A Novel Subnanosecond Monocycle Pulse Generator for UWB Radar Applications

A novel ultra-wideband (UWB) monocycle pulse generator with good performance is designed and demonstrated in this paper. It contains a power supply circuit, a pulse drive circuit, a unique pulse forming circuit, and a novel monopolar-to-monocycle pulse transition circuit. The drive circuit employs wideband bipolar junction transistors (BJTs) and linear power amplifier transistor to produce a high amplitude drive pulse, and the pulse forming circuit uses the transition characteristics of step recovery diode (SRD) effectively to produce a negative narrow pulse. At last, the monocycle pulse forming circuit utilizes a novel inductance L shortcircuited stub to generate the monocycle pulse directly. Measurement results show that the waveform of the generated monocycle pulses is over 76V in peak-to-peak amplitude and 3.2 ns in pulse full-width. These characteristics of the monocycle pulse are advantageous for obtaining long detection range and high resolution, when it is applied to ultra-wideband radar applications.


Introduction
Ultra-wideband (UWB) technology has received significant interests for a lot of applications, for example, nondestructive evaluation (NDE) of highway structures [1], geophysical prospecting, short-range in-building communications [2], vital sign detection [3], and so on.In these ultra-wideband communication and radar applications, step, Gaussian, and monocycle pulses are very important.And pulse generators of fixed pulse durations are used.These pulse types have a common characteristic of extremely wide instantaneous bandwidth and can be transmitted without carrier.Among them, the spectrum of the monocycle pulse contains very little low frequency and no DC components.It facilitates the design of other system components such as antennas, amplifiers, and receivers.What is more, this pulse type matches well with the transmitting antenna and owns better signal characteristics that can reduce distortion for the transmitting and receiving signals [4].
Several methods are used to produce monocycle pulses: one is using transmission line pulse forming networks or short-circuited stubs [5] and another is using RC or LC differentiators [6] to generate monocycle pulse.However, these methods usually used behind a SRD sharpener will introduce pulse distortion and much ringing into the output waveform.Therefore, how to reduce ringing behind the monocycle pulse is the key problem.
In this paper, a novel UWB monocycle pulse generator is showed.It contains power supply circuit, drive circuit, SRD pulse forming circuit, and monocycle pulse forming circuit.The drive circuit uses bipolar junction transistor to speed up the trigger signal and uses wideband linear power amplifier transistor to produce drive pulse with high amplitude.SRD pulse forming circuit uses two SRDs connected in series with a pulse-shaping network, which employs a Schottky diode.Monocycle pulse forming circuit consists of an inductance L short-circuited stub and an additional ringing suppression circuit, which combines two Gaussian pulses with opposite phase to form a monocycle pulse and reduce the ringing level of the output waveform effectively.The structure diagram of this novel generator is shown in Figure 1.The output monocycle pulse is over 76 V in peak-to-peak amplitude and 3.2 ns in pulse width.The ringing level is below 1.5%.These good performances make the UWB radar system obtain high resolution and long detection range for penetrating applications.

Power Supply Circuit
This monocycle pulse generator needs two kinds of power supply voltage.They are produced by the power supply circuit.The specific circuit diagram is shown in Figure 2. The implemented step-up voltage regulator 1 of type LM2577-ADJ is manufactured by National Semiconductor.The type of fast recovery diode 5 is MR851, which is manufactured by ON Semiconductor.The output voltage  CC2 is 56 V, while the input voltage  in is 15 V (8 = 100 kΩ, 9 = 1 kΩ).The implemented positive voltage regulator 2 of type ZR78L12 is manufactured by Zetex Semiconductors.It can change 15 V into +12 V as  CC1 .

Drive Circuit
This design consists of two bipolar junction transistors 1 (BFG35) and 2 (BFG31), a wideband linear power amplifier transistor 3, and energy storage capacitors.The type of 3 is B3-28, which is manufactured by ASI Semiconductor.Drive circuit uses the switching characteristic of 1 and 2 to shaping the trigger signal step by step.Then, it uses the power transistor 3 with stronger driving ability to produce the drive pulse with high amplitude and short pulse duration, which is hardly produced by normal RF transistor and microwave transistor.The specific circuit diagram is shown in Figure 3.It can offer a negative drive pulse to the input of the SRD pulse forming circuit.

