Question T4B12
From subelement T4 - T4B
What is the result of tuning an FM receiver above or below a signal’s frequency?
Why is this correct?
FM receivers use frequency discrimination to convert frequency variations into audio signals. When tuned off-frequency, the receiver's discriminator circuit operates on the slope of the signal rather than at the optimal center point. This causes nonlinear demodulation, resulting in distorted audio output. Unlike AM where off-tuning might change pitch or create heterodynes, FM's frequency-to-amplitude conversion process specifically produces audio distortion when not properly centered on the signal.
Memory tip
Remember that FM demodulation is fundamentally different from AM. In FM, the information IS the frequency variation, so precise tuning is critical for clean audio recovery. Off-frequency tuning in FM always means distorted demodulation, not frequency shifts like in other modes.
Learn more
In practical operation, this explains why FM repeaters and simplex contacts require precise frequency control. Modern transceivers use crystal-controlled synthesizers to maintain frequency accuracy within tight tolerances. The FCC's emission standards for FM include frequency deviation limits partly because receivers depend on accurate center frequency alignment. This is why frequency coordination is essential for repeater systems—even small frequency errors can cause audio quality degradation across the coverage area.
Think about it
Why do you think FM systems are more sensitive to frequency accuracy than AM systems, and what does this tell you about the trade-offs between different modulation methods?