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T7A

PRACTICAL CIRCUITS

Station equipment: receivers, transceivers, transmitter amplifiers, RF preamplifiers, transverters; Basic radio circuit concepts and terminology: sensitivity, selectivity, mixers, oscillators, Push-To-Talk (PTT), VFO, modulation

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T7A011 of 11

Which term describes the ability of a receiver to detect the presence of a signal?

Why Sensitivity is the measure of how weak a signal a receiver can still detect and turn into usable audio. It is usually specified in microvolts or dBm for a given signal-to-noise ratio, so a receiver rated 0.2 microvolts is more sensitive than one rated 1 microvolt. It depends mostly on the gain and noise figure of the front end stages.
Watch out Selectivity is the other common receiver spec, but it describes how well the receiver rejects adjacent signals, not how weak a signal it can hear. RF gain is just a front-end level control, and total harmonic distortion is an audio or transmitter purity measure.
Sensitivity = Sense the weak signal. Selectivity = Select one signal out of many.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A022 of 11

What is a transceiver?

Why The prefix trans- (transmitter) plus -ceiver (receiver) says it directly: a transceiver is one box that both transmits and receives, usually sharing circuitry such as the VFO, filters and antenna connector between the two functions. Nearly every modern amateur radio is a transceiver, which is why you press PTT to switch the same unit from receive to transmit.
Watch out The choice about converting frequencies to another band describes a transverter, which takes a transceiver's output on one band and shifts it to another, and the impedance-matching choice describes an antenna tuner.
TRANSmitter + reCEIVER = TRANSCEIVER. Transverter is the one that 'converts' bands.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A033 of 11

Which of the following is used to convert a signal from one frequency to another?

Why A mixer is a nonlinear stage that combines an incoming signal with a local oscillator signal and produces outputs at the sum and difference of the two frequencies. Selecting one of those products with a filter moves the signal to a new frequency, which is how superheterodyne receivers convert an incoming signal down to an intermediate frequency and how transmitters move a low-frequency signal up to the operating band. Transverters and frequency multipliers rely on the same mixing principle.
Watch out An amplifier is tempting because it is also a signal-processing stage, but it only increases amplitude and leaves the frequency unchanged. A phase splitter produces two outputs of opposite phase, and an inverter reverses polarity or converts DC to AC.
Mix two frequencies, get sum and difference. Mixing changes frequency; amplifying only changes size.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A044 of 11

Which term describes the ability of a receiver to discriminate between multiple signals?

Why Selectivity is a receiver's ability to separate a desired signal from others nearby in frequency, and it is set mainly by the bandwidth and shape of the receiver's filters (IF filters, crystal or DSP filters). A narrow CW filter of a few hundred hertz gives high selectivity because it rejects signals just a kilohertz away, while a wide FM filter passes much more of the band. Poor selectivity shows up as adjacent signals bleeding into the one you are trying to copy.
Watch out Sensitivity is the other common receiver spec, but it describes how weak a signal the receiver can detect, not how well it separates signals that are close together in frequency.
Selectivity = Separating signals; Sensitivity = Seeing weak Signals.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A055 of 11

What is the name of a circuit that generates a signal at a specific frequency?

Why An oscillator is the circuit that converts DC power into a repeating AC waveform at a chosen frequency, usually set by a crystal or an LC tank. Every transmitter and receiver needs one: the VFO that sets your operating frequency and the local oscillator that feeds a mixer are both oscillators. In short, it is the circuit that generates a signal at a specific frequency.
Watch out The modulator choices, reactance and phase, do not create a signal on their own; they take an existing carrier and vary it to impress audio or data onto it. A low-pass filter only removes frequencies above a cutoff, it generates nothing.
Oscillator = origin of the signal. Modulators change a signal, filters remove parts of it, only the oscillator makes one.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A066 of 11

What device converts the RF input and output of a transceiver to another band?

