3.3 Antennas and the Link Between a Transmitter and a Receiver

Chapter 3.3 Antennas and the Link Between a Transmitter and a Receiver

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 70 System Level Architecture and Design ConsiderationsExample 3.9: Specifying an ADCAn ADC is required to have a SNR of 30 dB and a signal bandwidth of 20 MHz. Ignoring any out of band interferers, give as much detail as possible about the per-formance the ADC would require. Solution: First let us assume that we are going to use a Nyquist rate ADC for this design and therefore the ADC will be clocked at 40 MHz and the OSR will be one. To give some margin in the design we will calculate the number of bits required for a SNR of 33 dB. In this case making use of (3.54), this ADC will need: 1.765.26.02SNRN-== Thus, a 6-bit ADC will be adequate. Next we can calculate the requirement on the reference clock jitter using (3.59). Again assuming 33 dB to give some margin: π--æöç÷èø==×1log200.18 ns2jitterjitterinSNRtf Therefore, this application will require a reference clock with better than 0.18 ns of jitter. 3.3  Antennas and the Link Between a Transmitter and a ReceiverIn general, information can be transmitted through cables, fiber, and other media. Transmitters and receivers designed with RFIC technology most often use air as the channel link between the transmitter and the receiver. In this case, there must be an antenna attached to both radios to transform between RF signals in the electronic circuits and EM radiation in the air. The antenna attached to the transmitter radi-ates the EM signal into the air. It is then the job of the receiving antenna to collect some fraction of the energy transmitted and provide this energy to the receiver so that the signal can be detected and processed. The design of antennas of all descrip-tions could take up a whole book by itself, however some very basic information will be given here in order to understand some of the limitations of transmitting information across a radio link. Antennas can either be isotropic, which means that energy radiates equally in all directions, or alternatively the antenna can be direc-tional, which focuses more energy in a particular direction in order to maximize the amount of energy that can be detected across the link. The amount of directivity that an antenna possesses can be thought of as an antenna gain. The antenna gain refers to the amount of excess energy transmitted in a given direction over and above that which would have been transmitted by an isotropic antenna. It can be shown that in free space the amount of power collected by a receiving antenna is given by [9]: