Characteristics of Image Frequency in FSK superhed receivers

This is continuation to my previous post on "Image Frequency in Superheterodyne Receiver".

In previous post we described about Image Frequency,

Suppose if some guy is transmitting an FSK signal in Image frequency near to your FSK receiver, what will happen?


Before answering to this question let we examine what is FSK all about.

FSK

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. [source: Wikipedia]

As the name suggest, to represent bit 0,1 we use different carrier frequencies. Which means by using one frequencies we define bit 0, same way using another frequency we define bit 1. When transmitting FSKsignal we see these two carrier frequencies representing bits 1, 0.

Eg)

In the above figure we can correlate that bit '1' is send in one frequency and bit '0' is send in another frequency. This way of modulating signal using frequency is called FSK.

In continuation to my previous examples, I select my desired frequency as 433.8 MHz, so my Image frequency is 434.2 MHz,

Means,  434 MHz (LO) +- 0.2 MHz (IF) = Desired Frequency (433.8 MHz)  or Image Frequency (434.2 MHz)

Here in desired frequency, we know that FSK require 2 frequencies and in example we choose 20 KHz frequency deviation.

Frequency Deviation:

Frequency deviation is the difference in frequency, between bit representing frequency (433.82 MHz or 433.78 MHz) with the base frequency 433.8 MHz.

In the pictorial representation we see 20 KHz freq deviation, and two frequencies selected to represent FSK are 433.78 MHz (low frequency) to denote bit 0, and 433.82 MHz (high Frequency) to denote Bit 1.


Now think about our Image Frequency, if we use the same Frequency deviation applied to Image frequency we get two FSK frequency (434.18 MHz and 434.22 MHz) as shown in picture below

434.18 MHz (low frequency) to denote bit 0, and 434.22 MHz (high Frequency) to denote bit 1.

Think of these FSK frequencies pass through our first part of Superhed receivers (mixer + filter).

As we all know from previous post,

IF = LO - Input signal, if input signal is lower than LO
IF = Input Signal - LO, if input signal is higher than LO

so the frequency components generated when 0 is send will be

desired frequency for bit  '0' :     434 MHz -433.78 MHz = 0.22 MHz (representing 0)
desired frequency for bit  '1' :     434 MHz -433.82 MHz = 0.18 MHz (representing 1)

Now think if our Image Frequency is passed with FSK into mixer what will happen

Image frequency  for bit '0' :     434.18 MHz -434 MHz = 0.18 MHz (representing 0)
Image frequency  for bit '1' :     434.22 MHz -434 MHz = 0.22 MHz (representing 1)


See the Magic,

In FSK signals desired frequency denoting bit '0' generates 0.22 MHz and same is generated by Image frequency denoting bit '1'.
Same way , in FSK signals, desired frequency denoting 1 generates 0.18 MHz and same is generated by Image frequency denoting 0.

So receiver will receive data is inverted form, if we send same data using Image frequency.

In other ways, if we send "10100011"  bits in Image frequency band we will receive inverse of these bits in our receiver like    "01011100".

I hope you understand the concept of inverting bits received in FSK because of Image frequency, next post we will describe about Image frequency characteristics in ASK reception

See you soon