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Why do we use I and Q signal instead of the original signal

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Full Member level 3
Mar 1, 2022
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in the following receiver, why don't we use a single mixer demodulating the original signal, instead of extracting I and Q signal and combine them to produce the original signal?


what are the advantages of using such technique?

Do you know the math behind this idea ? I mean why an oscillator has been used to drive two mixers by 90 degree phase difference ??
It's a homework for you..

There are many reasons for using the I and Q channels.
To mention just one, it comes down to signal processing theory.
In other words, it has to do with the composition of the signal.
Currently, signals with I and Q structure are used for communication.
To demodulate this signal, a demodulator with I, Q structure must be used.

The short answer is that you could if you had an RF sampling ADC and implement the mixers in digital. But you would still need the mixers.

In a system like this, I and Q channels are orthogonal to each other (they can't be represented as each other, they complete the I/Q plane). Think of mixer as multiplication, and let's say you only mix with 0 degree phase. This would multiply the information carried orthogonal to the mixing signal with 0 so you'd lose the Q channel information (Although technically you can do some non-linear stuff to mix them both but that's not the point). You can find the proof in most textbooks, or do it yourself using trig identities.

Very simple explanation is, assume Q is information carried by a sin wave and info is not time varying, I is information carried by a cos wave and info is not time varying. If you multiply a sin with a sin at the same frequency/phase, you'd get a number, but if you multiply sin with a 90deg phase shifted sin (or in other words a cos wave which is our Q channel), you'd get 0. So you need two phases to cover all the information in I/Q plane.

Disclaimer: This is a very simple explanation. There is more to it, I'm just trying to give context.
짧은 대답은 RF 샘플링 ADC가 있고 믹서를 디지털로 구현하는 경우 가능하다는 것입니다. 그러나 여전히 믹서가 필요합니다.

이와 같은 시스템에서 I 및 Q 채널은 서로 직교합니다(서로 표시할 수 없으며 I/Q 평면을 완성함). 믹서를 곱셈으로 생각하고 0도 위상으로만 혼합한다고 가정해 봅시다. 이렇게 하면 믹싱 신호에 직교하는 정보를 0으로 곱하여 Q 채널 정보를 잃게 됩니다(기술적으로는 둘 다 믹싱하기 위해 일부 비선형 작업을 수행할 수 있지만 그게 요점은 아닙니다). 대부분의 교과서에서 증명을 찾거나 삼각 ID를 사용하여 직접 증명할 수 있습니다.

매우 간단한 설명은 Q가 사인파에 의해 전달되는 정보이고 정보가 시변하지 않는다고 가정하고, I는 코사인파에 의해 전달되는 정보이고 정보가 시변하지 않는다고 가정합니다. 동일한 주파수/위상에서 sin에 sin을 곱하면 숫자를 얻을 수 있지만 sin에 90deg 위상 편이된 sin(또는 다른 말로 Q 채널인 cos 파동)을 곱하면 ' d는 0이 됩니다. 따라서 I/Q 평면의 모든 정보를 다루려면 두 단계가 필요합니다.

면책 조항: 이것은 매우 간단한 설명입니다. 더 많은 것이 있습니다. 저는 단지 맥락을 제공하려고 합니다.

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