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2016 Extra Class study guide: E4A - Test equipment

Posted: 11 Mar 2016 01:00 PM PST
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E4A Test equipment: analog and digital instruments; spectrum and network
analyzers, antenna analyzers; oscilloscopes; RF measurements; computer
aided measurements

An instrument that amateur radio operators frequently use when
experimenting or when debugging equipment is the oscilloscope, or simply
just “scope.� Oscilloscopes have become more common in amateur radio shacks
as the price has fallen and the technology has moved from analog to digital.

Analog oscilloscopes use amplifiers, filters, and other analog signal
processing circuits to display an input signal on a cathode-ray tube, or
CRT. Digital oscilloscopes, on the other hand, use an analog-to-digital
converter to convert the input signal into a series of numbers, which are
then processed by a computer and displayed on an LCD screen. All of these
choices are correct when talking about the advantages of a digital vs.
analog oscilloscope: (E4A05)

Automatic amplitude and frequency numerical readout
Storage of traces for future reference
Manipulation of time base after trace capture


One of the most important oscilloscope specifications is its bandwidth. The
bandwidth of an oscilloscope determines the maximum frequency at which the
oscilloscope can accurately measure a signal. While the characteristics of
the analog signal processing circuits determine the bandwith of an analog
oscilloscope, sampling rate is the parameter that determines the bandwidth
of a digital or computer-based oscilloscope. (E4A01) Similarly, the
analog-to-digital conversion speed of the soundcard determines the upper
frequency limit for a computer soundcard-based oscilloscope program. (E4A04)

Because digital oscilloscopes sample an input signal at discrete time
intervals, it is possible to fool them into displaying an incorrect
waveform. This phenomenon is called aliasing. The effect of aliasing in a
digital or computer-based oscilloscope is that false signals are displayed.
(E4A06) When using a computers soundcard input to digitize signals, the
highest frequency signal that can be digitized without aliasing is one-half
the sample rate. (E4A09)

Oscilloscope probes

When making measurements at RF frequencies, it’s important to connect the
probe’s ground connection as close to the location of the measurement as
possible. Keeping the signal ground connection of the probe as short as
possible is good practice when using an oscilloscope probe. (E4A11) Keeping
this connection as short as possible reduces the noise picked up by the
probe and reduces the inductance of the connection, which in turn, makes
the measurement more accurate..

Good quality passive oscilloscope probes have an adjustable capacitor in
them that needs to be adjusted so that the probe capacitive reactance is at
least nine times the scope input capacitive reactance. When this capacitor
is adjusted properly, we say that the probe is properly compensated, and
the scope will display the waveform with as little distortion as possible.

How is the compensation of an oscilloscope probe typically adjusted? A
square wave is displayed and the probe is adjusted until the horizontal
portions of the displayed wave are as nearly flat as possible. (E4A13)
High-quality oscilloscopes will have a special square-wave output
specifically for the purpose of compensating probes.

Spectrum analyzers

Spectrum analyzers display the amplitude of signals in the frequency domain
Frequency is the parameter a spectrum analyzer would display on the
horizontal axis. (E4A02) The drawing below shows typical displays from an
oscilloscope and a spectrum analyzer. Spectrum analyzers are very useful
for troubleshooting problems. For example, a spectrum analyzer is used to
display spurious signals from a radio transmitter. (E4A03)

Because spectrum analyzers are sensitive instruments, you need to be
cautious when using them. For example, an important precaution to follow
when connecting a spectrum analyzer to a transmitter output is to attenuate
the transmitter output going to the spectrum analyzer. (E4A12) Not doing so
could damage the spectrum analyzer because its input circuits are not
designed to handle high power.

Antenna analyzers

One of the instruments that I think every amateur radio operator should
have (or at least have access to) is the antenna analyzer. Antenna
analyzers are versatile instruments that allow amateur radio operators to
easily make antenna measurements, as well as other impedance measurements.
They can even be used as low power RF signal generators.

An antenna analyzer is the instrument that would be best for measuring the
SWR of a beam antenna. (E4A08) Actually, it’s the best instrument for
measuring the SWR of any kind of antenna. That’s what they’re made for!
When measuring antenna resonance and feed point impedance with a portable
antenna analyzer, connect the antenna feed line directly to the analyzers
connector. (E4B11)

An advantage of using an antenna analyzer compared to an SWR bridge to
measure antenna SWR is that antenna analyzers do not need an external RF
source. (E4A07) What this means is that you don’t need to connect your
transmitter to the antenna to tune it. This is because they have an
internal RF signal generator.

Frequency counters, logic analyzers

To measure the frequency of a signal, you use an instrument called a
frequency counter. When selecting a frequency counter, an important
specification is the maximum frequency. If you want to measure the
frequency of a signal whose frequency is higher than the maximum frequency
of your counter, you might use a prescaler. The purpose of a prescaler
function on a frequency counter is to divide a higher frequency signal so a
low-frequency counter can display the input frequency. (E4A14)

Most frequency counters work by counting the number of cycles of a signal
during a given time period. An alternate method of determining frequency
used by some counters is period measurement plus mathematical computation.
An advantage of a period-measuring frequency counter over a direct-count
type is that it provides improved resolution of low-frequency signals
within a comparable time period. (E4A15)

The proper operation of a digital circuit depends on the output state of
many nodes at specific times in a circuit. To ensure that a circuit is
working properly, or to troubleshoot a circuit, you may want to use a logic
analyzer. A logic analyzer displays multiple digital signal states
simultaneously. (E4A10)

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