If you have a coaxial cable, it may be better to directly transmit your baseband through it . If you insist in using RF (higher propagation loss), you can use a simple diode modulator; the diode opens and closes by baseband current (OOK). The on/off ratio can be >10 dB depending on RF carrier frequency. For 40 Mb/s, PIN diode modulators can be used; commercial models switching speed is 10-30 ns, just good for your 40 Mb/s. You can use a RF switching transistor as a RF amplifier, with the base (or gate) bias driven by your baseband signal. Hittite and Mini-circuits also offer MMICs that can be used.