Hi,
switching ON and OF is like mixing. You will see the sidebands.
Not only at 1kHz apart, but - because square wave includes overtones - you will also see 3kHz, 5kHz, 7kHz...
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You can do an experiment: (even excel is good for this)
generate your carrier frequency. and generate a second "sideband frequency" (Carrier + 1kHz) then add both. What is the result?
Klaus
Both situations are mixing. Both will produce sidebands, but may differ in how strong the sidebands are relative to the carrier in the overall output spectrum. The first case sounds like the carrier will be suppressed entirely, while in the second case the carrier will remain.hello,
An AM modulated signal is composed of an unmodulated carrier and two modulated sidebands.
I am struggling to understand how it works.
If I mix a 1KHz audio signal with an RF carrier, the carrier plus two 1KHz apart from it sidebands will occur. This is clear.
However if I have an unmodulated carrier and I manage to switch it on and off at 1KHz range, will two sidebands appear that are 1KHz apart from the carrier?
There is no mixing in the second case, just switching the carrier on/off.
a mixer is very much like an on/off switch
On-off switching (OOK modulation) is a non-linear operation. As said it's the equivalent to 100% AM with a square wave modulation signal.but I see that there are switching mixers out there, in contrast to the non-linear mixing process that occurs in diodes or semiconductors.
Mixing can be ambiguous unless the kind of 'mixing' involved is clearly stated or defined.
In AM theory, they often start everybody off with 'multiplication' ..... pure mathematical multiplication. So here, mixing would mean 'multiplying'. cos(w1.t)xcos(w2.t) ..... if you mathematically expand it out using the cos(A)cos(B) expansion, you end up with cos([w1+w2]t) + cos([w1-w2]t), which is mathematically two sinusoids, one of them with frequency w1+w2, and the other frequency at w1-w2. This isn't 'AM' as such. It is called DSB-SC....aka double sideband suppressed carrier. If w1 is the carrier frequency, then the carrier mathematically interacts with the message to yield the two 'sidebands'.
You only get 'AM' if you add some DC voltage or constant voltage to the message .... ie.. instead of cos(w2.t) being the message, you add some DC ...such as 1V DC.... to get a message signal of 1 + cos(w2.t). So if you do this, then the product between the carrier and the message (containing DC), will yield something similar to before.... ie two sidebands, but you also get the cos(w1t) in there as well. This results in a 'carrier' component between the two sidebands. This is called 'AM'.
But, even for DSB-SC, with apparently no visible carrier..... the original carrier signal is actually responsible for the two side-bands, as it is the whole interaction that puts those side-bands there.
Now, for a single carrier that is switched on and off at some frequency. This is multiplication as well, but it's multiplying a sinewave with a higher frequency square-wave, where the square wave has levels of +1 and -1. The result is a waveform that looks like time-domain DSB-SC, except the edges are vertical. However, the signal can turn into something that really looks like DSB-SC (when it goes through a transmission medium that has limited bandwidth).
Mixing can be ambiguous unless the kind of 'mixing' involved is clearly stated or defined.
In AM theory, they often start everybody off with 'multiplication' ..... pure mathematical multiplication. .
I don't see what this mean. The two functions are needed at all time in order to form the output.I believe (not so sure!!) that the term mixing came up because the two functions are allowed to interact for some time (tau)- if the interaction time (tau) is zero, there will be no (AM) modulation.
Both you and me (#5) are guilty of oversimplification. Mixing should be considered as a convolution or faltung (original term) of two functions.
I don't see what this mean. The two functions are needed at all time in order to form the output.
Mixing, for two functions f(t) and g(t) is defined as the integral (f(t-tau).g(tau)).d(tau)
Thank you very much for the explanation!
As previously stated , switching as well as multiplication in time domain (operation of an ideal amplitude modulator) is already a non-linear operation, both in terms of mathematics and electronics. But there's in fact an impact of using non-ideal diode mixers instead of ideal multipliers. You get additional higher order intermodulation products that would be avoided by an ideal modulator.but I see that there are switching mixers out there, in contrast to the non-linear mixing process that occurs in diodes or semiconductors
Multiplying with +/- 1 is a possible mixing scenario, but different from the OOK modulation described in post #1.
OOK spectrum includes carrier frequency and multiple side bands at +/- the fundamental and odd harmonics of the modulation square wave, +/- 1 kHz, +/- 3 kHz, +/- 5 kHz and so on.
I expect the OP is well confused by now.
How about someone drawing a simple spectrum caused by switching a 1khz tone on and off?
(and mentioning the result will depend on what equipment is used and how good the filtering is
and what level the spectrum analyzer is resolving)
And maybe pointing out the internationally acceptable levels for transmission that exist?
I know at least 2 of you here are better qualified than me to do this.
Just KISS it.
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