Threshold voltage generator

Threshold circuit for selecting video signals derived from small stationary and moving objects amidst an abundance of clutter in a harbor area. The threshold voltage generator (1) of this circuit is suitable for detecting the temporal average .DELTA.P of the positive peak value and the temporal average .DELTA.N of the negative peak value from the AC component of the applied video voltage (V.sub.DC +V.sub.AC) and for generating modified voltages (l+k)(V.sub.AC -.DELTA.P) and k(-V.sub.AC -.DELTA.N). The modified voltages together with the inverted video voltage are combined to form the threshold voltage V=V.sub.DC +.DELTA.P+k(.DELTA.P+.DELTA.N) being k times the average peak-peak value above the average peak value of the video voltage, where k is a constant.

BACKGROUND OF THE INVENTION 
The invention relates to a threshold circuit for video signals, as obtained 
with a radar apparatus, for example. 
Such a threshold circuit is widely known and is used for selecting strong 
video signals from other weaker video signals, while a large amount of 
clutter signals are eliminated. The term clutter signals is used to 
describe the collection of unwanted echo signals which smear the radar 
picture and render it impossible to identify wanted echo signals. Usually 
of interest are echo signals from moving targets; the remaining echo 
signals are therefore labelled as clutter when can be eliminated by means 
based on MTI and pulse Doppler techniques. This is not the case, however, 
with a harbor radar used to observe activities in the harbor area along 
and on the water. Here, not only of interest are echo signals from moving 
targets, but also from stationary targets; not only large structures, such 
as moored vessels and harbour buildings, but from smaller objects as well, 
such as buoys and beacons. In particular, the latter type of objects are 
difficult to identify on a radar screen amidst an abundance of clutter 
signals from rain and/or the water surface. In a harbor radar, the video 
signals from clutter cannot be eliminated by the abovementioned means, as 
this would also suppress wanted video signals from stationary targets. In 
the case in question, the clutter signals should be eliminated by means of 
a threshold circuit, where it is of great importance to select a suitable 
threshold level, as pointed out in Skolnik's Radar Handbook, Chapter 5.8. 
A too high threshold voltage constitutes the risk that video signals of, 
say, buoys and beacons are eliminated with the clutter signals, whereas a 
too low threshold voltage would tend to designate an excess of video 
signals as originating from genuine targets, thus causing an excessive 
false alarm rate. It is therefore important to select such a threshold 
value so as to result in a maximum number of detections of genuine targets 
with maximum clutter elimination and a minimum false alarm rate. 
Since the clutter strength is not constant, but is dependent, on the one 
hand, upon factors usually varying slowly in value, such as wind-force, 
wind direction, harbor-traffic density, and length of the water waves, and 
on the other hand, upon factors which may vary rapidly in local places, 
such as the radar bearing relative to the direction of the waves, 
variations in wind-force and bow and stern waves, it is advantageous to 
adapt the threshold voltage continuously to the varied circumstances 
during the radar observations. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide a solution for the 
above problem. According to the invention, the threshold circuit as set 
forth in the opening paragraph comprises a threshold voltage generator 
containing a detection and multiplication circuit, a combining circuit 
connected to the detection and multiplication circuit, and a low-pass 
filter connected to the combining circuit, whereby the detection and 
multiplication circuit detects, from the applied video voltage (V.sub.DC 
+V.sub.AC), both the temporal average .DELTA.P of the positive peak value 
of the AC component V.sub.AC for modifying V.sub.AC to obtain a first 
output voltage (1+k)(V.sub.AC -.DELTA.P), where k is a predefined factor, 
and the temporal average .DELTA.N of the negative peak value of the AC 
component V.sub.AC for modifying the inverted AC component (-V.sub.AC) to 
obtain a second output voltage k(-V.sub.AC -.DELTA.N); the two output 
voltages, together with the inverted video voltage (-V.sub.DC -V.sub.AC) 
are applied to the combining circuit and the low-pass filter connected 
thereto to produce a threshold voltage: 
EQU V=V.sub.DC +.DELTA.P+k(.DELTA.P+.DELTA.N)=V.sub.DC +.DELTA.P+k.V.sub.pp, 
where V.sub.pp is the average peak-peak value of the AC component in the 
video voltage. The threshold level so obtained is a defined factor k times 
the average peak-peak value V.sub.pp above the average peak value of the 
video voltage. 
