Subelement F: Receivers— Topic 45: IF Amplifiers
Question 3-45F1
Element 3 (GROL)What degree of selectivity is desirable in the IF circuitry of a wideband FM phone receiver?
Explanation
Wideband FM (WBFM) signals, commonly used for local repeater operation, employ a significantly larger frequency deviation compared to narrowband FM (NBFM). This wider deviation means the modulated signal occupies a much broader spectrum, generating more numerous and significant sidebands.
To properly receive and demodulate a WBFM signal without distortion, the Intermediate Frequency (IF) circuitry must have a sufficiently wide bandwidth to pass these sidebands. An IF selectivity of 15 kHz is appropriate for amateur radio WBFM, accommodating typical deviations (e.g., ±5 kHz) and the full range of modulating audio frequencies.
Filters of 1 kHz are suitable for CW, 2.4 kHz for Single Sideband (SSB), and 4.2 kHz for NBFM or some digital modes. These narrower bandwidths would severely clip the WBFM signal's sidebands, leading to high distortion and poor audio quality.
Related Questions
3-44F5 If a receiver mixes a 13.8 MHz VFO with a 14.255 MHz receive signal to produce a 455 kHz intermediate frequency signal, what type of interference will a 13.345 MHz signal produce in the receiver?3-44F6 What might occur in a receiver if excessive amounts of signal energy overdrive the mixer circuit?3-45F2 Which one of these filters can be used in micro-miniature electronic circuits?3-45F3 A receiver selectivity of 2.4 kHz in the IF circuitry is optimum for what type of signals?3-45F4 A receiver selectivity of 10 KHz in the IF circuitry is optimum for what type of signals?