Why A transverter is a frequency converter that sits between a transceiver and the antenna, mixing the radio's transmit output up (or down) to a different band and converting received signals on that band back to the radio's operating frequency. It contains a mixer and a local oscillator, plus a low-power transmit stage, so a 28 MHz HF radio can work 144 MHz or 1296 MHz. The name itself is a blend of transmit and converter, since it handles both directions.
Watch out High-pass and low-pass filters only block or pass ranges of frequencies, they cannot shift a signal to a new band; a filter changes what gets through, not where in the spectrum it lands.
Transverter = TRANSceiver + conVERTER: it moves both TX and RX to another band.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A077 of 11

What is the function of a transceiver's PTT input?

Why PTT stands for Push-To-Talk. The PTT line is a control input that keys the radio: pulling it to ground (usually through a microphone switch, foot switch, or an external device such as an amplifier or computer interface) tells the transceiver to switch from receive to transmit. It is an active-low control line, so opening the circuit returns the radio to receive.
Watch out A CW key connects to the separate KEY jack; that also causes transmission, but the PTT line is the general transmit-enable control used for voice and data modes, not for forming code characters.
PTT = Push To Talk. Ground it and you're transmitting; release it and you're listening.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A088 of 11

Which of the following describes combining speech with an RF carrier signal?

Why Modulation is the process of impressing information, such as speech, onto a radio frequency carrier by varying one of the carrier's properties: its amplitude (AM, SSB), its frequency (FM), or its phase. The carrier by itself conveys no information; the audio has to change some characteristic of it so the receiver can recover the speech by demodulating. Every voice mode name, AM, FM, SSB, is just a description of how that modulation is done.
Watch out Oscillation is tempting because an oscillator generates the carrier in the first place, but generating a steady signal is a different step from adding voice information to it. Impedance matching transfers power efficiently between stages, and low-pass filtering removes unwanted high frequency content such as harmonics.
Modulation = MODify the carrier with your voice. Oscillator makes the carrier, modulator puts your words on it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A099 of 11

What is the function of the switch which selects either SSB or CW-FM on some VHF power amplifiers?

Why On a VHF brick amplifier, that switch is inside the amplifier only, so it cannot change what the radio transmits. It configures the amplifier itself: in SSB position the RF-sensing transmit/receive relay gets a hold-in delay so the amp does not chatter between syllables of speech, and the bias is set for linear amplification. In the CW/FM position the amp is set for constant-carrier, full duty-cycle operation with fast keying.
Watch out The tempting answer is that it changes the transmitted mode, but the mode is generated in the exciter or transceiver; the amplifier just faithfully amplifies whatever it is fed.
The amp doesn't make the mode, it adapts to it: SSB position adds relay hang time, FM/CW position sets full-carrier operation.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A1010 of 11

What can be added to the output of a transceiver to increase the transmitted output power?

Why Transmitted power is increased by feeding the transceiver's output into an RF power amplifier, a stage that takes the low-level drive signal (often 5 to 100 watts) and amplifies it to a higher output level. In amateur practice this external box is called a linear amplifier or just an amp, and it must preserve the modulation waveform for SSB and digital modes. Note that the total station output still has to stay within the power limits of 97.313.
Watch out A potentiometer is just a variable resistor used to adjust voltage or level in low-power control circuits, and it would only waste RF power as heat; an 'impedance multiplier' is not a real station component, though impedance matching networks do exist and they transform impedance without adding power.
More watts out means more amplification: only an RF power amplifier adds power. Resistors and matching networks never create power.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7A1111 of 11

What is the function of the Variable Frequency Oscillator (VFO) circuit in a transceiver?

Why A VFO is the tunable oscillator that establishes the operating frequency of a radio. In the receiver it feeds the mixer to select which incoming signal gets converted to the IF, and on transmit it sets where the signal is radiated, so the same circuit determines both receive and transmit frequency in a transceiver. When you turn the main tuning knob, you are changing the VFO frequency (in modern rigs usually a synthesizer or DDS doing the same job).
Watch out The choice about injecting a variable frequency for CW reception describes the BFO (beat frequency oscillator), which is a separate fixed oscillator near the IF that turns CW and SSB into audible audio. Automatic frequency control is a correction circuit that nudges tuning to stay on signal, not the primary tuning source.
VFO = the tuning knob. Variable Frequency Oscillator sets where you listen and where you talk; BFO is the one that makes CW audible.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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