HamSandwich

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G4B

AMATEUR RADIO PRACTICES

- Tests and test equipment

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G4B011 of 13

What item of test equipment contains horizontal and vertical channel amplifiers?

Why An oscilloscope displays a signal as a plot of voltage versus time on a screen. To do that it needs a vertical channel amplifier to scale the input signal's amplitude and a horizontal channel that sweeps the trace across the screen (or amplifies a second input in X-Y mode). No other common bench instrument uses that two-axis deflection architecture.
Watch out A signal generator produces test signals rather than displaying them, and ohmmeters and ammeters are single-value measuring instruments with no display axes at all.
Horizontal + vertical = a screen with two axes, so it's the scope.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B022 of 13

Which of the following is an advantage of an oscilloscope versus a digital voltmeter?

Why An oscilloscope plots voltage against time, so it shows the actual shape of a signal: rise and fall times, ringing, distortion, modulation envelopes, pulse widths and phase relationships. A digital voltmeter collapses all that into a single number, typically a DC level or an RMS/average value of an AC waveform. That makes the scope the right tool whenever you need to see what a complex, non-sinusoidal waveform is actually doing.
Watch out Greater precision is tempting, but a good DVM is usually far more accurate for a simple voltage reading; a scope's amplitude accuracy is typically only a few percent. Impedance measurement needs an impedance bridge or antenna analyzer, not a scope alone.
Scope shows the picture, meter shows the number. Waveform shape = oscilloscope.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B033 of 13

Which of the following is the best instrument to use for checking the keying waveform of a CW transmitter?

Why A keying waveform is amplitude plotted against time, so you need an instrument that displays a signal's shape in the time domain. An oscilloscope does exactly that, letting you see the rise and fall times of each dot and dash. Too-fast rise/fall edges produce key clicks (splatter), and the scope trace shows whether the shaping is soft enough to keep the sidebands narrow.
Watch out A field strength meter only shows relative RF level with no time detail, and a sidetone monitor just generates an audio tone for the operator's ear, telling you nothing about the actual transmitted envelope. A wavemeter indicates frequency, not waveshape.
Waveform means shape versus time, and shape versus time means scope.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B044 of 13

What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal?

Why An oscilloscope displays the instantaneous voltage applied to its vertical input, so to see the modulation envelope of your transmitted signal you must feed it a sample of the actual transmitter output. That output is far too powerful for a scope's input, so it is taken through a directional coupler, RF sampling probe, or resistive divider to attenuate it to a few volts. The resulting trace shows the RF carrier filled in with the shape of your audio, letting you spot flat-topping or overmodulation.
Watch out Looking at the local oscillator or the balanced mixer output only shows a stage inside the rig, not the amplified, filtered signal actually going to the antenna, and an external oscillator carries no modulation at all.
Envelope check = watch what you actually transmit, turned way down. Attenuated RF output into the vertical input.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B055 of 13

Why do voltmeters have high input impedance?

Why A voltmeter is connected in parallel with the point being measured, so it forms a parallel path for current. If its input impedance is low, it draws appreciable current and changes the voltage it is trying to read, especially in high-impedance circuits. Making the input impedance high (typically 10 megohms or more in a modern DMM) keeps that drawn current tiny so the circuit behaves almost as if the meter were not there.
Watch out High impedance does not extend the voltage range or improve resolution; those come from the range divider network and the number of digits in the analog-to-digital converter. Safe measurement of high voltages depends on insulation and probe rating, not on input impedance.
Voltmeter goes in parallel, so it should sip, not gulp: high Z means low loading.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B066 of 13

What is an advantage of a digital multimeter as compared to an analog multimeter?

Why A digital multimeter displays a numeric reading to several digits, so you can resolve a value far more finely than you can by eyeballing a needle against a printed scale on an analog meter. That extra resolution is what the pool calls higher precision. Analog meters still have their uses, but reading exact values is not one of them.
Watch out Faster response is actually an analog strength: the moving needle shows trends and lets you peak or null an adjustment, while a digital display's numbers jump around and update only a few times per second.
Digital = more digits = higher precision. Analog = moving needle = better for peaking and nulling.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B077 of 13

What signals are used to conduct a two-tone test?

Why A two-tone test feeds two audio tones into an SSB transmitter and looks at the RF envelope or spectrum for intermodulation products, which reveals amplifier linearity. The tones must not be harmonically related (the classic pair is 700 Hz and 1900 Hz) so that any distortion products land on frequencies clearly separate from the tones themselves. If the tones were harmonically related, the IMD products would fall right on top of the harmonics and you could not tell distortion from the test signal.
Watch out Two tones of the same frequency shifted 90 degrees describes quadrature or I/Q signals used in phasing modulators, not a linearity test; square waves are already full of harmonics and would swamp the measurement.
Two-tone test: two tones, not harmonically related, classically 700 Hz and 1900 Hz.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B088 of 13

What transmitter performance parameter does a two-tone test analyze?

