While studying for his U.S. General class license, my son wondered about the "balanced modulator" referred to by a few questions in the FCC pool.
Quoting KB6NU's General Class study guide to give a sense of the two questions:
Filters are also used in amateur radio transmitters. A filter is used to process signals from the balanced modulator and send them to the mixer in a single-sideband phone transmitter. (G7C01) A balanced modulator is the circuit used to combine signals from the carrier oscillator and speech amplifier and send the result to the filter in a typical single-sideband phone transmitter. (G7C02)
Coming from the SDR world, I'm much more familiar with "mixers" being used for frequency conversion, and I tend to think of SSB from that perspective (i.e. raw baseband that's simply been upconverted). Other test pool questions want me to conceive of SSB as generated by an AM signal that's somehow had its carrier [sharply!?] filtered out afterward. Neither of those sounds like this.
In short, the term "balanced modulator" is unknown to me (despite working through the same exam pool a few years ago ;-] ) and my initial web searches on a small screen didn't turn up very much information either. Among Google results on a bigger screen today, the Armstrong phase modulator does come up but that seems more for FM than SSB, and, as far as I can tell, only uses a balanced modulator rather than explaining what one is….
What are the core principle(s) of a balanced modulator? What is the significance of them being "balanced"? In broad strokes how would one design/build one in practice? Are they still used in commercial SSB transceivers and/or modern kit radio circuitry?