Learn about multiplexer filters
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Learn about multiplexer filters

Posted Date: 2024-01-19

This chapter will take a closer look at multiplexer filters and how they are used in a variety of applications. You'll learn how multiplexers help designers create more complex wireless products.

Learn about multiplexers

A multiplexer is a set of radio frequency (RF) filters that are grouped together without loading each other to achieve a high degree of isolation between outputs. Multiplexers are used in the RF front-end close to the power amplifier (PA), which has a great impact on carrier aggregation (CA); antenna multiplexers are used behind the RF front-end to simplify routing to the antenna . Multiplexer filters can contain multiple filters, all integrated into a single package. Figure 1 provides a schematic diagram of several types of filters embedded in a complex module design. These individual duplexers, bandpass filters, and notch filters shown in the figure can also be part of multiplexer groups, called quadplexers, pentaxers, and hexaplexers, as shown in Figure 2 Show.


Figure 1: Bandpass filter, diplexer, notch, and multiplexer in functional block diagram

A simple duplexer can be combined with other duplexers and filters to create more complex multiplexer designs. Just like frequency division duplex (FDD) bands are 1 and 3, time division duplex (TDD) filters are band 41.

Today's wireless systems and devices must support more features, so they require smaller components. Integrating multiple non-overlapping filter bands into a multiplexer can help reduce the component count and size of the radio frequency front end (RFFE). Figure 2 shows some types of multiplexer designs that help reduce the overall RFFE component count to meet the size reduction requirements of new devices. Multiplexers enable system designers to optimize, shrink and simplify the overall design to meet system requirements.

Multiplexers are a very good solution and in many cases are the only practical solution for carrier aggregation (CA) combinations using closely spaced frequency bands. A multiplexer integrates all the transmit and receive filters required to aggregate component carriers (CCs) into a single component, providing the necessary isolation and allowing multiple CCs to be connected to the antenna simultaneously.


Figure 2: Types of multiplexer filters

To achieve the required performance, the multiplexer filters must be carefully co-designed and matched. Figure 3 shows the hexaplexer RF insertion loss for each filter. As you can see, each duplexer in this multiplexer (sixplexer) is matched for optimal insertion loss and passband. Additionally, the design of the individual filters ensures that they do not load each other.

TIPS

With a high level of integration, a well-designed multiplexer can bring even more benefits to mobile device engineers. Consolidating multiple filters into a single component saves critical printed circuit board space compared to using discrete filters.

There has been some discussion about the use of tunable filters in these complex systems. However, using tunable filters may not improve system functionality because all systems today require the use of multiple RF paths simultaneously. To accommodate this multipath capability, tunable filters can become more complex, so manufacturers have been pushing for more advanced multiplexer designs for more complex designs, such as eight multiplexers, which have eight RF Functional path.


Figure 3: B1+B3+B7 six-plexer filter insertion loss measurement

Understand multiplexer isolation and cross-isolation

As mentioned earlier, the degree of isolation and cross-isolation are important parameters worthy of attention. Isolation is the ability of a filter to prevent signals from appearing at a certain node in an RF circuit or system. For example, a high level of isolation needs to be provided in the transmit or receive path to prevent individual signals from interpenetrating.

Cross-isolation refers to isolation across frequency bands, as shown in Figure 4. The duplexer requires significant attenuation of the Tx signal at the corresponding Rx frequency output. For multiplexers such as quadplexers, the Tx signal needs to be significantly attenuated at both receive outputs. Likewise, the Tx signal at the Rx frequency must now be isolated at both Rx outputs to control noise in the Rx band. When you consider all things considered, there are eight significant isolations in a quadplexer versus only two in a duplexer. Figure 4 shows the isolation measurement between Tx and Rx for Band 1 (top graph). For the same components, we show cross-isolation measurements between Band 3 Rx and Band 7 Tx (graph below). Cross-isolation is a measurement between two different Tx to Rx bands in the same component, and isolation is a measurement between Tx and Rx in the same band.


Figure 4: Measurement of Band 1 Rx isolation, and Band 1 and 7 Tx cross-isolation

Multiplexers help achieve the required cross-isolation between aggregated RF paths to enable simultaneous communications on all aggregated carriers while attenuating out-of-band (OOB) signals for each path.

Learn about multiplexers and carrier aggregation

CA is another complex feature of RFFE. Advances in filter technology are key to driving the adoption of carrier aggregation (CA) based on Long Term Evolution (LTE), 4G and 5G. Advances in filter materials in coupling, aluminum nitride reinforcement, and material doping help achieve high OOB attenuation, low return loss, and the cross-isolation required to enable multiplexer filter CA.

