Quiz 5: Transmitters

Trade & Technology

Frequency agility is the capability of a system to quickly alter its operating frequency with no extensive retuning. It is used in such systems where there is need to change the operating frequency but with little efforts. For example - Citizen Band (CB) radio which has around 40 available channels, so it requires rapid tuning between these channels. Therefore, the transceiver used in CB radio should be capable enough to support frequency agility. Whereas for broadcast transmitter frequency agility is not required, this is because stations using broadcast transmitter changes frequency very rarely and if these stations are willing to change the frequency then it is very time consuming process, as it requires wide changes in antenna systems and also retuning of the transmitter. Frequency agility can be used or desired in the following conditions: 1. When there is sudden need to change the operating frequency with not much retuning. For example-CB radio. 2. In systems where there is need to change the operating frequency so as to overcome the atmospheric effects, jamming and mutual interference with nearby sources.

The signals which are emitted outside the required frequency or bandwidth by the transmitter are known as spurious signals. These signals are produced by the transmitter at frequencies, other than the carrier frequency and sidebands of modulation scheme used. So, harmonics of the operating frequency are nothing but the spurious signals. Harmonics are also produced by the amplifiers when they are overdriven (becomes nonlinear), for example- Class C amplifiers. Sometimes, these spurious signals hinder the performance of transmitters by interfering with other transmissions. So it is always good to filter all the frequencies, other than the assigned transmitting frequency. So, Harmonics are the spurious signals which are commonly generated by the transmitter.

The signals which are emitted outside the required frequency or bandwidth by the transmitter are known as spurious signals. Harmonics of the operating frequency are nothing but the spurious signals. Sometimes, these harmonics hinder the performance of transmitters by interfering with other transmissions. So it is always good to filter all the frequencies, other than the assigned transmitting frequency. Following are the reasons which can describe the need of suppression of harmonics and spurious signals for a transmitter: 1. These spurious signals can lead to overheating of the equipment; as a result equipment will stop functioning and will eventually fail. For example harmonics can cause capacitor overheating, transformer overheating etc. 2. Spurious signals in the form of harmonics have tendency to interfere with the other transmissions. This can lead to noisy transmission and data loss or corruption. For example interference in telephone lines which causes false operation. 3. These spurious signals can cause false operations of a system which can lead to tripping of the circuit. 4. Large number of harmonics and spurious signals can together, affects the output of the transmitter and hence making it inefficient to work.