er is found in this chapter. A modulator is a device that turns your digital bit stream into a signal that is ready to be sent over the communication channel. Generally speaking, you'll transmit your information signal over a channel (for ex- ample, copper wire, coaxial cable, or as an EM wave through the atmosphere). That channel will only allow certain frequencies to make it to the receiver (that is, it will act as a band pass filter (BPF)) The modulator's job is to turn the information bits into a waveform that can make it to the receiver. I bet you can guess what a demodulator does. It sits at the receiver side and turns the incoming waveform created by the modulator back to the original bit stream. 5.2 Modulators There are two types of modulators: baseband modulators and bandpass modulators. 5.2.1 Baseband Modulators Baseband modulators are devices that turn your bit stream into a waveform centered around 0 Hz (Figure 5.2). You'd use this type when your channel allows frequencies around 0 Hz to get safely to the receiver. There are many baseband modulators, each one creating its own unique waveform around 0 Hz to send across the channel. Baseband modulator .10110 Figure 5.2 A baseband modulator Getting It from Here to There: Modulators and Demodulators NRZ Modulators One kind of baseband modulator is called an NRZ modulator, short for non-return-to- zero. As you may have already guessed from the name, with an NRZ modulator the waveform created never returns to 0 (zero) volts. Let's look at a couple of modulators from this family. The first-born, which went on to great success in many academic and business circles, was the popular NRZ-L. You can see what the NRZ-L modulator does to a 0 and to a 1 in Figure 5.3. A 0 stays at +V volts for the bit duration T6, and a 1 stays at -V for a bit duration T . Not nearly as popular is the NRZ-M (the black sheep of the family, I suspect). Here, we send either +V volts for a bit time Tb, or -V volts for bit time Tb If the bit is a 0, the signal level doesn't change. If th