Sensitivity is one of the most important characteristics of a receiver — it is the ability to detect the presence (or absence) of a signal. It is often specified as the minimum input signal level required to produce a usable output (for example, a specified signal-to-noise ratio).

Selectivity is different: it describes the receiver's ability to receive a desired signal in the presence of other signals on nearby frequencies. A receiver with poor selectivity might let strong adjacent signals interfere with the one you want to hear.

RF gain refers to how much the receiver's front end amplifies incoming radio-frequency signals. While increasing RF gain can raise the signal level, sensitivity is the formal measure of how small a signal the receiver can detect; simply having gain does not by itself define sensitivity because noise and other factors also matter.

Total Harmonic Distortion describes how faithfully an amplifier reproduces the shape of a signal (how much harmonic distortion it adds) and is not related to the receiver's ability to detect a signal.

Memory aids:

• Sensitivity = detect (how small a signal can be detected)
• Selectivity = select (separate the desired signal from others)
• RF gain = amplification amount in the front end

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A transmitter converts information into radio waves; a receiver converts radio waves back into information. For example, the radio in your car is a receiver that picks up a broadcaster’s transmitter on the same frequency. A transceiver combines both a transmitter and a receiver in one device, which makes it ideal for two‑way communication because the same unit can both send and receive signals. Transceivers are the more common type of radio used by amateur operators, though some hams also use receive‑only equipment.

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A mixer is a device that combines two signals so that the output contains components at the sum and the difference of the two input frequencies. That property is what allows a signal to be converted from one frequency to another (for example, in receivers and transmitters using frequency conversion).

For example, mixing a 14.250 MHz signal with a 2.4 kHz signal produces outputs at the sum and difference frequencies:

Sum: 14.250 MHz + 2.4 kHz

• = 14,250,000 Hz + 2,400 Hz
• = 14,252,400 Hz
• = 14.2524 MHz

Difference: 14.250 MHz - 2.4 kHz

• = 14,250,000 Hz - 2,400 Hz
• = 14,247,600 Hz
• = 14.2476 MHz

These sum and difference products are why a mixer is used to change a signal from one frequency to another.

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Selectivity is the ability of a receiver to discriminate a desired signal from other adjacent signals. If the intermediate-frequency (IF) circuits do not adequately filter out signals on nearby frequencies, those signals will also be detected and will interfere with reception of the wanted signal.

Other related terms:

• Discrimination ratio: not a standard term for receiver performance and does not specifically describe how well a receiver separates adjacent signals.
• Sensitivity: a measure of how well weak signals can be detected and amplified above the noise floor; it describes the minimum signal level a receiver can handle, not how well it rejects nearby signals.
• Harmonic distortion: distortion that produces integer-multiple frequencies (harmonics) of the original signal; this describes waveform fidelity, not the ability to separate multiple signals.

Memory aids:

• "Selectivity" sounds like "select" — the receiver must select the desired signal and reject others.
• Remember: good selectivity lets you "discriminate" between signals.

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An oscillator is a circuit that produces a periodic signal at a specific frequency — it generates the carrier or timing signal used in transmitters and many electronic systems.

A reactance modulator is used to vary the reactance in a circuit to impose modulation (for example frequency modulation) on an existing signal; it does not by itself generate the carrier. A phase modulator changes the phase of an existing signal to add information, rather than creating the signal. A low-pass filter attenuates frequencies above a cutoff frequency and does not generate any signal.

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A transverter is a device that shifts the RF input and output of a transceiver to a different frequency band. It does this by using frequency conversion (mixing with a local oscillator) so signals from a transceiver on one band are translated to another band for transmission and reception.

Other common RF components do not perform band conversion:

• A high-pass filter passes frequencies above its cutoff and attenuates lower frequencies; it does not change the signal frequency.
• A low-pass filter passes frequencies below its cutoff and attenuates higher frequencies; it also does not change the signal frequency.
• A phase converter changes the phase relationship of a signal but does not alter its frequency.

Memory aids:

• Transverter — "trans" (across) + "verter" (convert) = converts from one band to another.
• High-pass / Low-pass filters — think "pass" certain frequencies, not "change" them.
• Phase converter — changes phase only, not frequency.

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The PTT input is an external control that switches the transceiver from receive to transmit when activated. While the PTT input is not asserted the radio remains in receive; asserting (activating) the PTT input causes the radio to stop receiving and begin transmitting, and releasing it returns the radio to receive mode.

Memory aids:

• PTT = Push To Talk
• Activation is often done by grounding the PTT input (closing a switch), such as the microphone's transmit switch.

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A modulator is the circuit that "modulates" or encodes the RF carrier frequency with the speech signal in a transmitter. The keyword "combines" indicates that the voice and carrier are put together, which should help you envision a circuit that modifies the carrier (for example by varying its amplitude, frequency, or phase) so it carries the audio information.

• Memory aid: Think of "modulation" as putting information onto a carrier wave so it can be sent over the air.

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The switch selects the amplifier's internal operating conditions so the amplifier runs correctly for the chosen modulation type. SSB and CW need the amplifier to operate in a more linear fashion (to preserve amplitude and phase information and to avoid distortion during keyed or single-sideband operation), while FM has a constant-amplitude signal and the amplifier can be biased for more efficient, less-linear operation. The switch changes biasing and related circuitry so the amplifier provides the proper performance and protection for the selected mode.

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An RF power amplifier is a device that increases (amplifies) the RF signal produced by a transceiver, producing greater transmitted output power by drawing additional power from its own power supply and delivering that extra RF power to the antenna.

A potentiometer only varies resistance/voltage and cannot increase the RF output power — at best it can attenuate or control levels. An impedance multiplier (or other passive impedance network) does not create additional power either; it only changes impedance relationships and can help with matching, but it cannot increase the total RF power available from the transmitter.

Memory aids:

• "Amplifier = increases power" — an amplifier adds power from its power supply.
• "Pot = pot down" — a potentiometer adjusts level, it doesn’t make more power.
• "Matching helps transfer, not create power" — impedance networks improve transfer but don’t generate extra RF power.

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The VFO is the part of the radio that sets your operating frequency — it’s what you’re actually tuning when you spin the main dial. So its job is to set the receive and transmit frequency.

Why is it called a Variable Frequency Oscillator?

• Oscillator: it generates a steady RF signal (a “carrier”).
• Frequency: that signal has a specific frequency.
• Variable: you can change that frequency as you tune around the band.

The other choices are things a radio might do, but they’re not the specific definition of the VFO’s job.

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