G4D: Signal Measurement and Processing – Ham Radio General License Study Guide

G4D covers speech processing, S meter readings and decibel relationships, and the rules for operating SSB near band edges. These are practical topics every General class operator applies when setting up and using their station.

The exam draws from topics including what a speech processor does, how it affects an SSB signal, what happens when it is incorrectly adjusted, what an S meter measures, how a 20 dB over S9 signal compares to an S9 signal, how many dB one S unit represents, how much power change moves the meter from S8 to S9, what frequency ranges are occupied by LSB and USB signals at specific carrier frequencies, and how close to a band edge a carrier can be placed when using 3 kHz wide sideband.

Speech Processors

A speech processor is a signal processing circuit used in the transmit audio chain of an SSB transceiver. Its purpose is to increase the apparent loudness of transmitted voice signals. It does this by compressing the dynamic range of the audio — raising the level of quieter speech passages while limiting the peaks. The result is a higher average audio level going into the transmitter.

On an SSB signal, the transmitted power varies moment to moment with the loudness of the voice. A speech processor increases the average power of the SSB signal. The peak power is limited by the transmitter's capability, but the average power rises because the processor fills in the quieter parts of the speech, keeping the transmitter working harder on average.

When a speech processor is incorrectly adjusted — typically set too high — the results are harmful to the transmitted signal quality:

• Distorted speech — over-compression makes the audio sound clipped and harsh
• Excess intermodulation products — overdriving the transmit chain creates spurious signals adjacent to the desired signal
• Excessive background noise — the processor amplifies background noise between speech syllables along with the voice

All three effects occur simultaneously with an incorrectly adjusted processor. The correct setting is one that gives a modest increase in average power without audible distortion.

S Meters and Signal Strength

The S meter on a receiver indicates received signal strength. It displays the strength of the incoming RF signal at the receiver's input. S meters are calibrated in S units, typically from S1 (very weak) through S9 (strong signal) and then in decibels above S9 for exceptionally strong signals (S9+10, S9+20, etc.).

A signal reading 20 dB over S9 is 100 times more powerful than a signal reading S9. This comes directly from the decibel relationship: 20 dB corresponds to a power ratio of 10^(20/10) = 100.

S Units and Decibels

Each S unit on a properly calibrated S meter represents a change of 6 dB in signal strength. Six decibels corresponds to a power ratio of approximately 4 (10^(6/10) ≈ 3.98).

This means that to increase the S meter reading on a distant receiver from S8 to S9 — a change of one S unit, or 6 dB — the transmitting station must increase its power output by approximately 4 times. For example, going from 25 watts to 100 watts would produce roughly one S unit of improvement at the receiving station.

Sideband Operation Near Band Edges

An SSB signal occupies a 3 kHz bandwidth. The carrier frequency displayed on the transceiver is not the center of the signal — it is one edge. For LSB, the signal extends 3 kHz below the carrier frequency. For USB, the signal extends 3 kHz above the carrier frequency.

This means you must account for the full signal bandwidth when operating near a band edge:

Operating with the carrier too close to a band edge causes part of the transmitted signal to fall outside the amateur allocation, which is a rules violation. The 3 kHz minimum separation keeps the entire signal within the permitted segment.

G4D Practice Questions