 All 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!
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Note: Some material based on books have quoted passages.
Return to RF Cafe Quiz #63
 This
quiz is based on the information presented in
Envelope Tracking Power Amplifiers for Wireless Communications, by Zhancang Wang.
"Envelope tracking technology is seen as the most promising efficiency enhancement technology for RF PA for 4G
and beyond wireless communications. More and more organizations are investing and researching on this topic with huge
potential academic and commercial values. This is the first book on the market to give complete introduction, theory,
and design considerations on envelope tracking for wireless communications. This resource presents microwave design
engineers with a full introduction to the subject and covers underlying theory and practical design considerations.
Note: Some of these books are available as prizes in the monthly
RF Cafe Giveaway
I am continually amazed at the brilliance of people who work tirelessly to advance the state of the art of technology,
medicine, transportation, and many other areas. They manage to succeed in spite of myriad obstacles placed in their
collective way by infuriating levels of government regulation and government-imposed societal manipulation
(e.g., political correctness). Scanning through the many engineering books I receive
from Artech House, Cambridge University Press, et al, is a truly humbling experience as I witness the incredible talent
and determination of my fellow humans. We owe them a great debt of gratitude. This book is a prime example of which
I write.
On a comical note, I noticed that the translation to English while overall excellent, did produce some notable
instances where similes did not come out quite right; e.g., on page 100 the description of something being 'the short
pole' [in the tent] came out as "'the short board' to limit overall ..." Being the 'long pole' in the tent is often
also used (but not in this book).
1. What happens to the power from an RF amplifier's DC power supply?
d) It gets converted to RF power
The function of an RF power amplifier is to convert direct current (DC) power to radio frequency (RF) power by
amplifying s given RF signal.
(see page 1)
2. What are the two types of efficiency definitions for an RF power amplifier?
b) Power added efficiency (PAE) and drain/collector efficiency
Efficiency, η, is
a measure of how much DC power is converted to RF power.
(see page 2)
3. Where did the Class S power amplifier get its name?
b) The 'S comes from its founder's (A.M.
Sandman) last name
The class-S concept was the name of a design published by A.M. Sandman in 1982 for use with all-digital RF transmission.
(see page 21)
4. What is the distinguishing feature of a Doherty amplifier?
d) Load impedance varies according to the power level
The basic principle of Doherty PA operation is based on load modulation, with which the load impedance varies according
to the power level.
(see page 26)
5. What makes envelope tracking (ET) so attractive in mobile communications?
a) High efficiency over a wide range of power levels
Envelope tracking (ET) exhibits highly efficient operation over a broad power range and its ability to handle frequency
agility as exhibited in 4G band fragmentation.
(see page 51)
6. What is the name given to the diagram that portrays the parameter tradeoff in envelope tracking
(ET) PAs?
c) Envelope tracking merit pyramid
The envelope tracking merit pyramid (see illustration) has four key vertexes for overall system performance and
implementation tradeoff for various application scenarios.
(see page 53)
7. What is the overall envelope tracking system efficiency most dependent upon?
a) Accurate delay compensation between the RF signal and the envelope power supply
Although the efficiency of an ET PA can be increased by having the supply voltage signal provided to the PA track
the instantaneous power being produced by the PA. the overall system performance is dependent on accurate delay compensations
between the RF signal and the envelope provided.
(see page 111)
8. When is the nonsynchronous impedance transformer a good option for use?
c) In applications where a ¼-wave transformer is physically too long
In applications in which a quarter-wave impedance transformer is physically too long and some bandwidth can be
sacrificed, the nonsynchronous transformer can be a good alternative.
(see page 205)
9. What is a common method used in digital front ends (DFEs) to mitigate efficiency degradation
due to high peak-to-average-power-ratio (PAPR) signals?
d) Crest factor reduction (CFR) algorithms reduce PAPR before entering the RF power amplifier stage
To increase the efficiency, CFR algorithms cab be used tin DFE to reduce the PAPR of the signal before it enters
the RF PA.
(see page 231)
10. How does envelope tracking (ET) boost RF PA efficiency?
b) By replacing a fixed DC supply with a dynamic supply voltage that tracks the RF envelope shape
ET boosts the efficiency of RF PAs by replacing the fixed DC supply by dynamic supply voltage, which closely
tracks the RF envelope shape.
(see page 283)
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