AMATEUR PRACTICES
Test equipment: analog and digital instruments; spectrum and network analyzers, antenna analyzers; oscilloscopes; testing transistors; RF measurements
How does a spectrum analyzer differ from an oscilloscope?
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A spectrum analyzer measures ionospheric reflection; an oscilloscope displays electrical signals
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A spectrum analyzer displays the peak amplitude of signals; an oscilloscope displays the average amplitude of signals
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Correct AnswerA spectrum analyzer displays signals in the frequency domain; an oscilloscope displays signals in the time domain
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A spectrum analyzer displays radio frequencies; an oscilloscope displays audio frequencies
a spectrum analyzer gives a visual display of the amplitudes of signals within a limited frequency range - the vertical axis shows signal amplitude while the horizontal axis shows frequency - an oscilloscope is a visual display which shows amplitude on the vertical axis and time on the horizontal axis
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Which of the following parameters would a spectrum analyzer display on the horizontal axis?
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SWR
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Q
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Time
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Correct AnswerFrequency
Frequency.
A spectrum analyzer places the frequency on the horizontal axis and the magnitude of the frequency, usually in dBm, on the vertical axis.
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Which of the following parameters would a spectrum analyzer display on the vertical axis?
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Correct AnswerAmplitude
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Duration
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SWR
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Q
The amplitude axis is often represented in a non-linear fashion. For example it may have a logarithmic scale calibrated in dBs. This allows a large range of amplitudes to be represented on a single axis
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Which of the following test instruments is used to display spurious signals from a radio transmitter?
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Correct AnswerA spectrum analyzer
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A wattmeter
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A logic analyzer
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A time-domain reflectometer
A spectrum analyzer measures the magnitude of an input signal across a wide frequency range. With a spectrum analyzer you can see the signal strength of a signal on a given frequency but also (usually on a graph) the signal strength of signals of nearby frequencies as well.
In this way you can see the "bandwidth" of a signal (how "wide" the signal is, meaning the difference between the highest frequency and the lowest frequency the signal can be picked up on) and also pick out any other nearby signals.
Spurious signals can be identified by watching the spectrum analyzer to see if when the main signal appears other spikes appear in nearby frequencies. Those are not part of the main signal and are unimportant, thus they are "spurious" or false.
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Which of the following test instruments is used to display intermodulation distortion products in an SSB transmission?
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A wattmeter
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Correct AnswerA spectrum analyzer
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A logic analyzer
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A time-domain reflectometer
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Which of the following could be determined with a spectrum analyzer?
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The degree of isolation between the input and output ports of a 2 meter duplexer
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Whether a crystal is operating on its fundamental or overtone frequency
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The spectral output of a transmitter
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Correct AnswerAll of these choices are correct
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Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR?
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Antenna analyzers automatically tune your antenna for resonance
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Correct AnswerAntenna analyzers do not need an external RF source
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Antenna analyzers display a time-varying representation of the modulation envelope
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All of these choices are correct
An antenna analyzer has its own built in signal source.
Hint: "Antenna analyzer" and "SWR" are in both the question and answer.
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Which of the following instruments would be best for measuring the SWR of a beam antenna?
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A spectrum analyzer
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A Q meter
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An ohmmeter
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Correct AnswerAn antenna analyzer
An antenna analyzer is a piece of equipment that analyzes different aspects of an antenna. The specific capabilities vary between different analyzers, but one capability that all have in common is the ability to measure the Standing Wave Ratio, or SWR, of an antenna.
The SWR of an antenna is the simplest indicator of how well an antenna matches the transmitter system on a given frequency.
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Which of the following describes a good method for measuring the intermodulation distortion of your own PSK signal?
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Correct AnswerTransmit into a dummy load, receive the signal on a second receiver, and feed the audio into the sound card of a computer running an appropriate PSK program
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Multiply the ALC level on the transmitter during a normal transmission by the average power output
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Use an RF voltmeter coupled to the transmitter output using appropriate isolation to prevent damage to the meter
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All of these choices are correct
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Which of the following tests establishes that a silicon NPN junction transistor is biased on?
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Measure base-to-emitter resistance with an ohmmeter; it should be approximately 6 to 7 ohms
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Measure base-to-emitter resistance with an ohmmeter; it should be approximately 0.6 to 0.7 ohms
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Measure base-to-emitter voltage with a voltmeter; it should be approximately 6 to 7 volts
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Correct AnswerMeasure base-to-emitter voltage with a voltmeter; it should be approximately 0.6 to 0.7 volts
When measuring the P-N forward biased junction, the forward voltage should be around 0.7V over a range of currents.
This is just a property of silicon NPN (bipolar) junction transistors that you have to remember applies to silicon NPN transistors and diodes. Other types may have other voltage drops.
Silly hint: point SEVEN goes with SILICON. (There's a similar question on the General exam where they ask about a silicon transistor and the correct answer is 0.7)
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Which of these instruments could be used for detailed analysis of digital signals?
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Dip meter
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Correct AnswerOscilloscope
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Ohmmeter
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Q meter
An oscilloscope is an instrument that measures voltage and displays it graphed by time. Because of this, it's possible to do detailed analysis of digital signals by graphing the change in voltage during the time period in which the signal in question occurred.
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Which of the following procedures is an important precaution to follow when connecting a spectrum analyzer to a transmitter output?
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Use high quality double shielded coaxial cables to reduce signal losses
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Correct AnswerAttenuate the transmitter output going to the spectrum analyzer
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Match the antenna to the load
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All of these choices are correct
Transmitters normally put out enough power to radiate from an antenna; the signal thus produced is enough to travel to another antenna, potentially quite a distance away, and produce a signal in that antenna strong enough for a receiver to pick it up.
Spectrum Analyzers are generally intended to work with a receiver, or as a receiver with an antenna so if you transmit too much power into them you're likely to blow something up. Thus, to connect a spectrum analyzer to a transmitter output it is important to reduce (or attenuate) the transmitter output before it gets to the spectrum analyzer.
Hint: spectrum and transmitter are both in the question and correct answer.
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