Fundamentals of Receivers and Transmitters: Amplitude Limiting, Squelch Systems, and Modulation Theory

Amplitude Limiter Performance and Operation

Amplitude limiting within a receiver occurs when the input signal reaches a specific magnitude where the output level stabilizes despite further increases in input.

The Limiting Process

1. Threshold of Limiting (Value 2): This represents the specific point at which limiting begins.
2. Saturation (Values 3 to 4): As the input increases from value 3 to value 4, there is no corresponding rise in output voltage. Instead, the output current flows for a progressively shorter duration of the input cycle. This behavior is analogous to a Class C amplifier.
3. Flywheel Effect: To maintain a sinusoidal output voltage despite the output current flowing in pulses, the flywheel effect of the output tank circuit is utilized.
4. Upper Limit (Value 5): When input voltage increases sufficiently to value 5, the angle of current flow is reduced to a degree that less power is delivered to the output tank, causing a reduction in output voltage. Value 4 marks the upper bound of the effective limiting range.

Practical Performance Specifications

The range of input voltages for satisfactory operation is finite, bounded by the threshold point on one end and the point of excessively reduced output current flow on the other.

• Typical Input Range: In a practical limiter, Value 2 (threshold) might correspond to $0.4\,V$ peak-to-peak, while Value 4 might correspond to $4\,V$ peak-to-peak.
• Output Stability: The output remains approximately $5\,V$ peak-to-peak for any input between $0.4\,V$ and $4\,V$.
• Nominal Operating Point: A practical limiter is typically fed a voltage in the center of its range, approximately $2.2\,V$ peak-to-peak ($\approx 0.8\,V_{rms}$).
• Interference Rejection: This provides a variation range of $1.8\,V$ peak-to-peak, requiring spurious amplitude variations to be exceptionally large relative to the signal to bypass the limiter.

Double Limiter Systems

Standard amplitude limiters may fail if signal-strength variations push the average amplitude outside the limiting range. To address this, practical FM receivers often employ a Double Limiter.

• Configuration: Two amplitude limiters are arranged in cascade.
• Performance Gain: If the first limiter has a range of $0.4\,V$ to $4\,V$ and its output drops slowly (e.g., reaching $0.6\,V$ output at $20\,V$ input), and the second limiter has a range of $0.6\,V$ to $6\,V$, the effective limiting range is extended to cover all input voltages between $0.4\,V$ and $20\,V$.

Squelch (Muting) Systems

Squelch systems, also referred to as muting, quieting, Quiescent (Quiet) AGC, or CODAN (Carrier-Operated Device Anti-Noise), are used to suppress noise in the absence of a carrier signal.

Necessity of Squelch

When no carrier is present, Automatic Gain Control (AGC) disappears, causing the receiver to operate at maximum sensitivity. This results in the amplification of atmospheric and internal noise, creating a loud, disagreeable sound. Squelch is vital for systems like those used by police, ambulances, and coast radio stations where receivers must be monitored constantly for sporadic transmissions.

Squelch Circuit Operation

The squelch circuit typically consists of a DC amplifier (T2) that manages the first audio amplifier (T1) based on the AGC voltage.

• No Carrier State: When the AGC voltage is zero or low, the DC amplifier (T2) draws current. This creates a voltage drop across its load resistor ($R_1$), which biases the audio amplifier (T1) to cutoff. No noise or signal passes to the output.
• Carrier Present State: Once a carrier is received, the AGC voltage becomes sufficiently negative to cut off the DC amplifier (T2). Consequently, T2 stops drawing collector current. T1 then operates under its own self-bias (provided by the bypassed emitter resistor $R_2$ and base potentiometer resistors), allows audio to pass normally.
• Manual Adjustment: Resistor $R_3$ is a dropping resistor that ensures the DC voltage to T1's collector and base is higher than the voltage at its emitter. By manually adjusting $R_3$, operators can vary the cut-in bias of T2, choosing the AGC threshold at which the muting is deactivated.

