Application Support

Approach in selection of capacitors for base station issues

In the design of base stations, which are becoming progressively smaller and are using even higher frequency bands, it is difficult to compromise due to the number of components that can be mounted on limited substrate space, and restrictions in the size and operating temperature of components.

To solve these issues, Murata Manufacturing Co., Ltd. presents a lineup of small capacitors with excellent high frequency characteristics. These capacitors can reduce the number of mounted components needed, and whose operation is guaranteed at high temperatures.

High-Q capacitor guaranteed to 150℃ 0402 inch size High-Q capacitor High voltage,High capacity KRM series

Murata Icon X A capacitor that can be used where temperatures exceed 125℃ near the PA circuit and in high temperature environments

GQM   

GQM series (X8G characteristics) High-Q capacitor guaranteed to 150℃


For DC cut and matching 


Heat generation of circuit boards and parts can increase the temperature of the base station PA. In particular, the heat generation of the amplifier transistor can increase, and the nearby positioned DC cut and matching capacitors can heat up. In addition, when amplified electricity is received, the heat generation of the capacitor itself can increase.


Conventional measures and their limitations


Most High-Q capacitors used for DC cut and matching have an upper limit of 125℃ in the operating temperature range. This has been resolved by making efficient use of capacitors guaranteed to 125℃ that reduce the ambient temperature by radiating heat using a heatsink design and that control the heat generation of the capacitor by devising a way to use a DC cut capacitor.


DC cut

With the recent trend of providing multiple outputs (increased Tx), the number of parts has increased, and as the size of the base station decreases, there is less space to use for a heat sink to radiate the heat. In addition, as the usage frequency band becomes higher, the heat generation of the parts increases, and the circuit design reaches a limit in terms of controlling the heat generation of the capacitor and the ambient temperature to 125℃ max.

Murata offers a lineup of High-Q capacitors that include conventional 125℃-guaranteed capacitors (C0G characteristics) and 150℃-guaranteed capacitors (X8G characteristics). This reduces the restriction on the ambient temperature of DC cut and matching capacitors for greater design freedom. 

150℃ guarantee, for the latest lineup
List of product numbers
in the 150℃-guaranteed High-Q capacitor GQM series (X8G characteristics)

Murata Icon X Improved high frequency Q value in a compact size with low capacity

GQM 

GQM series 1005 size High-Q capacitor

 

For matching


While base station PAs increasingly have more on-board parts due to multiple-output (increased Tx) specifications, there are continued demands to retain or decrease the footprint of the device, making it more important to realize a dense circuit board design. Although it would be ideal to reduce the size of the matching capacitor, generally a smaller capacitor means a low Q value and rated voltage. In addition, because as the frequency band increases, the Q value decreases, a capacitor with the highest Q value is preferred. However, it is structurally difficult for capacitors with a small capacitance selected for their high frequency to achieve the improvement effect of the Q value compared to standard specifications, especially for High-Q specifications.

Capacitance used for matching

Murata offers a lineup of High-Q capacitors that include the conventional 1608 size as well as the 1005 size. By using a special structure and materials, a higher Q value is achieved compared to standard specifications (GRM series), even for capacitors with a low capacitance value. A denser design of the high-frequency PA circuit can be achieved using a compact matching capacitor and high Q value. 


Comparison with 1.5pF product

Small size and high-Q,for the latest lineup
List of product numbers in the 1005 size High-Q capacitor GQM series

Murata Icon X Effectively utilizing the limited board area 2-layer high capacity capacitor

KRM 

High voltage, high capacity KRM series (2 layers) with space-saving design 


For decoupling of transistor Vdrain


As GaN high-frequency, high-power transistors gain in popularity for the PA design of base stations, PA performance has significantly improved, with higher power than conventional Si LDMOS transistors and the capability of handling high-temperature and high-speed (high frequency) operation. This means that while transistor performance has improved, there is a need for greater durability in rigorous usage environments with respect to peripheral parts.


How small can the occupied space of a ceramic capacitor be?


Fluctuations in operating temperature can greatly affect the selection of the Vdrain decoupling capacitor, which requires a large capacitance for stable PA operation.

While the advantage of an electrolytic capacitor is the large capacitance that can be obtained per unit, there is a risk of reliability in an environment of long-term continuous use at high temperatures, as with a base station PA. Today, the design that is widely adopted achieves the capacitance needed by connecting 10 to 20 3225-size chip multilayer ceramic capacitors (125℃-guarantee, 50 to 100 Vdc, 4.7 to 10 uF) in parallel for Vdrain decoupling.

However, recently, we are seeing designs that use GaN transistors to enable large voltage operation and that operate at a drain voltage 48 V, increased from 28 V. Because ceramic capacitors with a high dielectric constant have characteristics that reduce the effective value of capacitance as the applied DC voltage increases (DC Bias characteristics), the number of capacitors connected in parallel must be increased to secure the capacitance. Meanwhile, as the number of on-board parts increases due to multiple-output (increased Tx) specifications, the area of the decoupling capacitor must be reduced.

Risk of reliability at high temperature


Murata offers a lineup of capacitors with metal terminals that can achieve a high capacitance while occupying a small area as a result of creating multilayer ceramic capacitors with two layers. Large (5750) chip multilayer ceramic capacitors tend to be avoided due to cracks caused by mechanical stress or cracks while soldering caused by the temperature cycle, but metal terminals can absorb that stress and successfully minimize those risks. The large capacity and space-saving design of capacitors for transistor Vdrain decoupling can provide greater design freedom.


Advantages of MLCC

List of product numbers
in the high-voltage, high-capacity KRM series (2 layers) with space-saving design