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.