RF front-end designers have always been in need of passive devices that serve in many functional blocks in their radio designs. These devices include 90 degree hybrids, power dividers and directional couplers, to state a few. There are many others that can also be considered. Key challenges for the suppliers of these parts have been to maintain high RF performance while continuing to make them smaller and drive out cost. Also, the ability for these devices to have a high degree of repeatability and be easily integrated into higher level functions is certainly desirable. These high performance passives are used in wireless infrastructure, point-to-point radio, RFID readers, repeaters, instrumentation and many other ISM band applications. An example might be a direct conversion transceiver or base station transceiver where they might be responsible for dividing and combining the signals in a balanced amplifier or coupling a signal for error correction in a linear power amplifier (see Figure 1).
Figure 1 Highlighted passive components in a block diagram of direct conversion transceiver and basestation transmitter.
For any of these applications, the behavior of the individual passive component will play a serious role in the overall RF front-end performance. Lying in the critical path of RF signals, hybrids, directional couplers and n-way dividers are all required to have low insertion loss and VSWR as well as sufficient power handling capability. In addition, hybrids and power dividers need high isolation along with excellent phase and amplitude balance, while directional couplers need to maintain high directivity. Clearly, performance is not something that can be sacrificed for the sake of size and cost.
At Valpey Fisher, we recently established a Microwave Products Group to focus on developing and bringing these high performance passives and integrated modules to market. The approach chosen is a semiconductor- based monolithic design.
Figure 2 IC passive elements and IC cross-section of RFIPD.
At the core of these microwave devices is a state-of-the-art foundry, capable of producing highly repeatable structures on high resistivity silicon wafers. This technology is known as our RF Integrated Passive Device (RFIPD) platform (see Figure 2). It offers a significant advantage compared with alternative-technologies –miniaturization, performance and reduced cost. The processes have been optimized to achieve high quality factors inductors with Qs of greater than 30 at 1 GHz and capacitors with ESD ratings greater than 250 V human body model. In addition, the power handling of these devices is quite adequate for a majority of applications as, depending on the specific product type, handling 4 W CW has been demonstrated. In addition, lower thermal resistance packaging containing an exposed lead frame paddle could potentially allow for higher powers.
The combination of the material sets chosen, innovative modeling software as well as utilizing a standard MMIC process provide a higher performance device with excellent repeatability at a lower cost. Some additional advantages are the ability to realize a series of devices across a broad frequency range while maintaining a small package form factor. This is a challenge for LTCC or stripline as the sizes of these devices change more drastically with frequency. Also, being a semiconductor die allows for higher levels of integration utilizing existing multi-chip module (MCM) manufacturing capability. Some examples of these functions can include components such as voltage variable attenuators, phase shifters and vector modulators.
This process has been implemented to realize a series of products that fall into two main categories, namely single function discrete monolithic passives and higher functionality multi-chip module components. The first category includes devices such as 90 degree hybrids, two-way power dividers, directional couplers and fixed attenuators. These devices can all fit in a miniature 1.5 × 2.0 mm plastic leadless package. Also a four-way power divider has been demonstrated in a 3 × 3 mm package form factor. The frequency of operation ranges from as low as 500 MHz to as high as 8 GHz depending on the specific device type. Taking a look at just a few typical examples of RF performance, the 90 degree hybrids consistently provide 1.1:1 VSWR, isolation between 28 and 33 dB and a phase balance of 1 degree. Fixed attenuators have been shown to operate from DC to 8 GHz with an extremely flat attenuation response and low reflection. Two-way power dividers have amplitude and phase balances of ± 0.1 dB and ±1.0 degrees, respectively, a 1.1:1 VSWR while also maintaining low loss and very good isolation. These performances demonstrate some significant improvements over other types of solutions. Some other key advantages to the monolithic approach is the small form factor and the ability for these devices to have the same inputs and outputs within a specific product type across a broad frequency range. With today's multi-band requirements, this provides the designer with flexibility during implementation.
Another category of solutions utilizes a Multichip Module or MCM process. Here, the monolithic passive devices can be integrated with various other semiconductors to create higher-level multi-function devices. In this case, one can leverage the high performance of the monolithic passive with a controlled manufacturing approach to realize components with increased performance and reduced variability. A good example is a highly repeatable, low distortion voltage variable attenuator where the 90 degree hybrid is combined with PIN diodes and all the additional bias and decoupling components to provide a fully integrated solution. This provides the designer with an easy drop in solution, which requires just a single 0 to 5 V control line to fully operate the device. In addition, the manufacturing approach enables the ability to apply tight controls on all of the elements that cause the greatest variability. This results in at least a 2× improvement in the attenuation profile repeatability. At the nominal 20 dB point the maximum variation in the attenuation is ±1 dB. Other RF performance characteristics of note due to this approach are the IIP3 of +45 dBm due to the PIN diode implementation and the low VSWR and insertion loss, which can be directly attributed to the monolithic 90 degree hybrid.
The monolithic approach of RFIPDs, whether discrete functions or integrated into higher level MCM components, are a very good choice for many applications. They are a cost-effective alternative that offers smaller size and extreme repeatability while maintaining high RF performance.
Valpey Fisher Corp.