Microwave pulse generator

Details Microwave pulse generator Microwave pulse generator

2002-11-21

2004-08-03

Mis, David (Department: 2817)

Oscillators

With synchronizing, triggering or pulsing circuits

C331S099000, C331S1170FE, C331S1170FE, C331S175000

Reexamination Certificate

active

06771139

ABSTRACT:

The invention pertains to a microwave pulse generator for generating microwave pulses in the nanosecond range according to the characteristics of the preamble of Claim
1
.
Such microwave pulse generators are customarily employed for precise distance measurement in radar systems, particularly pulse radar systems.
Such microwave pulse generators are described in, for instance, DE 197 02 261 C2. The microwave pulse generators there have the advantage that the circuitry expense is considerably reduced in comparison to other microwave pulse generators by generating pulses of a suitable duration that are provided for the voltage supply of a microwave oscillator. Moreover, the known arrangement does not require any expensive charge-coupled diodes. The pulse in the nanosecond range, which determines the duration of the actual microwave pulse, can be generated by a simple pulse-shortening stage.
In a refinement, the coupling of pulse-shortening stage and oscillator can be improved by a driver stage and/or a decoupling stage. Should the pulse-shortening stage be constructed such that it generates an inverted output signal, this can be compensated by an inverting driver stage. The decoupling stage can advantageously be implemented by a circular stub.
The microwave pulse generator from DE 197 02 261 C2 will be explained further on the basis of three figures. In
FIG. 1
, a pulse generator, of which the output signal is fed to a pulse-shortening stage
2
, is designated
1
. Pulse-shortening stage
2
generates pulses in the nanosecond range, which are fed to the input of a driver stage
3
. The signal amplified by driver stage
3
is fed to a decoupling network
4
, which is connected at its output end to the supply input of a microwave oscillator
5
. The output signal of microwave oscillator
5
can be picked off at an output terminal
24
.
The pulse generator supplies a pulse train with a predetermined pulse period. According to
FIG. 2
, downstream pulse-shortening stage
2
can have an input terminal
6
, which is connected via a resistor
7
to the base of an npn transistor
11
and via a resistor
8
to the base of an npn transistor
10
. A capacitor
9
between resistor
8
and the base of transistor
10
is connected to ground. The collector of transistor
10
is connected to the base of transistor
11
, and the emitter of transistor
10
is connected to ground. The emitter of transistor
11
is likewise connected to ground. The collector of transistor
11
forms the output circuit of the pulse-shortening stage and is coupled to the input circuit of downstream driver stage
3
. For this purpose, the collector is connected to supply voltage terminal
17
via series circuit consisting of three resistors
12
,
13
and
16
. The center tap of the series circuit of resistors
12
and
13
is connected to the base of pnp transistor
18
and the center tap of the series circuit of resistors
13
and
16
is connected to the emitter of transistor
18
. Its collector is connected to an output terminal
19
. Resistor
16
is connected to ground at both ends via decoupling capacitors
15
and
16
.
According to
FIG. 3
, the microwave oscillator has a supply terminal
20
. The latter is connected via a resistor
21
to a circular stub A and a &lgr;/4 line B. At its output end, &lgr;/4 line B is wired to output terminal
24
via a capacitor
23
and is connected to ground via the load path of a field-effect transistor
25
and a resistor
26
connected in series thereto. The gate terminal of field-effect transistor
25
is connected to ground via an inductor
27
.
Output
19
of driver stage
3
is connected to supply terminal
20
. A pulse train with a predetermined period is supplied to input terminal
6
. The incoming pulse from pulse generator
1
is shortened in pulse-shortening stage
2
to length t
p
. This is done in the embodiment of
FIG. 2
by virtue of the fact that the positive edge of the incoming pulse switches transistor
11
into the conductive state upon exceeding its base-emitter potential. Thereby, voltage divider
12
,
13
,
16
is powered and thus sufficient voltage is dropped across resistor
13
to switch transistor
18
into the conductive state. At the same time, the positive edge of the incoming pulse is delayed via RC element
8
,
9
by the time defined by it. By selecting the fast transistor appropriately, this delay time can be adjusted from fractions of a nanosecond to the length of the incoming pulse. After this delay time has elapsed, transistor
10
is switched to become conductive, so that the voltage at the base of transistor
11
is reduced to the saturation potential of transistor
10
. Transistor
11
thus returns to the high-ohmic state and thereby also blocks transistor
18
. Accordingly, a short pulse of length t
p
is available at output
19
and can moreover be loaded low-ohmically. Network
14
,
15
,
16
serves only to block the operating voltage that is applied to terminal
17
. Pulse-shortening stage
2
and driver stage
3
complement one another in the present example by each inverting the signal to be processed, whereby a noninverted signal can be picked off at output
19
.
The signal thus obtained, with a pulse duration corresponding to the duration of the microwave pulse, is furnished to the microwave oscillator via terminals
19
and
20
. The microwave oscillator consists of a transistor
25
, embodied in the present example as a gallium arsenide field-effect transistor. A suitable bipolar transistor could also be used, however. Furthermore, the inductor is embodied as an inductive TEM line segment. The resonant circuit of oscillator
5
is composed of this line segment
27
and the internal transistor capacitance between gate and drain for FETs, or base and collector for a bipolar transistor. Together with the transistor capacitance, line segment
27
constitutes a series resonant circuit that can be tuned via the length of line
27
. The phase condition for the start of oscillation is additionally fulfilled by this. Resistor
26
is required to reduce the Q of the resonant circuit so that a rapid starting of oscillation is guaranteed. A resistor
21
is inserted in the feed line between driver stage
3
and decoupling network
4
to limit the current through transistor
25
. Capacitor
23
serves to block the supply voltage and thus decouples the output signal of the oscillator.
Microwave oscillator
5
is designed such that it generates a CW signal upon application of a supply voltage to terminal
20
at the resonant frequency of the determining resonant circuit. In matching the line length of line segment
27
, care should be taken to consider the transformed component of the self-inductance of resistor
26
parallel to inductor
27
.
As already described, the supply of voltage to the microwave oscillator is accomplished by a pulse of length t
p
. In order to decouple the pulse-shortening stage and the downstream driver stage
3
, it is fed via the decoupling network consisting of a circular stub A and &lgr;/4 line B to microwave oscillator
5
.
To achieve a rapid oscillation onset and decay behavior, the source terminal must be connected to ground via a resistor
26
. This resistor
26
reduces the Q of the resonant circuit sufficiently that the oscillator has achieved its maximum amplitude after half the pulse length, that is, precisely at the maximum of the pulse amplitude. From there on, the pulse amplitude, and thus also the amplitude of the microwave oscillation, decreases until the pulse amplitude has again reached zero.
The coherence of the microwave oscillation is achieved because the pulse supplying oscillator
5
, roughly a nanosecond in duration, has a small rise time in the vicinity of 250 ps and thus already couples a spectral energy component at the resonant frequency into the oscillator. Thus the initial phase of the microwave signal is firmly shaped.
The microwave pulse is decoupled at output
24
of oscillator
5
via capacitor
23
. Here, however, the shortened pulse is superimposed on the microwave pulse, bu

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