by Nathan M0NXD — What happens when your mic gain is too high on SSB
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Drive / PEP Meter
Signal Info
Understanding SSB Splatter
Single Sideband (SSB): SSB transmits only one sideband of an AM signal, suppressing the carrier and the other sideband. This halves the occupied bandwidth and concentrates all power into useful signal. USB (Upper Sideband) is used above 10 MHz; LSB (Lower Sideband) below 10 MHz — by convention.
Filter Width: The IF crystal/roofing filter determines the transmitted audio bandwidth. Standard is 2.7 kHz (300–3000 Hz audio). Narrower filters (1.8–2.4 kHz) reduce interference in contests but cut high-frequency audio. Wider filters (3.0 kHz) sound fuller but occupy more spectrum.
PA Linearity & IMD: Unlike FM (constant envelope), SSB has a varying envelope. The PA must faithfully reproduce this envelope. When overdriven, the PA compresses or clips peaks, generating intermodulation distortion (IMD) — spurious products at frequencies like 2f1−f2 and 2f2−f1 that splatter into adjacent channels and even the opposite sideband.
Two-tone Test: The standard way to measure PA linearity. Two equal tones (here 700 + 1900 Hz) are fed through the transmitter. A clean PA shows only the two tones; a distorted PA shows 3rd, 5th, 7th order IMD products extending beyond the filter passband. IMD3 of −25 dBc or worse indicates significant splatter.
Hard vs Soft Compression: A hard clip (Class C, saturated PA) creates sharp waveform discontinuities with strong, high-order IMD extending far from the signal. A soft PA (well-biased Class AB) compresses gradually, producing lower-level IMD concentrated in the 3rd and 5th orders. A perfectly linear PA produces no IMD at any drive level — but is physically impossible at unlimited power.
ALC (Automatic Level Control): Most transceivers have ALC that reduces gain when the PA approaches compression. Properly set, ALC prevents splatter. But if mic gain is so high that ALC is constantly active, the compression itself degrades audio quality. Set mic gain so ALC barely flickers on voice peaks.
Real-World Tips: Set mic gain so the ALC meter barely moves on voice peaks. Speak across the mic, not into it. Use speech compression sparingly — it increases average power but also increases IMD. On HF, your splatter can wipe out stations 3–5 kHz away. A good target is IMD3 ≤ −30 dBc.