SSB Clarifier Control

A Clarifier is a specific control on Single-Sideband (SSB) transceivers used to adjust frequency so the recovered audio matches the original modulating signal frequencies.

• Purpose: To ensure the received voice sounds natural.
• Effective Range: Typically $\pm 600\,Hz$ to $\pm 1500\,Hz$.

Transmitter Fundamentals

Transmitters are classified by their modulation type and method of transmission.

Continuous Wave (CW) Transmitter

• Usage: Primarily for radiotelegraphy using Morse code (short and long RF pulses).
• Advantages: Features a narrow bandwidth, requires less output power, and maintains high intelligibility under severe noise conditions (e.g., thunderstorms).

Amplitude Modulated (AM) Transmitter

• Principle: The instantaneous amplitude of the RF output is varied in proportion to the modulating signal.
• Components:
  • Oscillator/Buffer/Power Amp: Standard RF generation stages.
  • Microphone: Converting AF (voice) to electrical energy.
  • Modulator: Amplifies the AF signal to the level required to fully modulate the carrier.
  • Power Amplifier (PA): Combines the RF carrier and modulating signal.

Frequency Modulated (FM) Transmitter

• Principle: The frequency of the resultant wave varies with the instantaneous amplitude of the modulating signal.
• Components:
  • Varicap: Connected across the oscillator tank circuit; its reactance varies based on the modulating signal.
  • Frequency Multiplier/Power Amp: Used to increase frequency and amplitude respectively for transmission.

Single-Sideband (SSB) Transmitter

• Principle: Suppresses the carrier and one of the two identical sidebands, transmitting only the Upper Sideband (USB) or Lower Sideband (LSB) to improve efficiency.

Amplitude Modulation Theory and Math

AM is defined as a system where the carrier amplitude is proportional to the instantaneous amplitude of the modulating voltage ($V_m$), where the carrier frequency ($f_c$) is higher than the modulating frequency ($f_m$).

Frequency Spectrum and Bandwidth

• Sideband Frequency Formula: $f_{SB} = f_c \pm n f_m$
• First Pair ($n=1$):
  • Lower Sideband (LSB): $f_c - f_m$
  • Upper Sideband (USB): $f_c + f_m$
• Bandwidth Requirement: The total bandwidth required is twice the frequency of the modulating signal ($2 \times f_m$).

Modulation Index ($m$)

• Definition: The ratio of the modulating voltage amplitude to the carrier voltage amplitude.
• Formula: $m = \frac{V_m}{V_c}$
• Range: Typically between 0 and 1 (often expressed as a percentage).

Signal Clipping and Over-Modulation

If $m > 1$ (over 100% modulation), "clipping" occurs. Clipping is defined in several ways:

1. Overdriving: Flattening of the peaks of the sine wave peaks due to the amplifier being driven beyond capacity.
2. Voice-Operated Effects: Loss of initial or final parts of words or syllables due to non-ideal device operation.
3. Distortion: Severe deformation of speech signals resulting from limiting maximum amplitude.
4. Display/Video: In video, the shearing off of peaks (white/black levels) or sync signals; also known as "scissoring" in display contexts.

AM Signal Generation and Levels

Flywheel Effect

Generating an AM wave involves applying current pulses to a tuned circuit. Each pulse initiates a sine wave in the tank circuit proportional to the pulse size. This "flywheel effect" works best with high-Q tuned circuits and requires at least 10 pulses per audio cycle to create a smooth AM wave.

High-Level vs. Low-Level Modulation

• High-Level Modulation: Modulation is applied at the final output stage (e.g., plate-modulation or collector-modulation).
  • Pros: Better efficiency, lower distortion, better power handling.
  • Cons: Requires very high audio power.
• Low-Level Modulation: Modulation occurs at any stage prior to the output.
  • Pros: Requires lower modulating power.
  • Cons: Following stages must be linear RF amplifiers (e.g., Class B) to preserve the AM envelope, which are less efficient than Class C amplifiers.
• Broadcasting Note: In TV transmitters, grid modulation of the output stage is considered "high-level" because generating high video power for large bandwidths at the plate level is impractical.