Fundamentals of Radio Control - Basic Elements

Duplexer

Duplexer is a passive RF component which isolate Rx and Tx signals while they are using the same antenna.
The signal from to Tx port to Rx port is attenuated. The attenuation is called Tx Rejection. In addition, Tx noise contains spectral energy in Rx band. This needs to be rejected before reaching to the antenna port.  This is called Rx rejection. The rejection varies about 40dB to 60dB. 

The signal passing through from antenna to Rx port in downlink frequency band, and the signal  passing through from Tx port to antenna in uplink frequency band are subject to small loss, called Insertion Loss, which is generally about or below to 2dB.

RF Switch

RF switch is sued to multiplex multiple RF ports to one RF ports. In a common case, multiple RF RX and Tx ports are multiplexed to a single antenna. The key parameters are the insertion loss between RF ports and antenna, the isolation between RF ports. A few bit digital ports can be used to control the switch configuration.

Power Amplifier

Power Amplifiers, or in short PA, is used to amplifies the RF signal prior to antenna. RF signal amplification starts from mixer, and may continue with RF drivers within RFIC. The signal amplification level at output RFIC is less than the maximum allowable power levels from antenna, therefore an additional power amplification stage is needed in most cases.   In 3GPP WCDMA/HSPA standards, the maximum power level of a user equipment is defined to be 24dBm. RFIC in this field provides maximum of about 5 to 8 dBm.Assuming about 1dB loss from output of the power amplifier to the antenna port 17 to 20dB gain is required from PA.

One of the main concern in PA is the efficiency. In order to increase efficiency many techniques have been developed and commercialized. Discrete gain modes, adaptive discrete or continuous bias signal level, adaptive power supply to PA stages are some examples. Amplitude tracking is another method.A DC/DC converter can be used to provides variable power supply voltage to PA. All these controls are shown in basic form in the figure above.
Some PA or PA modules can support multiple bands, and this is controlled by an interface signal.

Power Detector and Maximum Power Control

The gain of the Tx path can be degraded due to temperature and therefore a calibrated gain mapping cannot help to make sure that PA is delivering maximum allowable power level when needed at the edge of cell coverage or more generally at the poor coverage areas. On the other hand, the maximum power level is also mandated by the regulations due to radiation hazard to human being and interference control within the wireless network. Therefore additional closed loop maximum power control can be used to solve these concerns, as shown above. Power detector samples the signal level at the output of PA and produces a voltage at its output, which can be sampled with an ADC. The curve can be obtained by the calibration which maps the reading level at the output of ADC to certain power level at PA. An algorithm determine a dynamically varying maxim gain control at the baseband/modem within RF software driver. This power control works with assumption of low sensitivity of power detector to temperature.

Low Noise Amplifier (LNA)

Every component at a receive path in an RF subsystem add some noise to the signal. LNA is an amplifier used at early stages on the receive path to amplify the signal before other components between LNA to the modem add noise. It may have programming for

• Band of operation
• Change the gain between multiple discrete gain levels.

Multiple gain stages are used to keep the different signal levels within the dynamic range and also to reduce power consumption at high received signal levels. At some RF subsystems there are external discrete LNA modules. At some other design these are located within RFIC.

Filters

Filters are used both on receiver and transmitter paths at RF system designs. They are two port components and they designed for specific band of operations. The filter passes the signal at designated band and attenuate all the other outside of the designated band. SAW (Surface Acoustic Wave) filters or BAW (Bulk Acoustic Wave) filters are commonly used in RF. BAW filters are relatively requires more complicated fabrication, and therefore they are not cheaper than SAW filter. However, BAW filters are known with sharper filtering. There are many parameters important in the characterization of a filter. The main ones important in RF radio control are:

• Insertion loss (less than 2dB in general, the loss within the designated pass band)
• Badwidth (The spectral with of the pass band)
• Rejection (The attenuation out side of the pass band)

No programming is needed.

Multiplexers

Multiplxers are used instead of antenna switches at some RF designs. It connects multiple RF signals to a single RF antenna, or single RF port. No programming is needed. Multiplexers are called according to the number of RF ports.Diplexer has two RF ports, triplexer has three RF ports, quadplexer has four RF ports and quintplexer has  five RF ports other than antenna port.


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