FM vs AM: Exploring Signal Quality and Power Efficiency

Historical Background and Experimental Setup

When analyzing the modulation schemes of FM (Frequency Modulation) and AM (Amplitude Modulation), we often come across questions about their transmission power requirements. An interesting insight into this topic was provided by Edwin Armstrong in the 1930s. As he was developing and characterizing FM, Armstrong conducted several experiments to compare these two methods. To ensure these comparisons were fair, he employed specialized equipment capable of generating controlled amounts of interference. He transmitted both AM and FM test signals on the same frequencies, which were around 44 MHz. These experiments laid the groundwork for understanding the relative strengths of FM and AM.

Efficiency and Signal Quality

Based on his experiments, Armstrong discovered that FM required significantly less power to achieve the same signal-to-noise ratio (SNR) compared to AM. This is because FM effectively masks interference, reducing its impact on the received signal. Consequently, Armstrong could transmit an FM signal with less noise at the same power level as an AM signal. This led him to label his proposed method as a noise reduction technique. This technique was a central aspect of his notable paper published in the Institute of Radio Engineers (IRE).

Bandwidth and Transmission Spectrum

However, there was a catch to Armstrong's findings. The FM signals that he experimented with required a large amount of spectrum space to achieve their best signal-to-noise ratios. This is why they needed to be broadcast on Very High Frequency (VHF). If FM signals were to occupy less spectrum space, their noise reduction advantages would be diminished. To be precise, if the deviation of an FM signal was kept low enough, it would be possible to broadcast it on the conventional AM band. However, such a signal would have no performance advantage over AM. This flexibility in bandwidth allocation was essential, especially when systems like AM stereo were being developed by RCA and Bellar in the late 20th century.

Instantaneous Transmit Power and Limitations

Understanding the transmit power in AM and FM requires some awareness of how these systems operate. In AM systems, the allowed peak power (such as 100 kW in the examples) must not be exceeded. However, the average output power can vary beneath the peak allowed. This variability is crucial, as it allows for efficient transmission of audio signals with changing amplitudes.

Another important factor to consider is the Effective Radiated Power (ERP), which takes into account the antenna system design and height above average terrain. ERP is essential for calculating the overall power that is radiated into space, but it does not directly relate to the input power. For a more detailed explanation of these concepts, please refer to resources such as Wikipedia or the The Amateur Radio Handbook. Additionally, Part 15 of the FCC code likely covers these aspects comprehensively.