In the first episode of the RF Modulation Schemes series, we refreshed our understanding of radio frequency (RF) modulation basic principles. This technique allows us to imprint and extract data to and from electromagnetic waves (carriers) that transport information between distant places.
You’ll recall that sinewave carrier signals have three parameters we can modulate: Amplitude, frequency, and phase. In this second episode, we’ll take an analog message and apply the three basic modulation techniques: Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM).
The carrier amplitude is modulated by the information.
In amplitude modulation, the analog message signal is multiplied by the carrier signal in a mixer. The carrier amplitudes vary with the amplitude of the information signal (M(t)); the visual result is a sort of envelope appears.
Figure 1: Analog amplitude modulator – time domain view (Image: Ivan Akira, Wikimedia Commons. Used under Creative Commons Attribution-Share Alike 3.0 Unported license.)
The mixer does the multiplication as follows, with carrier frequency ωc and message frequency ωm. The combined signal appears as:
Ac Sin(ωc.t) * Am Sin(ωmt) or:
Ac* Am [Cos(ωc-ωm)t - Cos(ωc+ωm)t]
One can observe the original frequencies ωc and ωm have disappeared and two new ones are generated:
(ωc-ωm) and (ωc+ωm).
They can be better visualized in the frequency domain:
Figure 2: Analog Amplitude Modulation – Frequency domain
There are various ways to implement an analog signals multiplier function. Here below is a typical one:
Figure 3: Possible analog amplitude modulator implementation (Image: しまでん, Wikimedia Commons. Used under Creative Commons Attribution-Share Alike 3.0 Unported license.)
To unpack the message at the receiver side, one needs to remove the carrier frequency from (ωc-ωm) and (ωc+ωm). This is done with an AM demodulator, also called an envelope detector. The simplest version of this circuit uses a diode and an RC filter.
Figure 4: AM demodulator (Image: Chetvorno, Wikimedia Commons. Public domain.)
Figure 5: In red, the AM demodulated signal (Image: Bas Duineveld, Wikimedia Commons. Public domain.)
The carrier frequency is modulated by the information.
The principle of FM modulation is that the carrier frequency changes (slightly) with the information signal. FM modulation is much more robust than AM because it’s less sensitive to amplitude perturbance. The useful signal acts on the carrier frequency and not on its amplitude. The disadvantage is that FM consumes more frequency band.
Figure 6: Principle of FM modulation (Image: Michel Bakni, Wikimedia Commons. Used under Creative Commons Attribution-Share Alike 4.0 International license.)
Many analog FM modulators use an LC oscillator with variable capacitance diodes, or Varicaps. These are diodes biased in reverse which exhibit capacitance changing with the applied voltage. Hence the produced LC frequency is modulated by the message input.
Figure 7: Typical FM modulator circuit (Image: Hapa, Wikimedia Commons. Public domain.)
FM modulation creates side frequencies of greater or lesser importance depending on the ratio between the carrier frequency variation and the message frequency. This is called the modulation index β.
β = Maximum frequency deviation / Maximum message frequency
More side frequencies will be seen as β increases. In general, when β <0.5, the FM transmission is called narrow band FM or NBFM. When β >0.5, the transmission is called wide band FM or WBFM. To obtain good information quality (i.e. audio), WBFM is a must.
You can see in the graphs below that the larger the modulation index, the more frequencies will be produced, and hence it will require greater bandwidth to support them.
Figure 8: FM modulation effect on bandwidth (Image: Jlpons, Wikimedia Commons. Used under Creative Commons Attribution-Share Alike 4.0 International license.)
At the FM receiver side, various circuit topologies exist to extract (detect) the message out of the carrier. A more detailed description of the most common FM receiver architecture can be found on the Analog Devices University site.
Figure 9: FM demodulation principle
In our present discussion, we present one typical example, well suited for chip integration since it does not use any transformer or resonant circuits; however, high-speed elements are required. This is the pulse-averaging discriminator. This method detects how often the FM signal crosses zero. Each crossing produces a pulse. The series of pulses reflect the combined frequencies of the carrier and the message. By low-pass filtering the signal, the message is then extracted.
Figure 10: Pulse-Averaging discriminator FM detector
The carrier phase is modulated by the information.
To complete the analog modulation triplet, we mention the phase modulator. In this scheme, the carrier phase is modulated by the message signal. The resulting signal looks very similar to a frequency modulation since both frequency and phase affect the angle in the carrier sinewave.
Figure 11: Graphic view of an analog phase modulated signal (Image: Potasmic, Wikimedia Commons. Public domain.)
However, at the receiver side, one must detect the angle deviations in a PM scheme; that’s much more difficult to do than detecting frequency variation. That’s probably the main reason analog PM is not widely used for transmitting analog audio signals via radio waves. But in digital, PM is much more popular.
We have learned (or refreshed) the three basic modes to modulate analog messages on a sinewave carrier by imprinting the instant input levels to its amplitude (AM), frequency (FM), or phase (PM). In today’s RF landscape, analog modulations have largely been phased out in favor of digital modulation techniques. AM and FM are only still used in some niche applications where message flow is not important, such as FM radio broadcasting.
