Best way to detect constant wave - working

Generic Usage oF Spectrum Analyzer 2

So coming back to second question on how we can quantify signal in spectrum analyzer.

As discussed in first part, spectrum analyzer showcases our signal in power level called decibel (dB).

What is dB, why not Volts?

We will try to answer this question in theoretical and practical way.

First Theory

dB   =    10 * LOG10 (P1/P2).

So decibel is a ratio of power, also this ratio is taken in logarithmic scale with base 10 not natural log.
that says decibel is a relative measurement, where as volts and watts are absolute measurements.

Practical Explanation: In above picture we  randomly selected 16 values of power P1 and made P2 as constant values. We tried to show these values in an oscilloscope (left bar diagram) and same inputs we tried to portrait decibel in right bar diagram.

We can see that right most diagram is easy to analyze and compare where as leftmost bar diagram is not speaking much about the signal strength.

Also as a thumb rule every time the P1is double we need to add 3 in dB scale. (Number 2,8,9,10) also every time if P1 is multiplied 10 times we need to add 10 in dB scale (1,3,4,5).

So as logarithmic property multiplication and division is converted to addition and subtraction.

                           

decibel		P2 Reference
dBm		1 milli Watt mW
dBu		1 micro Watt uW
dBmV		1 milli Volts mV
dBc		?
Suffix turns dB into absolute quantity

dBc - This is called decibel relative to the carrier.

Example, As you know about harmonics, i take 10 MHz signal and one of the harmonic of this signal is 20 MHz. dBc talks about the relative dB level of power in comparison with carrier power.

ie, suppose signal power of 10 MHz is 5 dBm, and for harmonic 20 MHz is -50dBm,  then we can say that at 20 MHz harmonic, signal is -55dBc. ie with respect to carrier power, 20 MHz harmonic power is 56 dBm less.

Generic Usage of Spectrum Analyzer

As name suggest, this post describes  usage of spectrum analyzer in layman perspective.

First we will think on below topics:

•  -  What is unique in spectrum analyzer than in oscilloscope
•  -  What is dBm, why to use that as a quantifying scale in spectrum analyzer
•  -  How to quantify signal, its strength and how far it can reach
•  -  What is bandwidth (RBW, VBW)
•  -  What is span and what is zero span
•  -  What is sensitivity of a system

Lets start,

1. What is unique in spectrum analyzer than in oscilloscope?

  To start with, Oscilloscope showcase characteristic of signal in time domain, means the signal plotted is amplitude in volts versus time in sec. where as spectrum analyzer showcase characteristic of signal in frequency domain, means the signal plotted is amplitude in dBm versus frequency in hertz.

When we connect the input to spectrum analyzer we can see the components of input signal we are receiving in terms of frequency.

Also this means, two signals (eg. sine wave) of two distinct frequency is passed to input of oscilloscope (after mixing), we will only able to see the cumulative effect of those signals in time domain else we need to have a FFT of the output signal to understand individual characteristics of each signal .

In spectrum analyzer, individual frequencies are tuned to check whether it is present in input signal. Here each frequency component and its strength in input signal is shown in spectrum analyzer.

Now we will go to second question related to quantifying signal in spectrum analyzer.

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

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