Search RFC: |                                  
Please support my efforts by ADVERTISING!
Serving a Pleasant Blend of Yesterday,
Today, and Tomorrow™

Vintage Magazines

Electronics World
Popular Electronics
Radio & TV News
QST | Pop Science
Popular Mechanics
Radio-Craft
Radio-Electronics
Short Wave Craft
Electronics | OFA
Saturday Eve Post

Formulas | Data

Electronics | RF
Mathematics
Mechanics
Physics

Calvin & Phineas

kmblatt83@aol.com

Archive | Sitemap

Resources

Articles | Radar
Cogitations
Magazines | AI
RF Museum
Software | Videos
Radio Service
Tech Notes

Entertainment

Crosswords
Humor | Podcasts
Quotes | Quizzes
Tech Comics

Parts | Services

1000s of Listings

About RF Cafe
Software: RF Cascade Workbook | RF Symbols for Office | RF Symbols & Stencils for Visio | Espresso Workbook
Please Support My Advertisers!
Transcat | Axiom Test Equipment - RF Cafe
Aegis Power | Centric RF | RFCT
Empower RF | Reactel | SF Circuits
Alliance Test | Isotec
Anatech Electronics RF & Microwave Filters - RF Cafe

Innovative Power Products (IPP) Directional Couplers - RF Cafe

Werbel Microwave power dividers, couplers - RF Cafe		 Tennode RF Connectors and Cables - RF Cafe

dB Control dB-9006 Magnum Opus Synthesizer - RF Cafe

Exodus Advanced Communications Best in Class RF Amplifier SSPAs

Please Support RF Cafe by purchasing my ridiculously low-priced products, all of which I created.

RF Cascade Workbook for Excel

RF & Electronics Symbols for Visio

RF & Electronics Symbols for Office

RF & Electronics Stencils for Visio

RF Workbench

T-Shirts, Mugs, Cups, Ball Caps, Mouse Pads

These Are Available for Free

Espresso Engineering Workbook™

Smith Chart™ for Excel
Innovative Power Products (IPP) 90 deg Hybrid Couplers - RF Cafe

Digital Communications Basics
Answers to RF Cafe Quiz #23
minimum height spacer

RF Engineering Quizzes - RF CafeAll RF Cafe Quizzes make great fodder for employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job. Bonne chance, Viel Glück, がんばろう, buena suerte, удачи, in bocca al lupo, 행운을 빕니다, ádh mór, בהצלחה, lykke til, 祝你好運. Well, you know what I mean: Good luck!

Click here for the complete list of RF Cafe Quizzes.

Note: Some material based on books have quoted passages.

Return to RF Cafe Quiz #23

This quiz challenges your knowledge of digital communications basics.

 

1. What is meant by binary frequency shift keying

    (BFSK)?

b)  Using two distinctly different frequencies to represent digital 0s and 1s.

For instance in the early days of Bell 300 Baud modems, the "0" (space) frequency was 1070 Hz and the "1" (mark) frequency was 1270 Hz.

RF Cafe - Quiz 232. The constellation to the right indicates what type of modulation?

a)  QPSK

QPSK has state transitions that can either pass through the origin (the diagonal) or directly from state to state (horizontal and vertical).

 

3. What is the primary benefit of offset QPSK (O-QPSK) over QPSK?

c)  Constant envelope power.

Offset QPSK does not permit the zero transition, which results in a constant power level (it never goes to zero). The disadvantage is that the method is less efficient because 4 of the 12 possible transitions are not permitted.

RF Cafe - Quiz 234. What is the phase shift for the QPSK modulator block diagram to the right?

c)  90°

The QPSK modulator inserts a 90° phase

shift at the local oscillator (LO) port of the

quadrature (Q) path.

RF Cafe - Quiz 23 (borrowed from Wikipedia)5. What is the image to the right an example of?

d)  An eye pattern

An eye pattern (also called an eye diagram) is a time domain representation of the binary state transitions of a digital broadcast (over-the-air or through a cable). Ideally, the patter closely resemble the modulation scheme's (binary PSK in this case) ideal constellation diagram. System timing instability, multipath, or other foams of noise result in the type of display shown.

 

6. What is the purpose of the lowpass filter (LPF) in the QPSK modulator shown in Question 4?

b)  Limits the bandwidth of the transmitted signal

It is necessary to limit the bandwidth of the transmitted signal both to conform to allowed spectral restrictions by governing authorities (FCC, Ofcom, etc.), to minimize noise content within the system, and to prevent aliasing during the demodulation/detection process.

RF Cafe - Quiz 237. What is the spectrum to the right an example of?

a)  Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum (FHSS) uses a pseudo-random (PR) number sequence to transmit on a fixed set of discrete frequencies (determined in time by the PR number) in order to minimize the probability of interference with the signal. The number of frequencies can be just a few, as shown here, or thousands.

 

8. Which of the following items affects the bit error rate (BER) of a received signal?

d)  All the above

All three add uncertainty to the detection process by possibly changing whether the threshold level detector decides the "bit" is assigned as a "0" or a "1"

 

9. Which of these issues presents a problem for zero-IF (direct conversion) receivers?

d)  All the above

Because the local oscillator (LO) operates at the same frequency as the input RF signal (at the center of the band), the 1x1 (1*LO - 1*RF) mixer product results in a 0 Hz (DC) signal that gets fed into the digital-to-analog converter, which results in an uncertainty in making a decision whether the signal bit qualifies as a "0" or a "1." The flicker noise of the LO diodes is at the baseband frequency, so it also affects uncertainty in binary bit decision making. Finally, unlike with most well-designed receivers that use an intermittent frequency (IF) prior to the baseband conversion, a direct conversion receiver will have 2nd-order mixer spurious products that fall inband, so the 2nd-order intercept point (IP2) will be of concern.

 

10. What is the Z-transform of the Dirac delta function, δ[n] ?

a)  1

The Dirac delta function is a singularity with an amplitude of 1 at n=0, and 0 everywhere else. It is used in sampling convolution to determine a signal's value at an instant in time. Since convolution in the time domain is equivalent to multiplication in the Z domain, a sampled signal is multiplied by "1" at a determined instant and therefore its amplitude is discovered without a scaling factor.

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, Radio-Electronics, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2026
Webmaster: Kirt Blattenberger
   BSEE - KB3UON - AMA92498
   USAF Radar Technician

About RF Cafe

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a lady's voice announced "You've Got Mail" when a new message arrived...

Copyright  1996 - 2026

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
AirplanesAndRockets.com

My Daughter's Website:
EquineKingdom