Communications - Decibel

Ref: SMAD Sections – 13 Communications Architecture

Introduction to Space Systems and Spacecraft Design Space Systems Design

Communications - Decibel Dealing with large and small signal levels Decibels -  powers of 10 Absolute power

P

dB

= 10Log

10

(P

2

/P

1 ) units - dbW Amplifier gain/loss units - W units – 1 W

P

dB

(dbW) = 10Log

10

(P

2

(W)/1(W)

units - dbm units - mW units – 1 mW

P

dB

(dbm) = 10Log

10

(P

2

(mW)/1(mW)

Introduction to Space Systems and Spacecraft Design Space Systems Design

Communications - Decibel Dealing with large and small signal levels Decibels -  powers of 10 P dB = 10Log 10 (P 2 /P 1 ) P dB (dbW) = 10Log 10 (P 2 (W)/1(W)

Introduction to Space Systems and Spacecraft Design Space Systems Design

P dB (dbm) = 10Log 10 (P 2 (mW)/1(mW) 3

Communications - Decibel Dealing with large and small signal levels Decibels -  powers of 10 Add decibel Output (dBW) = Input (dBW) + Gain(dB) Output (W) = Input (W) x Gain Multiply absolute values

Introduction to Space Systems and Spacecraft Design Space Systems Design

4

Communications - Decibel Dealing with large and small signal levels Decibels -  powers of 10 Component Loss (-Db) Add decibel Output (dBW) = Input (dBW) + (- Loss(dB)) Loss is value < 1.0

Output (W) = Input (W) x Loss Multiply absolute values

Introduction to Space Systems and Spacecraft Design Space Systems Design

5

Communications - Decibel

Introduction to Space Systems and Spacecraft Design Space Systems Design

6

Communications - Decibel

Comparing Power Levels using Decibels

P dB = 10Log 10 (P 2 /P 1 ) P 1 P 2 Amplifier – gain in power Attenuation – loss in power Let P 1 = 4 watts Let P 2 = 8 watts Let P 1 = 8 watts Let P 2 = 4 watts

Introduction to Space Systems and Spacecraft Design Space Systems Design

When (P 2 /P 1 ) > 0 dB is positive When (P 2 /P 1 ) < 0 dB is negative P 2 /P 1 = 2 ratio P(dB) = 3 dB P 2 /P 1 = 0.5 ratio P(dB) = -3 dB 7

Communications - Decibel

Absolute Power Levels using Decibels

P dBm = 10Log 10 (P(mw)/1mw) reference to 1mw = 10Log 10 (1mw/1mw) = 0dBm P dBw = 10Log 10 (P(w)/1w) reference to 1w = 10Log 10 (8w/1w) = 9dBw Every time it doubles, add 3dB Every time it is 1/2, subtract 3dB

Introduction to Space Systems and Spacecraft Design Space Systems Design

8

Communications - Decibel

Powers greater >= 1 watt

0 dBw = 1 watt 3 dBw = 2 watts 6 dBw = 4 watts 9 dBw = 8 watts 10 dBw = 10 watts = 1 watt x 10 1 20 dBw = 100 watts = 1 watt x 10 2 30 dBw = 1000 watts = 1 watt x 10 3 100 dBw = 10 M watts = 1 watt x 10 10

Introduction to Space Systems and Spacecraft Design Space Systems Design

9

Communications - Decibel

Powers greater <= 1 watt

0 dBm,= 1 milliwatt -3 dBm = 0.5 milliwatts -6 dBm = 0.25 milliwatts -9 dBm = 0.125 milliwatts -10 dBm = 0.100 milliwatt = 1 milliwatts x 10 -1 -20 dBm = 0.010 milliwatts = 1 milliwatts x 10 -2 -30 dBm = 0.001 milliwatts = 1 milliwatts x 10 -3 = 0.001 milliwatts = 1 microwatt

Introduction to Space Systems and Spacecraft Design Space Systems Design

10

Communications - Decibel

Introduction to Space Systems and Spacecraft Design Space Systems Design

11

Communications - Decibel

Questions?

Introduction to Space Systems and Spacecraft Design Space Systems Design

12