September 2015 9
not that hard). Thus, we should do troubleshooting in a
systematic and orderly way, such as signal injection/signal
tracing or bracketing the problem. Figure 11 shows a block
diagram of a superheterodyne radio similar to the AM/
FM radio you are familiar with. Each block of the diagram
represents a major section of the radio’s circuits, and each
section is composed of electronic components such as
resistors, capacitors, inductors, transistors, etc. On the
diagram, I have included six test points after each of the
major sections (numbered 1 to 6), with the antenna and
ground symbols shown to the left of the RF amp.
Bracketing is used to narrow down the location of the
problem in a complex system. This technique is based on
establishing “good” brackets where the signal is as it should
be, and “bad” brackets where the signal is either absent or
degraded.
Referring back to Figure 11, we can assume the
antenna and ground are good by making sure it is attached
properly (you can use a known good radio to ensure there
is a radio signal present, or use an RF signal generator
to send an input signal) to establish a good bracket. (By
convention, signals on diagrams flow from left to right.)
Let’s say that there is no signal coming from the
speaker, so this represents our bad bracket. We can
either move to a good bracket to the right, move the bad
bracket to the left, or both. For this case, let’s move the
bad bracket until we find a good signal. This way, we’ll
know that the problem is between the last bad bracket
and the last good bracket. At this stage, we use a
detailed schematic diagram to determine which
component(s) are causing the problem.
Signal insertion/signal tracing is used in conjunction
with a signal generator and a signal detector (DMM, VOM,
oscilloscope, logic probe, etc.). Again, in Figure 11, you
could insert a radio frequency signal into the antenna
connectors, then use an oscilloscope to see if there is an
audio signal at the speaker terminals and demodulator
output at the intermediate frequency (455 kHz for most
broadcast band sets), then at the radio frequency signal
(535 to 1,700 kHz for AM and 88 to 108 MHz for FM) at
the RF amp output and local oscillator frequency (carrier
frequency + 455 kHz).
You can see there is a broad range of frequency
covered in a “simple” AM/FM radio. The signal input to a
TV set is a visual pattern that appears on the screen of a
good TV. Seeing actual broadcast signals is difficult since
the broadcast signal pattern is not as periodic as that of a
signal generator.
Bracketing and signal injection/signal tracing can be
used on systems other than electronics, such as wastewater
treatment systems and mechanical systems. However, the
“test points” may not be as easily determined as they are in
electrical systems.
I hope this information whets your appetite for further
study into electronics to help you in your troubleshooting
efforts. NV
