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On the transmitter side, the digital I and Q data – which are already processed by a digital-to-analog converter, filtered and amplified – are up-converted by quadrature mixers to the carrier frequency of interest. After combining, the signal is again filtered to contain the spectral content of the signal in the required bandwidth stipulated by the emission mask. After that, it is applied to the power amplifier and transmitted over the air with an antenna (Figure 2).

Now let's get into the details for each of the blocks in the transmitter and receiver.

Antennas

Antennas are coupling circuits to space that radiate or receive information-bearing electromagnetic waves. In a receiving antenna, the EM wave impinging on the surface produces currents, which in a 50-ohm system are applied to an LNA for amplification and subsequent processing. On the transmitter side, the surface current density on the antenna produces a magnetic field around the antenna. If the current density is time-varying, an accompanying electric field is also produced; propagation takes place in a direction perpendicular to both the electric and magnetic fields. The total radiated power is given by a surface integral of the Poynting vector over any surrounding surface. The value of resistance that would dissipate this amount of power is called the radiation resistance, which is caused by the power radiated. The total resistance of the antenna comprises radiation resistance and resistance due to power loss. For high efficiency, the value of radiation resistance should be large.

Filters

Filters remove the effect of broadband noise and thereby increase the SNR of a desired signal. They are also used to select channels in multiple transmission environments and to remove image frequencies in broadband services and other out-of-band interference. In the transmitter, digital pulse-shaping filters are used for efficient utilization of the RF spectrum and externally to suppress RF splatter in adjacent channels.

Amplifiers

The RF signal at a receiver’s antenna is very small in magnitude. The IEEE 802.15.4 standard defines a minimum signal of -85 dBm = 3.16 pW, whose voltage in a 50-ohm system is 12.6 µV. At the detector, the typical signal requirement is at 1 mVp-p for detection and decoding of digital waveforms. To achieve this, low-noise amplifiers are used in the front end to amplify the signal up to the detection stage. The gain required in the receiver is usually between 60-90 dB – very high. Therefore, to avoid oscillations, this gain is distributed over different stages of the radio-frequency integrated circuit.

On the transmitter side, power amplifiers (PAs) are used to transmit the EM wave. PAs come in various classes and can be linear and nonlinear. They usually employ matching circuits between the output and the load. In practice, the output impedance of the active device is complex and varies with load; thus nonlinear complex impedance must be matched to a linear load. More often, the antenna impedance may be complex and vary with both the position of the transceiver and surrounding objects. This makes PA input and output matching a nontrivial task. In practice, a technique called load pull is applied to a matching circuit design. In this test, the output power is measured and plotted as a function of the complex impedance load seen by the transistor output stage. A tuner can accurately vary the output impedance while a power meter measures the power, keeping it constant. The impedance gives a contour on the Smith chart. As the output impedance varies, this changes the input impedance of the transistor, thus requiring the use of a second tuner such that the impedance seen by the generator remains constant.

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Source:  OpenStax, Senior project guide to texas instruments components. OpenStax CNX. Feb 12, 2013 Download for free at http://cnx.org/content/col11449/1.3
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