How a radio transmitter circuit mixes audio with carrier wave? - Page 1
Forum section: electronics / rf-microwave

author: prabhatkarpe4
author_rank: Contributor
timestamp: October 23, 2024, 11:00:57 am
content: I need to know how a radio transmitter circuit mixes audio signal with a radio wave (carrier wave)?Is this mixing called modulation?Why is an oscillator required in a radio circuit?

author: Andy Chee
author_rank: Super Contributor
timestamp: October 23, 2024, 12:51:02 pm
content: Yes, you can call this mixing modulation. Most commonly known are AM (amplitude modulation) and FM (frequency modulation).An oscillator creates the radio frequency carrier wave, for example, AM radio frequency might be 500kHz to 1.6MHz and FM radio frequency might be 80MHz to 108MHz.If you don't use a carrier, then you have to transmit the audio signal baseband 20Hz to 20kHz, which are *extremely* long wavelengths in radio waves.

author: TimFox
author_rank: Super Contributor
timestamp: October 23, 2024, 01:41:11 pm
content: Technically, AM might be considered mixing, but FM is a different process.If interested, you should consult the literature on FM, since the different methods of doing the modulation are quite interesting.Also, the spectrum generated by FM is complicated, involving Bessel functions, while the AM spectrum is simple.

author: A.Z.
author_rank: Super Contributor
timestamp: October 23, 2024, 02:05:12 pm
content: Quote from: prabhatkarpe4 on October 23, 2024, 11:00:57 amwhy don't you get some decent textbooks (there are a lot for free), STUDY them, and then make yourself some educated guess ?

author: CaptDon
author_rank: Super Contributor
timestamp: October 24, 2024, 08:57:08 pm
content: Stop and think for ten seconds, where would the carrier come from without an oscillator?? I'll give you a clue, where would the sound (audio carrier) of a whistle come from without a whistle? One simple explaination of AM modulation is raising and lowering the supply voltage to an oscillator (bad idea as it will also cause some FM shifting) or raising and lowering the supply voltage to the R.F. buffer or final amplifier thereby changing the amplitude of the carrier and generating sidebands.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 24, 2024, 09:21:41 pm
content: Modulation comes in many ways. The carrier is modulated in some way, and then the receiver must be able to process that modulation.FMAMDMRWiFi7 has a neat modulation schemeand many others

author: TimFox
author_rank: Super Contributor
timestamp: October 24, 2024, 09:52:22 pm
content: Back to simple modulation, for "AM".One way to get amplitude modulation is to use an analog multiplier, whereVout=XxY/H, whereXandYare the two input voltages andHis a constant factor.All three terms and the output are measured in Volts.The RF (carrier) signal isY(t) =Cx cos(wt), whereCis the constant amplitude of the RF carrier.It comes from the oscillator.Let the audio signal beA(t), where the waveformAhas 0 DC value (since it only goes down to, say, 20 Hz), and the peak-to-peak value is 2 V (so the waveform varies between -1 V and +1 V).For "regular AM" (which contains the carrier and both sidebands), letY(t) =Cx cos(wt) andX(t) = (1 V) +A(t) , which ranges from 0 to +2 V.When the audio signalA(t) goes through 0, the output is the carrierCx cos(wt) /H, remembering the scale factor.For "double-sideband suppressed carrier" (DSB), we drop the extra 1 V and simply letX(t) =A(t), giving the outputXY/H=Cx cos(wt) xA(t) /H.When the audio signalA(t) goes through 0, the output is 0.When the music stops, there is no output for DSB, while the AM output is the carrier power.The carrier carries no information, and can be considered wasted power.However, it allows the modulated signal to be de-modulated using a simple diode rectifier and low-pass filter, as found in normal AM radios.

author: iMo
author_rank: Super Contributor
timestamp: October 26, 2024, 09:59:00 am
content: https://www.eevblog.com/forum/beginners/newbies-please-read-before-posting-24542/

author: A.Z.
author_rank: Super Contributor
timestamp: October 26, 2024, 10:51:39 am
content: Quote from: iMo on October 26, 2024, 09:59:00 amtotally correct and agreed, but please, please; check the posts (and threads) history of the OP