Question about intermediate frequency

[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.

 

[NoTime]

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

 

[Averagesupernova]

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

 

[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.

 

[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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In a nutshell, here is what happens in the typical AM broadcast receiver: We'll use that because it is fairly well known and simple. All signals in the AM broadcast band arrive at the mixer, or convertor as it was called in the olden days. For this example I will assume that the station we want is at 570 Khz and is the ONLY one in the AM band. The local oscillator is tuned to a frequency that is 455 Khz above the wanted stations frequency. Depending on the type of mixer, a number of signals come out of it. More simple mixers will have 4 signals coming out in this example. They will be: 1) The local oscillator (LO) frequency, 2) The 1000 Khz signal that is wanted, 3) The sum of the LO and the station, 4) The difference of the LO and the station. We only want one of these and the job of the IF stages in the receiver is to selectively amplify these 4 signals. That means only the difference signal is amplified which is 455 Khz. The rest are suppressed. Now, the local oscillator in this example would be running at 1025 Khz. Now suppose that there is a signal on 1480 Khz also. The difference between the LO in this case and the 1480 Khz signal is 455 Khz. So the 1480 signal would also be converted and amplified in the IF. It's known as the image. Normally in AM receivers there is a tracking filter or preselector that tracks the LO and let's only the section of the band pass through that is desired. This supresses the image. Now you're right No Time, multiple IFs in and of themselves don't eliminate images. The key to that is a higher IF frequency. Take the FM band for example with a 10.7 Mhz IF. Notice that it is not possible for there to be an image from within the FM broadcast band. The math just doesn't work out. I don't think FM broadcast radios typically have more than one IF frequency anyway. But, a lot of commercial two-way radios and other higher performance receivers use several intermediate frequencies. I don't call them stacked, they are properly called single or dual conversion receivers. RF testing equipment will sometimes have 3 or more conversions. There is also something called a direct conversion receiver. Familiar with it?
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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.

 

[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.

 

[Averagesupernova]

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.

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.