2.2. UWB Pulse Generation
Subnanosecond pulse generation is a critical element of UWB communications systems. Today several techniques are available to generate UWB pulses, mostly from radar technology. Pulses with fast rise time can be generated by rapid activation and deactivation of semiconductor switches. One method for generating short-duration pulses is using light-activated silicon switching (LASS), where laser light is employed to activate semiconductor switches . However, these high-power devices are not very common in most communications applications.
Some of the more typical methods for generating subnanosecond pulses in communications applications are tunnel diodes (TDs), step recovery diodes (SRDs), and drift step recovery diodes (DSRDs). The pulses generated by these devices still need to go through appropriate pulse-shaping mechanisms to be converted to the desired UWB pulse shape for various applications.
In this section, we provide an overview of TDs, SRDs, and DSRDs. Optical activated switches will not be covered in this book; however, more information on this technique can be found in . Before starting our discussion on UWB pulse radiation and antennas in Section 2.3, we briefly explain a common Gaussian pulse-shaping technique.
2.2.1. Tunnel Diodes
Tunnel diodes have been successfully used for decades to generate narrow pulses for radar applications. In this type of diode, a very small biasing voltage can create current, based on the tunneling concept in quantum mechanics. In tunnel diodes, an increase in the forward voltage decreases the current and creates a negative resistance region, making the diode unstable. This instability causes the diode to switch to the forward point and generate a short-duration pulse. Tunnel diodes can generate pulses as short as 25 picoseconds. However, these pulses have low amplitude, on the range of 0.25 to 1.0 volt , which makes them unsuitable for UWB communications.
2.2.2. Step Recovery Diodes
Step recovery diodes, commonly called charged diodes, are the most common and reliable method for the generation of UWB pulses. An SRD in its simplest form is a PN junction that operates based on its ability to store and release charges. Figure 2-1 shows a typical PN junction: electrons reside in the negative region, N, and holes reside in the positive region, P. In other words, holes represent the absence of electrons and constitute the positive region of a PN junction.
Figure 2-1. A PN junction
In SRDs, a forward-bias voltage is applied to a PN junction, which causes the holes to move away from the positive voltage. At the same time, the electrons are attracted to the positive voltage and move toward the voltage source. This movement of holes and electrons creates positive and negative charges at the junction, called the depletion region, as shown in Figure 2-2.
Figure 2-2. A PN junction in forward-bias mode
As illustrated in Figure 2-2, once the forward-bias voltage is applied to the PN junction, charges are stored in the depletion region and the junction acts as a capacitor with low impedance. As the reverse-biased voltage is applied to the PN junction, the positive charges formed in the depletion region move toward the negative voltage, and negative charges move toward the positive voltage, as shown in Figure 2-3.
Figure 2-3. A PN junction in reverse bias
Once the charges are drained from the depletion region, the junction has high impedance and low capacitance. At this time, a short pulse is generated with duration equal to the time it takes to drain the charges. SRDs are capable of generating pulses 60 to 200 picoseconds long with amplitudes as high as 20 to 200 volts .
2.2.3. Drift Step Recovery Diodes
Drift step recovery diodes are capable of generating subnanosecond pulses with very high peak power on the order of 100 megawatts . Their basic operation is similar to that of SRDs with one major distinction. In SRDs, a continuous pumping current is applied to the PN junction for a time longer than the lifetime of the carrier (electron and hole). In DSRDs, the pumping current is a very short pulse rather than a con-tinuous current. Therefore, to generate a subnanosecond pulse using DSRDs, an impulselike current is applied first in the forward direction to charge the PN junction. Then the direction of the impulselike current is reversed rapidly, which causes the removal of the accumulated charge in the junction. This process generates high-power subnanosecond pulses at the diode's terminal that are suitable for UWB applications.
2.2.4. UWB Pulse Shaping
The narrow pulses generated by any of the methods described so far are either steplike or ramplike impulses that need to be shaped to the desired UWB pulse shapesuch as wavelet, Gaussian, chirp, and so onbefore transmission. To convert the impulses to the various pulse shapes, we need proper pulse-shaping circuitry. In this subsection, we describe a common method for converting a ramplike pulse generated by an SRD into a Gaussian monocycle.
A simple method for obtaining a Gaussian monocycle pulse shape is to superimpose a steplike pulse generated by an SRD with a delayed and inverted version of itself. Figure 2-4 shows a simplified, top-level block diagram of a Gaussian monocycle pulse-shaping circuit. Detailed information about the circuit components can be found in .
Figure 2-4. A block diagram of a common Gaussian pulse-shaping circuit
As shown in Figure 2-4, the steplike pulse generated by an SRD divides at point A. Hence, one steplike pulse propagates down "transmission line 1" and another similar steplike pulse travels toward "transmission line 2."
The pulse that has traveled through the first transmission line will be reflected back with an opposite polarity, due to short circuit to ground. Then the delayed, inverted pulse ultimately combines with the other steplike pulse and forms a Gaussian pulse shape. The duration of the formed Gaussian pulse is related to the time that it travels in the short-circuit path and consequently is related to the length of transmission line 1. The resistive matching network shown in Figure 2-4 prevents a large ringing effect in the generated Gaussian pulse.