Such a threshold circuit enables the elimination of the phenomena of 
continuous occurrence, like clutter, and to retain the phenomena of 
incidental occurrence.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1A shows the variation in the video voltage to be processed in 
inverted form over a certain period of time. This voltage, obtained from 
clutter signals (C) and a real-target echo signal (T), can be split into a 
DC component V.sub.DC and an AC component V.sub.AC. The object of the 
present invention is to generate such a threshold voltage V as to result 
in a maximum number of detections of genuine targets with maximum clutter 
elimination and a minimum false alarm rate. In view of the fact that the 
strength of the clutter signals from the water surface can vary frequently 
and abruptly, the desired threshold voltage V is such that the difference 
between the threshold voltage V and the average peak value (V.sub.DC 
+.DELTA.P) of the clutter-fouled video signal is a function of the average 
peak-peak voltage of the clutter signals V.sub.pp =.DELTA.P+.DELTA.N. The 
threshold voltage V can therefore be expressed by: V=V.sub.DC 
+.DELTA.P+k.V.sub.pp, where k is a suitable constant. Hence, the greater 
the decrease in clutter activity, the closer the approach of the threshold 
level to the video signal level. In the embodiment of FIG. 2A, the numeral 
1 denotes a threshold voltage generator producing the desired voltage 
V=V.sub.DC +.DELTA.P+k.V.sub.pp. 
Threshold voltage generator 1, consisting of a detection and multiplication 
circuit 2, an inverting circuit 3, a combining circuit 4 and a low-pass 
filter 5, delivers the desired threshold voltage to a gate circuit 6 for 
comparison with the video voltage. 
The video voltage, which in the embodiment in question is derived from a 
logarithmic receiver 7, is applied to the inverting circuit 3 and 
delivered as -(V.sub.DC +V.sub.AC) to combining circuit 4. Also the video 
voltage is applied to the detection and multiplication circuit 2 in which 
an AC detector 8, having a highpass filter characteristic, passes the AC 
component V.sub.AC of the video voltage. A phase splitter 9, connected to 
AC detector 8, generates two output signals, one signal being in phase and 
the other in antiphase with the input signal. 
The in-phase AC signal (V.sub.AC) is fed to a first peak detector 10 and is 
modified to form the signal V.sub.AC -.DELTA.P, where .DELTA.P is the 
temporal average of the positive peak value of the AC component V.sub.AC 
(see FIG. 1A). FIG. 2B shows an embodiment of a peak detector consisting 
of a capacitor C.sub.1 and an equivalent diode circuit D. Diode circuit D 
of this embodiment is a grounded FET switch F, which is controlled by the 
output signal of a grounded comparator K. After multiplication of the 
voltage from peak detector 10 by a factor (1+k) in unit 11 (see FIG. 2B), 
where k is a predefined value, the resulting voltage (1+k)(V.sub.AC 
-.DELTA.P) is fed to combining circuit 4. 
The out-of-phase AC signal (-V.sub.AC) is applied to a second peak detector 
12. This detector, which may be of the same type as the first peak 
detector 10, is to determine the temporal average .DELTA.N of the negative 
peak value of the AC signal V.sub.AC (see FIG. 1A) and thereby to modify 
the AC component to form the voltage -V.sub.AC -.DELTA.N. After 
multiplication by a factor k in unit 13, the latter voltage is also fed to 
combining circuit 4. 
In consequence of the three supplied voltages -V.sub.DC -V.sub.AC, 
(1+k)(V.sub.AC -.DELTA.P) and -k(V.sub.AC +.DELTA.N), combining circuit 4 
(see FIG. 1B) produces an output voltage: V.sub.1 =-{V.sub.DC 
+.DELTA.P+k(.DELTA.P+.DELTA.N)}. Since a peak detector also makes a fast 
response to variations in the video voltage with the detection of echoes 
from genuine targets, the output voltage of combining circuit 4 will 
therefore follow the variation in the peak values of the video voltage at 
a difference level of kV.sub.pp. Hence, also video signals from genuine 
targets will remain below the output voltage of circuit 4, so that this 
output voltage is not simply suitable as the threshold voltage! This 
situation is however prevented by connecting a low-pass filter 5 to 
combining circuit 4. In this way the clutter, which behaves as a 
continuous phenomenon (with slow variations in the peak voltage level), 
will remain fully below the output voltage of low-pass filter 5. On the 
other hand, a video signal of a genuine target, which behaves as an 
incidental phenomenon (with usually rapid variations in the voltage level) 
will ascend rapidly above the slowly increasing output voltage of low-pass 
filter 5. For this reason, the output voltage of low-pass filter 5 is 
suitable to function as the threshold voltage. 
