From Transceiver and System Design for Digital Communications, Second Edition

1.4 Transmitter

The transmitter creates, modulates and transmits the signal through space to the receiver (see Figure 1-1). The transmitter is responsible for providing the necessary power required for transmitting the signal through the link to the receiver. This includes the power amplifier, the transmitter antenna, and the gains and losses associated with the process, such as cable losses, to provide the effective radiated power (ERP) out of the antenna (see Figure 1-2).


Figure 1-2: The transmitter block diagram.

Power from Transmitter

The power from the transmitter is the amount of power output of the final stage of the power amplifier:


For ease of calculating the link budget, the power is in dBm or converted to dBm from milliwatts. Power in milliwatts is converted to power in dBm by


Therefore, one milliwatt is equal to 0 dBm. The unit dBm is used extensively in the industry and a good understanding of this term and other dB terms is important. The term dBm is actually a power level related to one milliwatt and is not a loss or gain as in the term dB.

Transmitter Component Losses

With most systems, there will be RF components, such as circulators and transmit/receive switches that enable the transceiver to use the same antenna for both transmit and receive. Also, if there are antenna arrays, (multiple antennas), there will be components that interconnect the individual antenna elements. Since these elements have a loss associated with them, they need to be taken into account when...

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Antennas
Antennas are structures or devices used to collect or radiate electromagnetic waves.
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RF isolators and RF circulators are passive devices used to control the propagation of an RF signal.
Fiber Optic Polarizers
Fiber optic polarizers (FOP) are placed inline to improve the extinction characteristics of fiber optic cable. They allow the transmission of only one polarization, blocking light in unwanted polarization states.
RF Power Meters
RF power meters measure the strength of an RF signal in dBm or watts and may also be called an RF Wattmeter. Typically RF power meters are used to measure the output of a transmitter.

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