12.4 Audio, Squelch & Receiver Performance Specs

12.4 Audio, Squelch & Receiver Performance Specs

Quick Answer: AF amps drive speakers; FM de-emphasis + TX pre-emphasis → flat audio + noise reduction. Squelch: noise-operated mutes on noise presence; CTCSS = tones; DCS = digital codes. Sensitivity = minimum usable level (e.g. µV for 12 dB SINAD). Selectivity = reject adjacent channels. AGC holds level. Dynamic range = max tolerable / MDS. Cross-modulation = unwanted modulation heard on the desired signal.

The IF and detector can be perfect, and the set still fails the bridge or cockpit if audio, squelch, or overload behavior is wrong. Key topics 049–050 close Topic 3-F.

Audio frequency stages

After detection, baseband audio is still small and often needs filtering, de-emphasis, volume control, and power amplification.

De-emphasis (FM)

FM transmitters apply pre-emphasis (boost highs) before modulation because FM noise power rises with frequency. The receiver applies complementary de-emphasis (high-frequency roll-off) in the audio path.

Purpose of de-emphasis in the receiver audio stage: when coupled with transmitter pre-emphasis, flat audio and noise reduction is received. Memorize both outcomes—not flatness alone.

U.S. land-mobile/broadcast tradition often uses a 75 µs time-constant network; the exact constant matters less on Element 3 than the pre-emphasis ↔ de-emphasis pair concept.

DSP in the audio/IF chain

What radio circuit samples analog signals, records and processes them as numbers, then converts them back to analog? The digital signal processing (DSP) circuit.

DSP term for noise subtraction circuitry: adaptive filtering and autocorrelation. Modern marine/aviation DSP sets use these tools for hiss reduction and interference cancellation beyond simple analog filters.

Squelch systems

Purpose

Purpose of the squelch control: mute the receiver audio output when no signal is present (or when only noise is present), eliminating constant open-channel hiss. Set the threshold just tight enough to quiet the channel—over-tight squelch hides weak but important calls (same operating discipline as Element 1 VHF panels).

What makes each type work

What causes a squelch circuit to function? Presence of noise (classic noise-operated squelch wording on the pool).

What makes CTCSS squelch work? Tones.

What makes Digital Coded Squelch work? Digital codes.

CTCSS does not encrypt; it only gates the speaker so co-channel user groups share a frequency without hearing each other. After the FM discriminator recovers audio + tone, the tone is stripped before the audio section while a decoder compares tone/code to the programmed value.

Sensitivity, selectivity, AGC, dynamic range

Sensitivity

Sensitivity of a radio receiver: the minimum signal level required to produce a usable output. Specs quote microvolts (or dBm) for a stated quality metric:

SINAD = Signal + Noise + Distortion over Noise + Distortion. Higher SINAD means cleaner recovered audio; the sensitivity number is the input level that achieves the stated SINAD—not the SINAD itself.

Limiting condition remains the noise floor (§12.1); noise figure of the RF stage largely sets how low that floor sits relative to thermal noise.

Selectivity

Selectivity: the ability to separate a desired signal from adjacent-channel signals. Primary implementation is IF bandpass filtering (§12.3). Do not confuse with sensitivity: a hot but broad receiver hears everything—including the channel next door.

Automatic gain control

Function of AGC (automatic gain control): maintain a relatively constant audio output level despite varying signal strength. Strong RF → reduce IF/RF gain; weak RF → increase gain. AGC/AVC voltage is often a DC level proportional to signal strength—usable for S-meters (block-diagram questions may point to the AGC rectifier node for DC monitoring of signal strength).

Without AGC, tuning from a weak offshore ship to a strong local coast station would blast the speaker or bury the weak one in the noise as gain stayed fixed.

Dynamic range (reprise with performance framing)

Dynamic range = ratio of largest tolerable input to minimum discernible signal. Related overload symptoms:

Cross-modulation interference: modulation from an unwanted signal heard in addition to the desired signal (pool definition). Worded another way: signals from a very strong station are superimposed on other signals being received.

Mobile and installation noise (performance in the real world)

Element 3 includes practical interference fixes that affect “effective sensitivity” even when the radio meets the bench SINAD spec:

These are vehicle-side cures; they do not replace a good RF ground or antenna placement, but they are the answers the pool expects for spark-related pulse noise.

Performance reading for GROL maintainers

When a marine VHF “won’t hear” or an HF SSB “hears trash”:

1. Measure or estimate sensitivity — antenna, coax, or front-end NF before condemning the IF.
2. Check squelch / CTCSS — tight squelch or wrong tone mimics a deaf receiver.
3. Check selectivity path — wrong IF filter after a repair, or alignment drifted.
4. Check AGC — stuck AGC can sound like desense or no audio on strong signals.
5. Strong-signal environment — pad the front end; look for IMD/cross-mod, not only “needs more RF gain.”
6. Audio chain — de-emphasis, DSP, volume, and speaker; FM that sounds tinny may be missing de-emphasis.

Exam-day checklist for 049–050

1. De-emphasis + pre-emphasis → flat audio and noise reduction.
2. DSP → sample/process as numbers/convert back; noise subtraction → adaptive filtering and autocorrelation.
3. Squelch purpose → mute when no signal; noise squelch keyed to noise presence; CTCSS → tones; DCS → digital codes.
4. Sensitivity → minimum usable level (µV / SINAD); selectivity → adjacent-channel separation.
5. AGC → constant AF vs varying RF; dynamic range → max tolerable / MDS.
6. Cross-modulation → unwanted modulation on desired signal; ignition fixes → series R on plug wires / ferrites on ignition leads.

With architecture, conversion stages, filters/detectors, and audio/performance specs in hand, Topic 3-F (041–050) is complete. Element 3 next turns to transmitters—the mirror image of much of this receiver chain.