What exactly are image signals and what is their origin?

Image signals.

I would like to know what exactly are image signals (in the context of superhetrodyne receivers) and what is their origin?

My textbook says that it is to be eliminated in the RF amplifier because if it reaches the mixer, it can never be eliminated.

The same textbook says that, the tuned stages of the IF amplifier eliminates the image signals as the lower local oscillator makes the image frequency to deviate further more from the desired IF.

These somewhat contradictory explanations have left me confused...Please help...

I would appreciate any explanations, etext, weblinks or anything that helps me clerify this idea.

Thank you.

Re: Image signals.

Image frequency is very important, espcially in multiband (broadband) receivers.

Lets say you are receiving a 900 MHz signal, and you have a 100 MHz IF frequency where you have tight channel filters. Lets say you chose a 1.0 GHz local oscillator frequency. So a 0.9 GHz -1.0 GHz = 100 MHz IF.

Now lets say some clown has a wireless device nearby at 1.1 GHz. Your reciever will gladly downconvert this signal also to a 100 MHz IF. This interference at the IF frequency will really mess up your digital processor, or make the audio sound bad if an analog signal!

The only way to stop such an interferror is to use front end filtering. If you had a 890 to 910 MHz bandpass filter between the input lownoise amplifier and the mixer, that filter would allow the desired signal thru, but attenuate the image frequency by many dB. Since the interferror signal may be very large compared to the desired signal, the rejection of the input filter has to be pretty good.

For noise figure reasons, it is also standard practice to have at least a modest RF input filter after the low noise amp.

Image signals.

How is the IF set to 100MHz or any other value ?

Do I understand that a local oscillator frequency is fixed, and the IF depends on the signal you chose to tune into ?

Re: Image signals.

You "set" the IF frequency by putting a bandpass filter into the IF chain. Whatever center frequency that IF Filter is tuned to is the frequency that the IF is "set" to. In rare cases, there can be multiple IF filters switched in or out, or more commonly "virtual" IF filters formed in a DSP processor with math. But in general, there is only one IF center frequency.

Once you have picked an IF center frequency, you move the LO frequency around until the RF signal you want is downconverted into the IF.

Re: Image signals.

Hi

The IF is set by your incoming signal and the LO. It is the difference between the two signals that determine the IF frequency: F_lo - F_rf = F_if, where F_rf is your incoming signal. So by choosing an LO you automatically choose an IF frequency or visa versa.

Of course you have to consider if you want an high or low IF. Especially the design of the image rejction filter and the IF filter after the mixing have taking place, is dependent of the IF frequency.

I have attached a page from some teaching materiale.

It is also possible to eleminate the image by using what is called an image rejcection mixer. Which very basicly is to use two double balanced mixers, like Gilbert-cell mixers.

Hope it helps.