From Robert F. Liebe and Robert A. Surette, "Combining Digital and Analog Signals for US IBOC FM Broadcasting," IEEE Transactions on Broadcasting, vol. 48, no. 4 (December, 2002), p. 361.
The above image represents an FM IBOC signal operating in what the authors call "extended hybrid" mode.
The triangular part of the signal is the analog portion; the squares to its right and left are the digital sidebands.
The solid-line tracing around the signal is a representation of the FCC's spectral emission mask rules.
Note that beyond ~100 kilohertz of either side of the center frequency (represented by the dotted line) the power level of FM signals must drop off precipitously to stay within the emission mask. It is in this area where the digital sidebands begin.
A lone analog FM signal occupies approximately 200 kilohertz of bandwidth on the FM dial. As this illustration clearly shows, FM hybrid analog/digital signals - what the industry currently calls "HD Radio" - occupy 398 kilohertz of bandwidth.
This has generated concern about increased interference between stations, both analog and hybrid. Field tests have confirmed these concerns are real, but there is not enough data to make conclusive projections on how serious the interference problem will become as more stations deploy digital broadcast technology.