I cannot find a simple (or really, not-so-simple) explanation of how mixing the high-frequency carrier wave with the lower-frequency input signal works...

Is it entirely due to constructive and destructive interference?

Also, are the carrier-wave and modifying wave created and broadcast on all the same equipment? Broadcast using the same antennae, etc.?

  • 2
    \$\begingroup\$ are you thinking AM or FM? \$\endgroup\$
    – Andy aka
    Jun 29, 2020 at 14:43
  • 1
    \$\begingroup\$ For starters, you should just search for "AM Modulation" on the web. AM is probably the easiest to understand with a minimum (if any) of math, and there are a lot of good references out there. Then come back with specific questions. \$\endgroup\$
    – SteveSh
    Jun 29, 2020 at 17:48
  • \$\begingroup\$ The question embraces indefinitely general area of interest; one cannot deduce genuine intention of OP here. Still, whoever has downvoted the answer here, s\he is disrespectful to the SE policies. Both answers are good and useful at any level of comprehension of the subject matter. \$\endgroup\$
    – V.V.T
    Jun 30, 2020 at 10:44
  • \$\begingroup\$ @V.V.T It can be difficult to ask a specific question in an unfamiliar area. However Kurt doesn't seem to have accepted any answers in a sample of his questions. Kurt, have a look at the help and accept answers that address your queries. If answers don't address your queries please add comments to indicate where you need further infomation. \$\endgroup\$
    – Graham Nye
    Jun 30, 2020 at 18:47

2 Answers 2


The process of getting an input signal to modify the carrier-wave of a radio signal is known as modulation. The input signal is called the baseband signal or modulating signal. The combined signal is called the modulated signal.

Wikipedia has an introductory article on modulation which briefly reviews a wide range of modulation types. You might want to follow up with SteveSh's excellent suggestion of looking at AM as a straightforward modulation scheme.

Is it entirely due to constructive and destructive interference?

No. Modulation doesn't work by just adding the instantaneous values of the modulating and carrier signals together. The mathematical description varies between modulation types. AM is the simplest. (If you don't get on with that Wikipedia description, as SteveSh points out, there are plenty of others.)

Also, are the carrier-wave and modifying wave created and broadcast on all the same equipment?

It varies but the number of equipment boxes doesn't affect the principles of modulation. You could do everything in a single box, like a mobile phone, or split the transmitter functions across multiple boxes.

The modern approach, used across the full range of modulation types, is to perform the modulation at low power after which the modulated signal is amplified in the transmitter and possibly a separate power amplifier. (As you've added a ham-radio tag the power amplifier is often known as a linear in amateur radio circles.)

The modulation might be done in a separate box known as an exciter, which will send a low power modulated signal to a separate transmitter to be amplified then sent to the antenna. Or the exciter and RF amplification functions can be combined in a single box, also known as a transmitter.

An older technique, used in older ham AM (and other modest power) transmitters is high level modulation where a baseband audio signal is amplified to a high power level in a separate audio amplifier chain and is used to modulate the RF carrier at high power in the final stage of the transmitter.

Broadcast using the same antenna?

Once the baseband signal and the RF carrier have been combined in the modulation process only a single modulated signal is sent to the antenna. There are no longer two separate signals to send to the antenna.


Most often, precise analog multipliers are used, so the carrier amplitude and phase are rather carefully modified. For arbitrary modulation, you use I/Q modulation and I/Q carrier, and linearly sum the 2 multiplier outputs.

Sometimes this multiplication is performed in the moderate 100MHz range, and then up-converted to RF.

If you need DENSE constellations, where precise magnitude/phase positions is crucial, this is the method.

I have seen less-precise modulations (not dense QAM) generated by carefully contrived energy cancelling at the RF/carrier. Tiny pulses, of the right time and width, are summed with the main carrier; this results in acceptable spectral shaping, and the FCC is happy.


Suppose you want FM modulation? ( I understand this, because I assisted in a transmitter for GSM cell signals, and GSM is minimum_shift_keying FM.)

By rotating the baseband I/Q phases at 10,000 cycles per second, you will offset the carrier ---- up or down, depending on UP phase or DOWN phase --- by that 10,000 Hertz.


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