1. Jun 7, 2006

### chingkui

I have been reading about demodulation. It seems like in practice, a high frequency signal is mixed to bring it down to lower IF, sometimes in multiply steps, for demodulation. When I was taking a digital communication class, all I learn was to bring that signal down to baseband and then detect it. Can anyone point out why IF is used in practice? where can I find the theoretical reason for doing this? Thanks.

2. Jun 7, 2006

### NoTime

Generally, its easier to maintain a constant bandpass characteristics with a fixed IF(width, shape, gain).
Stacking IFs is usually done if an extremly narrow bandwidh is wanted.

3. Jun 8, 2006

### Averagesupernova

Multiple IFs are usually used to avoid image frequencies and other unwanted signals from coming through the receiver.

4. Jun 12, 2006

### NoTime

Don't know that it does much for image frequencies.
The mixer circuit allows three input signals to show up in the first IF passband. One + the difference, one - the difference and the third at the IF passband.
They will all pass equally well to the second IF and RF design is needed to eliminate the undesired channels.

5. Jun 13, 2006

### Averagesupernova

No time, I'm not sure I understand your last post at all. And I'm not sure what you mean by 'stacking' IFs. Your description of what comes out of a mixer and how far the signals go into the IF chain is somewhat confusing also.
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I also might mention that the LO is not always higher than the wanted signal. Sometimes it is lower. This is known as low side injection. The other is logically called high side injection.

6. Jun 14, 2006

### NoTime

Hi Averagesupernova
Clarity isn't always my strong suit

I don't disagree with most of what you said.
I did have an issue with the image rejection part of your previous post.

While you correctly point out the operation of the mixer circuit, the point I was trying to make is that the mixer will also pass a signal at 455khz.
So the IF in your AM example, where LO=1025khz could potentially pass three different transmitted signals -> LO-570Khz=455khz, 1480khz-LO= 455khz and 455khz=455khz. In the last case the LO does not come into play and you essentially have direct conversion or what we used to call TRF.

Not sure where "2) The 1000 Khz signal that is wanted" came from.
I suspect it's a typo and you meant to put 570 Khz there.

7. Jun 14, 2006

### Averagesupernova

Yes, it is a typo. The receiver would in fact pass 455 Khz and operate as TRF you are correct. However, it is outside of the range of frequencies we want so I didn't consider it. If you want to get technical, there are hundreds of signals in a simple receiver that can get in when we don't want them. Consider the 2nd and 3rd harmonic of the LO mixing with some other signal. Again, the advantage of having a higher IF puts all the signals that can get in that we don't want farther away from the band of interest. Naturally it is easier to filter those out when they are farther away.