November
2004, Issue 172
Test Your EQ
Each
month, Test Your EQ presents some basic engineering
problems for you to test your Engineering Quotient.
What's your EQ?
|
Problem
1—You’re
building a basic 5-VDC power supply that will have a maximum
load of 1 A. You have a 10-VAC transformer, a bridge rectifier,
and a 7805 three-terminal regulator. You need to select
the primary filter capacitor that sits between the rectifier
and the regulator. How large should it be? 400, 2,000,
10,000 µF?
Answer
Problem
2—Using a capacitor that’s larger than necessary
will reduce the input ripple to the regulator, and will
therefore reduce the output ripple by a small amount as
well (although the line regulation of a three-terminal
regulator is pretty good). However, there are drawbacks
to using an oversize capacitor. Can you name some?
Answer
Problem 3—What is a limited-weight data code?
Answer
Problem
4—What are good applications for limited-weight
codes?
Answer
Problem
5—A common problem in communications systems
is that a serial datastream comes in at a known bit rate,
but the phase relationship to the local clock is unknown
and may be subject to variation over time (jitter and
wander).
The
general solution for this is to introduce a FIFO into
the receive data path, with the input side clocked by
the recovered receive clock and the output side clocked
by the local system clock.
But
a general-purpose FIFO (often a separate chip) can be
overkill for many applications; it can introduce excessive
amounts of latency (time delay) to the system overall.
How can you describe the smallest possible FIFO that would
do the job? For example, what are some of the characteristics
that such a FIFO would need to have?
Answer
Problem
6—You need to design a digital unit that will
fit into a larger system. This unit must maintain an accurate
timebase. The unit includes a processor with a crystal-controlled
clock that isn’t adjustable. The crystal has a nominal
accuracy of ±100 ppm. The processor also has a counter/timer
that can be clocked by the processor clock or an external
signal. The synchronization signal is an exact integer
multiple of 1 Hz, anywhere from 1 to 200 Hz,
but you don’t know the frequency in advance. The synchronization
signal is locked to a 1 PPS signal coming from a GPS receiver.
Can you come up with a scheme that allows time intervals
on the order of 1 to 10 ms to be measured with the
accuracy of the external reference?
Answer
Problem
7—What is the relationship between PCB copper
“weight” (e.g., 1, 2 oz., etc.) as typically used in America
and copper thickness as typically used in Europe?
Answer
