Some questions about "image problem."

Why has the image problem? the theorem of Razavi and Lee confused me.
it happened in LNA or Mixer or so on?
if RF=1G Hz,LO=900M Hz .what's the frequency of image?
What's the relationship between image problem and spurious response?

Hi a1919kimo,

Image problem is linked to mixer. In your case, I assume your IF=100MHz (RF-LO), the image will be at IM=800MHz folding in the same band than the wanted signal after downconversion (LO-IM). To remove the image your have to filter before the downconverter mixer.

Hope it helps.

Nass

thank's nass.
"LO-IM",where IM means intermodulation or so on?

hi a1919kimo,

IM stands for image (here at 800MHz), which folds into 900MHz-800MHz=100MHz after donwconversion. Hope it clarifies.

Nass

I know clearly.
Thank you so much.

I think this is the way it goes..........

in a receiver the LNA comes before the Mixer. The LNA will amplify your signal and will also create something called "IMD" or (Inter-Modulation Distortion). Imaging isnt a concern here.


Imaging involves only the mixer in the following way.......

Based on your example "RF=1G Hz,LO=900MHz"

When you say that your RF is 1GHz and your LO is 900MHz you are basically telling me two things:

1) The mixer output will be 1.9 GHz and 100MHz. Its obvious that your Intermediate Frequency will be 100MHz instead of 1.9GHz.

2) Your frequency of interest is 1GHz. Meaning that your "data" is modulated with a 1GHz carrier.

Now that I know your desired RF is 1GHz, your LO is 900MHz and your IF is 100MHz. There is one other RF(Image) frequency that combined with your LO will generate your IF of 100MHz. It is:

1) 800MHz. The difference of the 900MHz LO and the 800MHz Image will give a IF of 100MHz

Your data is not located at 800MHz carrier so this will show up as noise in your system if not filtered. Image Rejection Mixers are better for situations where the image frequency are close to the LO frequency. It will be harder to filter an image that is close to the frequency of the LO....but that is beyond the scope of this post.


I would say that the "spurious response" are the sums and differences of the LO and RF created by the mixer. Within the spurs are everything the mixer created.

Image is just a term used to describe a fake RF channel that when combined with the LO (via the mixer ) will give you the same IF that your desired/wanted RF channel would give you. One has the data modulated in the carrier the other has noise.