Transmitters, receivers and transceivers
Transmitters, receivers, and transceivers
This domain tests radio architectures and the impairments that set sensitivity, selectivity, and spectral cleanliness. Superheterodyne image response, IF choice, phase noise, PA linearity, and duplex operation are core.
Core rules
- In a high-side superheterodyne, LO=fRF+fIF and the image is fRF+2fIF. A low-side LO gives LO=fRF-fIF and image=fRF-2fIF.
- Receiver sensitivity: -174+NF+10logB+required SNR. Selectivity depends on IF filtering and phase noise, not only on RF preselection.
- Reciprocal mixing: strong nearby signals mix with LO phase noise and raise the noise floor. Image rejection needs RF filtering before the mixer.
- PA efficiency rises from Class A to AB to C, but linearity falls. Back-off improves ACLR and EVM at the cost of efficiency.
- In FDD, the duplexer isolates transmit and receive ports; in TDD, switching time and isolation set the turn-around constraints.
Worked example
A receiver tunes 400 MHz with a 45 MHz IF and high-side LO. LO=400+45=445 MHz. The image is at 445+45=490 MHz, equivalently 400+2(45)=490 MHz. If the RF filter gives only 20 dB rejection at 490 MHz and the image signal is -30 dBm, it appears at -50 dBm at the mixer input, potentially desensitizing a weak wanted signal. Increase image rejection or change the IF plan.
Traps
Picking the wrong image side, omitting IF filter loss, and quoting sensitivity without bandwidth or required SNR. Also distinguish average and peak power when checking PA limits, and do not confuse duplexer isolation with antenna isolation.