-cost manufacturing — but it also limits any future revisions and changes without a module redesign.

Because LTCC technology in itself is not always the optimum performance solution, there has been an increased interest in new integrated devices that have evolved from discrete solutions based on thin film technology.

Thin film solutions

Thin film technology, as a first step, goes back to a discrete format. The key difference with this technology is that it is based on photolithography and plasma-enhanced chemical vapor deposition (PECVD) processing. The photolithography gives extremely precise geometry, while the low-temperature PECVD process combines the benefits of high conductivity conductors with the use of highly stable dielectrics (e.g., SiO2) deposited on a stable alumina base. The technology also allows downsizing to 0201 size and further integration. The most basic element is the thin-film capacitor as shown in Figure 2.

Figure 2. Thin-film capacitor structure.

 

The system above is characterized by stable dielectric, single-layer construction, which eliminates harmonics, is readily modeled and is reproducible. Designs breadboarded on the bench will be precisely reproduced in mass production at the CEM, month to month and year to year. Because the parts are discrete circuit elements, there are no up-front design costs and full design flexibility is maintained throughout the program lifetime. Ultra-stable dielectrics usually have a trade off: low dielectric constant. But thin film is capable of far thinner dielectric films (~1 µm) compared to LTCC (50 µm to 250 µm), which results in a higher cap yield per mm2. The conductors used for the electrodes also have lower resistivity (~1 m▸/mm2) than their LTCC counterparts (10 - 20 m▸/mm2), giving greater RF power-handling capability. Because of their precision, these devices can be used in conjunction with LTCC packages, while any given LTCC module will have limited design flexibility. The small size and termination compatibility of discrete thin film devices means that they can be used to fine-tune an application or modification or tuned last-minute for Federal Communications Commission (FCC) compliance.

Figure 3 shows how these components are typically configured in a standard radio application.

Figure 3. Key design areas for thin film technology.

The LNA is one of the more critical sections in the receiver circuitry. To maximize the performance, it is essential to have stable biasing and accurate impedance matching. Thin film provides discrete capacitors and inductors with high Q, low equivalent series resistance (ESR), and accurate capacitance values (±.01 pf) and inductance values (±.1 nH).

By using this technology, a low-noise amplifier (LNA) can have a more accurate match over temperature and greater repeatability from board to board compared to both traditional discrete devices and wider-tolerance LTCC.

Figure 4a shows the deviation in S11 when comparing NP0 multilayer ceramic capacitor (MLC) vs. thin film capacitors. Notice that the thin-film capacitor response tracks with no variation between parts. This demonstrates exactly how precise the thin-film process can be from capacitor to capacitor and from batch to batch. This not only improves the quality of the LNA, it can actually improve the yield in manufacturing by eliminating the fine-tuning of circuits in production. Figure 4b shows the response at higher frequencies. Being single-layer devices, thin film shows no harmonic resonances.

Figure 4. Thin film vs NP0 MLC ceramic.

The same components can be used to accomplish the critical matching of the input and output of a power amplifier. By using low-loss, thin-film capacitors and inductors, more power can be sent to the amplifier transferred to the antenna. This results in improved performance and increased efficiency of the power amplifier, as well as improved temperature performance.

Antenna matching is also a critical design issue. The available real estate for the antenna is continually decreasing, which generally leads to a non-ideal form-factor design. This situation will almost always require an impedance matching circuit for the antenna. Thin film capacitors and inductors are ideal for this application, providing an accurate match of impedance to the antenna to maximize energy transfer under all conditions to minimize losses from the PA or to the LNA.

Beyond high-accuracy microminiature capacitors and inductors, thin-film PECVD technology also lends itself to integration. By combining both a capacitor and inductor element on the substrate, an inductor and capacitor (LC) low-pass filter (LPF) can be formed, as shown in Figure 5. These can be made in the same small form factors (0402 and up) and so use little board space while saving cost through component count reduction. These thin-film filters provide high out-of-band attenuation (greater than 30 dB) while maintaining the lowest insertion loss available to the RF designer (less than 0.3 dB). They can also be used to isolate the frequency of interest on the output of the mixer after conversion. The filters must be internally matched to 50 ▸ in order to achieve specified performance.

Figure 5. Thin film LPF and response for a 915 MHz LPF.

A directional coupler is a device that samples an RF/microwave signal while minimizing loss to the signal. Thin-film devices, based on back-to-back inductors, produce high-directivity, low-insertion-loss directional couplers. These couplers offer the highest amount of directivity found on the market today, in small package sizes down to 0402. In the diagram in Figure 6, the coupler is being used to sample the output and send the sample to a gain control circuit for the power amplifier. As with the LPF, the couplers are also capable of handling up to 3 W continuous power.

Figure 6. Directional coupler.

Directional couplers work from the principal of field coupling. The electric field produced by a transmission line in series with the signal is coupled onto an adjacent conductor through the air or dielectric medium. Coupler elements can be included within LTCC modules. The technology allows lumped elements, rather than coupled lines, to produce directional couplers to 10 dB. However, thin-film technology has a number of advantages in this area. The finer line widths maximize the coupling coefficient, making available hybrid couplers to 3 dB in 0603 size.

This coupler, with port configuration shown in Figure 7, is designed to couple 3 dB of power to another channel, with the addition of a 90 ° phase shift to the signal. This can be useful in designs using an in-phase and quadrature (I/Q) architecture where the channels are 90 ° out of phase. By using a hybrid coupler on the output of the oscillator, the local oscillator (LO) can be generated for both I and Q sections. It can also be instrumental when using two amplifiers to improve the linearity by splitting the power between the two circuits and then recombining after amplification. This reduces harmonic emissions, improves efficiency and increases gain from an amplifier.

Figure 7. Hybrid couplers used to improve linearity.

The latest stage in evolution is passive microcircuits (PMC). PMC goes to the next level of passive integration. Its key advantages are that it retains the minimum line width capability and line width precision of thin film, but increases the maximum stacking layers capability. This can be used for increased capacitor and resistor density per mm2 and added turn capability in inductor elements. Table 1 gives the prime characteristics for these materials.

The PMC process is ideal for integration of the passive content of the RF circuit to optimize PA or LNA performance for a given application. As with all integration, the higher the complexity, the more single-application specific the device.

In summary, designers have choices at the outset of the product cycle to choose from full integrated systems to full discrete solutions with their associated pros and cons. Both thin film and PMC are rapidly developing technologies, and a number of new devices are guaranteed to be emerging during the next few quarters.

Publicado por: Jahir Alonzo Linares Mora C.I: 19769430 CRF

Bibliografia: http://rfdesign.com/mag/508RFDF2.pdf