A threshold voltage generator which can be used to advantage, is obtained 
when the low-pass filter 5 is polarity-dependent, that is, if the filter 
time constant is large with the increase of the threshold voltage but 
small with the decrease of this voltage; this amounts to a slow rise of 
the threshold voltage in the presence of a video signal from a genuine 
target, but a very rapid fall of the threshold voltage after the 
disappearance of the signal (see threshold voltage V.sub.2 in FIG. 1B). 
Such a low-pass filter can be regarded as an RC circuit, of which the 
resistance branch is formed by a parallel circuit of both a high 
resistance R.sub.1 and a series circuit of a low resistance R.sub.2 with 
an equivalent diode circuit D. Diode circuit D is so incorporated that, if 
the filter input voltage is greater than the output voltage, the diode 
circuit is blocked and, conversely, if the filter input voltage is 
smaller, the circuit is conducting. Also in this case, diode circuit D may 
consist of a FET switch (F), controlled by a comparator K. 
The filter output voltage can now be applied as threshold voltage to gate 
circuit 6. A suitable gate circuit is, for instance, a linear amplifier, 
of which the output voltage is proportional to the difference between the 
applied video voltage and the threshold voltage, provided the video 
voltage is greater than the threshold voltage. However, in the embodiment 
in question, where the video signals are obtained from a logarithmic 
receiver 7, an amplifier having an exponential gain factor is used as gate 
circuit 6. 
To obtain a better range resolution in the echo detection, it is 
advantageous to blank the video signal during its rise and fall time. The 
threshold circuit 1 is thereto provided with a slope detector 14 for 
determining the slope of the video signal and a blocking circuit 15 to 
which the output of gate circuit 6 is connected via a delay means 16 (with 
a fixed delay of .DELTA.T.sub.2), the blocking circuit 15 being controlled 
by the output signal of slope detector 14. In the embodiment in question, 
slope detector 14 comprises means 17 for detecting a leading edge having a 
slope that exceeds a predefined value, and means 18 for detecting a 
trailing edge having a slope whose absolute value also exceeds the 
predefined value. In this embodiment, means 17 comprises a unit 19 for 
decreasing the video signal level by .DELTA.V, giving signal A, and a unit 
20 for delaying the video signal by .DELTA.T.sub.1, giving signal B. A 
comparator 21 fed with the output voltages of units 19 and 20 establishes 
whether and, if so, for how long the leading edge of signal A extends 
above the delayed signal B (see shaded part in FIG. 3A) and hence, whether 
the leading edge of a target echo meets the slope condition in order to be 
blanked. Similarly, it can be established whether the trailing edge meets 
the slope condition in order to be blanked. Means 18, therefore, 
comprises, in addition to unit 20, also a unit 22, connected to unit 20, 
for delaying the applied video signal by a time .DELTA.T.sub.1 and for 
decreasing the level of this signal by .DELTA.V, giving signal E. The 
directly applied video signal G and signal E are applied to a second 
comparator 23 to establish whether and, if so, for how long the trailing 
edge of signal E extends above the directly applied signal G (see FIG. 
3B), and hence whether the trailing edge of a target echo meets the slope 
condition in order to be blanked. The output signals of the two 
comparators 21 and 23 are combined in an EXCLUSIVE-NOR circuit 24 (see 
FIG. 3C) forming a control signal H for blocking circuit 15 to blank the 
video signal during the period the slope of the video voltage exceeds a 
certain value. The normal value of the slope is dependent upon the applied 
values .DELTA.V and .DELTA.T.sub.1. 
From FIGS. 3A and 3C it can be seen that control signal H of circuit 24 
appears a time .DELTA.T.sub.2 (where .DELTA.T.sub.2 &lt;.DELTA.T.sub.1) later 
than the start of the rising edge of the video signal G; a similar 
situation occurs with the falling edge of the video signal. For this 
reason a video signal obtained from gate circuit 6 is delayed by time 
.DELTA.T.sub.2 before it is applied to blocking circuit 15. The time 
.DELTA.T.sub.2 can be determined empirically. 
During the generation of the transmitter pulse, peak detectors 10 and 12 
are switched off and the FET switch F in the low-pass filter 5 is closed, 
permitting a faster adaptation of the filter-produced threshold voltage to 
the strong video signal directly after the transmitter pulse. The required 
signals t.sub.1 and t.sub.2, are derived from a timing unit 25 controlled 
by a synchronization signal S.