Why is FM operated at high frequencies when compared to AM ?

Answer 1

AM radio is the older technology. When FM technology was developed, it allowed higher frequencies (then unused) to be used.

Answer 2

The audio frequencies to be transmitted are used to modulate the carrier frequency. The high frequency end of audio is 18 to 20 KHz , the carrier would have to shift from its base frequency +/- 9to10KHz .

Compare the percentage shift from base frequency of +/- 10KHz against a low frequency in the AM band vs the high frequency in the FM band. The percentage shift in frequency is much smaller in the high frequency FM band.

The percentage in frequency shift is done in an oscillator circuit, It is far easier and much more stable to adjust an oscillator by a small percentage than by a larger percentage. Oscillators used tuned circuits of coils and capacitors to generate a particular frequency . The large the values of capacitance and inductance used typically resulted in lower frequencies. (Smaller capacitance equals a higher frequency)

Some Early FM mikes were capacitive type instead of resistive (carbon) type. These capacitive microphones only produced a small limited change in capacitance in response to audio input. The capacitive microphone would be connected to a Colpitts oscillator where this changing capacitance would be used to modify the base capacitance . The result is a cahnging frequency in response to audio input.

So in order to change the oscillator frequency by the required percentage to transmit audio with such a small change in capacitance requires a smaller oscillator capacitor which in turn generates a higher base frequency......


In addition high frequencies use smaller coils and capacitor which in turn are cheaper and smaller. This would also aid in producing smaller and cheaper radio receivers. It is most likely that these two reasons greatly influenced the location of FM within the radio spectrum.

Answer 3

FM radio stations have 200kHz of bandwith, with the carrier frequency in the middle. The range allows a broad range of audio frequencies to be represented allowing for better music. By contrast AM radio stations are allocated only 10kHz of bandwidth, with the carrier frequency again in the middle of the range.