Image Frequency in Superheterodyne Receiver

In radio communication mostly we use superheterodyne receivers (superhet) for reception. As you know, the main advantage of this technology is that by using mixer a received signal is converted to  a fixed frequency (Intermediate Frequency - IF), all processing is done using this fixed frequency to reduce complexity in developing other components in reception chain.

By using mixer, we will come across Image Frequency, What is this?

Here we will discuss about first part of superhet

Mixer will mix two input frequencies to give back one output frequency. Thus mixer gives out two different frequencies, one addition of input frequencies and other subtraction of input frequencies.

eg 1) Input signal 433.8 MHz mixed with Local Oscillator 434 MHz will give output as 

 867.8 MHz  (  = 434MHz + 433.8 MHz ) which intern will be filtered out in next filter stages (fig: 1)

     0.2 MHz  ( =  434 MHz  - 433.8 MHz )

eg 2) Input signal 434.2 MHz mixed with Local Oscillator 434 MHz will give output as
 868.2 MHz  (  = 434MHz + 434.2 MHz ) which intern will be filtered out in next filter stages (fig: 1)
     0.2 MHz  ( =  434 MHz  - 434.2 MHz ) or ( =  434.2 MHz  - 434 MHz )

Now we get to know that two input frequencies can pass through the superhet receiver filters which is configured in example as 0.2 MHz
In example these input frequencies are 433.8 MHz and 434.2 MHz, or in other sense

LO +- IF = input frequencies

Now the frequency other than your desired frequency (input frequency you  intent to send) is called Image Frequency, means if you intent to receive your original send data at 434.2 MHz your image frequency is 433.8 MHz and vice versa.

Next post we will discuss on characteristics of image frequency in FSK and ASK receivers, See you soon