SRD Pulse Forming Circuit
SRD is a kind of switching device which uses the chargestorage effect of minority carriers.There is steep impurity distribution area in its PN junction boundaries.It is the key component in the SRD pulse forming circuit.Transition time and minority carrier lifetime are the most important parameters of SRD.The transition time determines the minimum achievable pulse transition time, which is directly relevant to the rising time or falling time of output pulses.Minority carrier lifetime directly decides the peak amplitude of the output pulses by affecting the charge storage time under reverse-bias conditions.This design chooses appropriate  SRDs 2 and 3 connected in series to achieve a Gaussian pulse with maximum achievable amplitude and suitable pulse width.The MP4062 SRD in the P2 package [7] and BAT62-03W Schottky Diode (SD) in the SOD-323 package [8] are used in the SRD pulse forming circuit.The specific circuit diagram of the SRD pulse forming circuit is shown in part (a) of Figure 4.When the negative drive pulse does not come, the SRDs and Schottky Diode 1 are forward biased by +bias voltage and  CC2 , respectively.The minority carriers are stored near the PN junction of SRDs.When the negative drive pulse arrives, SD is reverse bias and cuts off immediately.It does not influence the circuit at the moment.The movement of the minority carriers forms reverse current by the reverse electric field effect.When the minority carriers are extracted completely, the reverse current quickly fell to minimum value, and the SRD turns off in a very short time.Thus a reverse current step is produced and a negative narrow pulse is generated at the cathode of 3.2 is used to store energy.1 is used to reduce the overshoot of the narrow pulse and also cut down the reflected wave formed by the poststage circuit.The 1N4148 in the SOD27 package are used for 4.

Measurement Result
The monocycle pulse generator is fabricated on FR4 glass epoxy substrate.The relative dielectric constant of the substrate is 4.5 and the thickness is 1.6 mm.A photograph of the generator is shown in Figure 5.The circuit is simple, and it is attractive for practical application.

Measurement Result
A TTL signal with a repetition frequency from 100 kHz to 10 MHz is used as trigger signal.The output voltage of the monocycle pulse generator has been measured choosing the following element values: 1 = 100 pF, 2 = 10 pF, 3 = 1 nF, 4 = 10 nF, 5 = 10 nF, 1 = 50 Ω, 2 = 3 = 200 Ω, 5 = 1 kΩ, and 2 = 10 uH.The output monocycle pulses measured by a wideband oscilloscope are showed in Figure 6.The trailing behind the main pulse is extremely small.The peak-to-peak amplitude ( p-p ), the rise time of the peak-to-peak amplitude (  ), pulse full-width (  ), and the ringing level at 1 MHz and 3 MHz PRF are summarized in Table 1.The normalized power spectrum of the output monocycle pulse calculated using the Fourier transform with Matlab is shown in Figure 7.
The 20 dB bandwidth is 1.25 GHz.And the output monocycle pulses at different PRF are shown in Figure 8.It shows that the influence of the PRF on the shape of the output waveforms is negligible.This monocycle pulse generator can work well up to 10 MHz.

Conclusion
This monocycle pulse generator contains a power supply circuit, a drive circuit, a SRD pulse forming circuit, and a monocycle pulse forming circuit.The key technology of this  pulse generator is drive circuit and the monocycle pulse forming method.The drive circuit connects BJTs and wideband linear power amplifier transistor together to produce drive pulse with high amplitude.The monocycle pulse forming circuit makes inductance  equivalent be a short-circuited microstrip line and utilizes the reflection characteristics of  the short-circuited transmission line to synthesis of a monocycle pulse.Moreover, an additional circuit consists of a capacitance and a diode connected in series to reduce the ringing level.As the measurement results showed, this monocycle pulse generator produces a monocycle pulse with good symmetry and low ringing level below 1.5%.Its peak-to-peak amplitude is 76 V, and its pulse full-width is approximately 3.2 ns.Therefore, this design can be used well for UWB radar application such as vital sign detection, penetrating thick and lossy building walls or ground with high resolution and good detection range.

Figure 1 :
Figure 1: Structure diagram of the discussed monocycle pulse generator.
This design uses inductance 3 as a short-circuited microstrip line.The specific circuit diagram of the monocycle pulse forming circuit is shown in part (b) of Figure4.The negative narrow pulse at the cathode of 3 propagates in two directions away from SRDs.One is to the output port and the other is to the short-circuited inductance 3.The latter is reflected back with inverted polarity as echoingcharacteristics of the short-circuited transmission line and then propagates to the output with time delay.Finally, a monocycle pulse is formed by the synthesis of the delayed inverted narrow pulse and the pulse propagating unchanged from the SRDs to the output port.An additional ringing suppression circuit is placed at the end of the design.It consists of capacitance 5 and diode 4.It reduces the trailing of the monocycle pulse effectively.

Figure 5 :
Figure 5: The photograph of the monocycle pulse generator.

Figure 6 :
Figure 6: The output pulse waveforms of the monocycle pulse generator at different PRF.

Figure 7 :Figure 8 :
Figure 7: Calculated power spectrum of the output pulses normalized to the peak value.