![](https://csdnimg.cn/release/download_crawler_static/10217850/bg13.jpg)
2.1 A Brief History of UWB Systems and their Applications 11
circuit is ready to detect another fast pulse at the input. In conclusion this circuit
works as a monostable circuit, that when a small amplitude short pulse is pre-
sented to its input, produces a large amplitude pulse with a long duration at the
output. This output pulse can be used to measure the time between the transmitted
pulse and the return pulse created by the target (echo) and therefore, determine the
distance of the target. The clock signal that triggers the transmission of the short
pulse can be applied to a transmission line with the necessary time delay for the
pulse to travel to target and back to the receiver; this delayed clock can be used to
sample the output of the receiver, to determine if a target is present at the prede-
termined distance. The sensitivity of this receiver is determined by the tunnel di-
ode, if this device is biased closer to its turning point, the amplitude value of the
input pulse necessary to trigger the diode is reduced. This means that the circuit
threshold has been lowered and it will be more sensitive to any input pulses, but at
the same time it will be more prone to trigger on unwanted signals, i.e. circuit
noise.
The second type of receiver described in [8], called a constant false-alarm rate
receiver, improves the sensibility of the single-hit receiver by processing several
hits before signaling the detection of the target. The receiver is essentially the
same, except that the bias current of the tunnel diode is adjusted by a feedback
loop. The objective is to bias the diode as close as possible to the threshold of the
negative resistance region, to increase the sensitivity of the receiver. The feedback
loop is controlled by the number of hits detected when no signal is present. This is
archived by radiating the pulse on the rising edge of a clock and sampling the out-
put of the single-hit receiver on the falling edge of the clock. As the sensitivity of
the receiver is increased the number of false hits increases, the feedback loop will
adjust the bias current of the tunnel diode until the number of false hits is equal to
3 over 32 observation time instants. This means that the probability of a false hit is
around
(1/16) , the probability of 32 consecutive false hits would be equal to
32
(1/16)
which is a very small number. The output of the single hit receiver is
sampled when the clock is present and the number of hits obtained is added, a tar-
get is considered to be present when this sum exceeds 25, this will eliminate most
of the false hits. A proximity radar for detecting incoming ammunitions, using this
receiver, is described in [18].
In the early 1990's, McEwan [19] developed a low cost, low power impulse ra-
dar system, using discrete components mounted on a small PCB; this sensor is
called Micropower Impulse Radar (MIR). This radar sensor uses impulses with a
duration between 1 and 2 ns. These pulses are generated by applying a step signal
to a step recovery diode. The radar transmitter emits these pulses with a random
interval to reduce the probability of interference between different radar sensors.
Each sensor has a random noise generator that modulates the period of the square
wave that is used to generate the short pulses. A simple block diagram of the im-
pulse transmitter is shown in Fig. 2.4