SSB — How Your Voice Gets On Air
Why SSB?
Regular AM transmits a carrier plus two sidebands — but the carrier carries NO information (it's just a reference), and both sidebands carry the SAME information. That's wasteful!
SSB removes the carrier and one sideband, saving power and bandwidth:
- Power: All transmitted power goes into the actual voice signal (AM wastes 2/3 on the carrier)
- Bandwidth: SSB uses ~2.4 kHz instead of AM's ~6 kHz
This is why SSB is the standard voice mode for HF amateur radio.
The Filter Method — How Most Radios Generate SSB
This is the classic approach, step by step:
- Your voice → Microphone → Audio amplifier
- Balanced modulator: Mixes your audio with a carrier frequency. The key trick: it produces BOTH sidebands but suppresses the carrier. You now have DSB-SC (Double Sideband, Suppressed Carrier).
- Crystal filter: A very sharp band-pass filter that passes ONE sideband and rejects the other. This is where the sideband selection happens.
- Mixer: Shifts the SSB signal from the filter frequency up to your operating frequency.
- Power amplifier (Class AB): Boosts the signal to your desired output power.
- Low-pass filter → Antenna: Removes harmonics before transmission.
- Below 10 MHz → use LSB (Lower Sideband)
- 10 MHz and above → use USB (Upper Sideband)
The Phasing Method — The Software Approach
Instead of using an expensive crystal filter, this method uses phase-shifting to cancel one sideband mathematically. It requires precise 90° phase shifts in both the audio and RF paths.
This approach is popular in SDR (Software Defined Radio) because phase shifting is easy to do precisely in software.
What the Balanced Modulator Actually Does
Think of it this way: it's a mixer that's specifically designed so the carrier leaks through as little as possible. The output contains the two sidebands but (almost) no carrier. That's why it's called "balanced" — the circuit is balanced so the carrier cancels itself out.