RF Signals and Modulation

Question 1

Define frequency.

Answer 1

The number of times a signal makes one complete up and down cycle in 1 second.

Question 2

What is a radio band?

Answer 2

A contiguous range of frequencies.

Question 3

In the 2.4Ghz band, how far apart are the channels?

Answer 3

5MHz

Question 4

Define signal bandwidth.

Answer 4

It is the width of the frequency space required within the band.

Question 5

Law of Zero

Answer 5

A value of 0dB means that the two absolute power values are equal.

Question 6

Law of 3s

Answer 6

A value of 3dB means that the power value of interest is double the reference value.

Question 7

Law of 10s

Answer 7

A value of 10dB means that the power value of interest is 10 times the reference value.

Question 8

What is an isotropic antenna?

Answer 8

It is a reference antenna that doesn’t exist. It is a tiny point that radiate RF equally in all directions.

Question 9

What does EIRP stand for?

Answer 9

Effective Isotropic Radiated Power

Question 10

What is the calculation for EIRP?

Answer 10

Tx Power (dBm) - Tx Cable Loss (dB) + Tx Antenna Gain (dBi)

Question 11

What is the difference between EIRP and ERP?

Answer 11

ERP uses the dBd for the antenna gain rather than dBi. dBd is 2.15dBi.

Question 12

What does RSSI stand for and what is its measurement?

Answer 12

Recieved Signal Strength Indicator (measured in dBm)

Question 13

What does SNR stand for and how is it calculated?

Answer 13

Signal to Noise Ratio (RSSI - noise floor)

Question 14

What is the recommended SNR for data and voice?

Answer 14

20+dB for data and 25+dB for voice.

Question 15

What does FHSS stand for and what is the channel width?

Answer 15

Frequency-Hopping Spread Spectrum (1MHz)

Question 16

What does DSSS stand for and what is the channel width?

Answer 16

Direct-Sequence Spread Spectrum (22MHz)

Question 17

What does OFDM stand for and what is the channel width?

Answer 17

Orthogonal Frequency-Division Multiplexing (20MHz)

Question 18

How many channels in the 2.4GHz range and how many can be used in the US? How many in Europe?

Answer 18

There are 14. 11 used in US. 13 used in Europe.

Question 19

What is the Barker 11 code?

Answer 19

11 bit sequence that represents 1 bit.

Question 20

In the Barker code what is a chip and what is a symbol?

Answer 20

Chip is a bit in the sequence. Symbol is a complete group of chips that represent a data bit.

Question 21

What is CCK stand for and what is it?

Answer 21

Complementary Code Keying (encoding method that takes either 4 or 8 data bits at a time to create a 6-bit symbol)

Question 22

What does DBPSK stand for and what speed can be achieved?

Answer 22

Differential Binary Phase Shift Keying (1Mb)

Question 23

What does DQPSK stand for and what speeds can be acheived?

Answer 23

Differential Quadrature Phase Shift Keying - 2Mb using Barker 11, 5.5Mb using CCK 4, 11Mb using CCK 8

Question 24

How many subcarriers does OFDM use?

Answer 24

64 total
12 guard - help to set channels apart
4 pilot - equally spaced to help evaluate noise
48 data

Question 25

What is phase?

Answer 25

A measure of shift in time relative to the start of a cycle.

Question 26

What 2 aspects of the RF signal does QAM alter?

Answer 26

Amplitude and phase.

Question 27

What does QAM stand for?

Answer 27

Quadrature Amplitude Modulation