As the number of CA CCs increases, the importance of multiplexers will continue to increase. For example, aggregating three or more CCs significantly increases the likelihood of using closely connected bands. These opportunities increase the possibility of using multiplexers, such as quadplexers or hexaplexers.

CA enables microwave devices to achieve higher data rates by combining two or more carrier signals. CA becomes more complex as more bands are added to the combination options. 5G defines hundreds of new combinations of up to 16 CCs, each with a continuous bandwidth of up to 100MHz and a total aggregate bandwidth of around 1GHz.

CA can be divided into three categories, as shown in Figure 5:
» Intra-band continuous aggregation: used when the spectrum modules are adjacent spectrums in the same frequency band.
» Intra-band non-contiguous aggregation: Used when spectrum modules are independent of each other within the same frequency band.
» In-band aggregation: Spectrum aggregation in different frequency bands. These frequency bands may be widely separated or connected together.


Figure 5: Three types of CAs

Linear multiplexer filters with high OOB rejection support the use of multiple CA combinations. By using the latest filter technology, such as BAW, engineers can use multiplexers with minimum insertion losses of less than or approximately 1 decibel (dB). This low insertion loss minimizes negative impact on power amplifier current consumption and device battery life. Enhanced filter technologies like BAW also provide excellent band isolation and cross-isolation for optimal system-level performance.

It is important to note that CA has a greater chance of causing interference. Multi-band signals may interfere with each other due to insufficient filter attenuation. System desensitization can also occur when there is insufficient isolation or cross-isolation between the Tx and Rx paths, resulting in reduced receiver sensitivity.

The cryogenic compensation filter technology used in multiplexers is ideal for optimizing isolation, attenuation and OOB attenuation performance across frequency bands, helping designers address these challenges.

Additionally, in CA applications, RFFE switches used with filters require ultra-high linearity and low insertion loss. Because increased insertion loss can lead to increased system noise, designers must adopt a receiver path. If the insertion loss is poor, it will also reduce the overall PA efficiency, reduce battery life and device signal range.

Mobile devices use high-speed up/down links to transmit video and data. The sensitivity of the receiver will be affected by noise, which in turn affects signal reception. This noise can cause system desensitization. Desensitization is a decrease in receiver sensitivity due to noise sources, often generated by the same equipment radio.

EN-DC multiplexers and carrier aggregation

4G LTE-Advanced Pro supports up to 5 CCs. 5G New Radio (NR) CA supports up to 16 contiguous and non-contiguous CCs and can aggregate new 5G bands of spectrum up to approximately 1GHz, as shown in Figure 6. Dual connectivity allows user devices to send and receive data simultaneously. Coupled with CA, the data capacity of the network can be increased.


Figure 6: LTE and 5G NR CA

Dual connectivity is also widely used in new 5G devices. Dual connectivity enables user devices to be on two cell groups (primary evolution node B) simultaneously [eNB] and secondary node eNB) to send and receive data on multiple CCs. When using Evolved Universal Terrestrial Radio Access (E-UTRA) (which is the radio interface of 3GPP) and 5G NR Dual Connectivity or E-UTRA NR Dual Connectivity (EN-DC), the network can leverage the 4G and 5G spectrum to add users throughput, provide mobile signal reliability, and support eNB load balancing.

Non-standalone and EN-DC

The 5G Radio Access Network (RAN) is designed to work with existing 4G LTE networks. The 3GPP Release 15 standard allows for multiple NR deployment options such as non-standalone (NSA) and standalone (SA). The scheme used by the NSA is very similar to that of the CA. It uses a combination of LTE anchor frequency bands for control and 5G NR frequency bands to provide faster data rates, as shown in Figure 7. NSA is a deployment model that provides 5G services without an end-to-end 5G network. EUTRA and NR dual connections are used. In EN-DC, LT and 5G NR carriers are used simultaneously. When using EN-DC, one device transmits two high-power RF signals.

This configuration requires integrating two complete uplink signal paths into the smartphone's small device area. This poses a significant challenge, namely the need to avoid two strong signals interfering with each other. In these cases, high isolation filtering must be achieved.


Figure 7: LTE and 5G NR CA

A high degree of isolation between these two signal paths is critical to limiting intermodulation products and meeting OOB emission specifications. In addition, the use of highly isolated multiplexers between signal paths enables coexistence between the two uplink signals, thereby increasing efficiency and allowing smartphones and mobile network operators to save battery power and energy.


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