Question

Please answer the following questions.

1. Please explain why we need image rejection filter or image rejection mixer and when we need with detail explanation.

2. Please introduce a couple of methods to remove image signals.

Answer 1

1.

Ans: Image rejection is very important criteria in designing . IF based receiver uses the Variable frequency oscillator and signal of variable frequency oscillator will shift the received spectrum down to the equivalent centered spectrum with an intermediate frequency.And this shifting is done by RF multiplication process .As as result this multiplication process affects many things not only the received spectrum but also the symmetrically located spectrum with respect to variable frequency oscillator's frequency . which means the due to multiplication one signal shifts down to the IF and another signal shifts up to the IF .

Due to this the Frequency information in the desired frequency spectrum get corrupted by the frequency information in the image signal .Therefore this symmetrical located image spectrum is a serious obstruction. Because this signal will not be in the control of the wireless system you are designing .Thus we must do something to remove or mitigate the effect of this image signal because, The quality of the system reception will totally dependent on the behavior of the signals near the image frequency which is now unpredictable .

Therefore in order to mitigate this effects of image spectrum image rejection filters or image rejection mixers are introduced in Heterodyne receivers . They are always kept before the mixers. Therefore it will suppressed the image spectrum before it shifts to the intermediate frequency .

2.

Ans : Methods to remove Image signals

To remove image signals or image noise is very important task in image processing. The major goal in image restoration is to remove the noise signal or image signals from the image in such a way that original image is clearly visible. Therefore in this modern world removing the image signals is a well known problem in the field of photography or publishing where image was some what degraded but it need to be printed and published .

There are two approaches to remove the image or noise signals

a.Linear Filtering

b. Non linear  Filtering

a). Linear Filtering :

This method is use to remove certain type of noise . This method uses convolution method , it will convolve the original image with mask that represents the Low pass filters or smoothing operation. By this method Filter removes the noise from the image . This method will have some disadvantages it will blur this sharp edges of the image , destroy the lines and other fine details of the image because the output of this process performing sum of two inputs is as same as performing the operation on the inputs individually and adding them .There fore by this we can conclude that this method can be fast and linear  but fails to keep the details of the image

b). Non linear Filtering :

In this method the output is not linear with respect to the input. But it will preserve the details of the image .Limitation of linear filtering method is over come by this strategy .It has many application specially in the filed where the Noise signals are not additive