Why A two-tone test feeds two non-harmonically related audio tones (for example 700 Hz and 1900 Hz) into an SSB transmitter and the RF output is examined on a spectrum analyzer or oscilloscope. If the amplifier chain is perfectly linear, only the two RF tones appear; any nonlinearity mixes them and creates intermodulation distortion products at other frequencies, which show up as extra spikes and as splatter on the air. The level of those IMD products relative to the two tones is the measure of linearity.
Watch out Carrier and unwanted sideband suppression is checked with a single tone, where you can see the carrier and opposite-sideband residuals as separate spectrum lines; two tones would just clutter that measurement.
Two tones in, only two tones out = linear. Extra spikes = IMD.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B099 of 13

When is an analog multimeter preferred to a digital multimeter?

Why An analog meter's moving needle gives you a continuous visual sense of which way a reading is heading, so when you tune a circuit for a peak or a null you can watch the needle swing and stop at the turning point. A digital display just throws up a string of numbers that jump around and lag, making it hard to see the trend while your hand is on the adjustment. That is why analog meters survive in tuning, peaking, and nulling work like adjusting a trap or dipping a stage.
Watch out High precision favors the digital meter, which typically resolves 3.5 digits or more and has no parallax or needle friction error. Frequency is measured with a counter, not a multimeter, and logic circuits are checked with a logic probe or scope.
Needle for trends, digits for numbers: watch the swing when peaking or nulling.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B1010 of 13

Which of the following can be determined with a directional wattmeter?

Why A directional wattmeter (or directional coupler) separates the power traveling toward the antenna from the power reflected back down the line. Knowing forward and reflected power gives you the reflection coefficient, rho = sqrt(Pref/Pfwd), and SWR = (1 + rho) / (1 - rho). That is exactly how an inline SWR/power meter works, so standing wave ratio falls right out of the two readings.
Watch out Front-to-back ratio compares radiation off the front and back of an antenna, which requires a field strength meter and a distant signal source, not a meter inserted in the feedline.
Forward minus reflected: two power readings in the feedline equal SWR.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B1111 of 13

Which of the following must be connected to an antenna analyzer when it is being used for SWR measurements?

Why An antenna analyzer is self-contained: it has its own low-power signal generator and impedance/SWR detector built in, so the only thing it needs is the load you want to measure, meaning the feed line and the antenna at its far end. You disconnect the radio and connect the antenna system to the analyzer's port, then sweep frequency to see where SWR is lowest. In fact the transmitter must stay disconnected, since RF from a transmitter can destroy the analyzer's sensitive front end.
Watch out Connecting a transmitter is the tempting answer because that is how an in-line SWR meter works, but an SWR bridge needs an external RF source while an analyzer supplies its own and can be damaged by transmitter power.
The analyzer is the transmitter and the receiver. All you add is what you are testing: feed line and antenna.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B1212 of 13

What effect can strong signals from nearby transmitters have on an antenna analyzer?

Why An antenna analyzer works by injecting a very low-level signal (often well under a milliwatt) into the antenna and measuring the tiny reflected voltage. The antenna under test is also a receiving antenna, so RF from a nearby transmitter arrives at the analyzer's detector at a level comparable to or greater than its own test signal, and the detector cannot tell the difference. The result is erratic or falsely high SWR and impedance readings. The cure is to test when neighboring stations are quiet, or to check whether readings change when the other station keys up.
Watch out The choices about desensitization, intermodulation and harmonic generation describe receiver front-end overload problems in a superheterodyne receiver, not the simple diode or bridge detector in an analyzer, which is just being swamped by unwanted received power.
An analyzer's test signal is microwatts; a neighbor's signal is watts. Whoever is louder wins the reading.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4B1313 of 13

Which of the following can be measured with an antenna analyzer?

Why An antenna analyzer is a low-power signal source plus an impedance bridge: it sweeps a frequency range and reports the complex impedance (R and X), SWR, and resonant frequency of whatever is connected to its port. Because coax is just another load at the analyzer's terminals, you can measure a cable's characteristic impedance, electrical length, and velocity factor by testing it open or shorted at the far end. What it cannot do is evaluate radiation properties, since those require measuring fields at a distance.
Watch out Front-to-back ratio and antenna gain are radiation-pattern measurements that need a test range with a distant source and a reference antenna, not a bridge at the feedpoint. Transmitter power output is measured with a wattmeter, and feeding transmitter power into an analyzer can destroy it.
Analyzer sees only what's at its connector: impedance, SWR, resonance. Anything about the far-field pattern needs a test range.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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