Modulators – Pulse or interrupted continuous wave modulator
Reexamination Certificate
2001-11-05
2003-06-24
Pascal, Robert (Department: 2817)
Modulators
Pulse or interrupted continuous wave modulator
C332S111000
Reexamination Certificate
active
06583678
ABSTRACT:
This application claims Paris Convention priority of DE 100 59 585.5 filed Nov. 30, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention concerns an electrical circuit for pulse modulation of a carrier signal. The circuit comprises a pulse generator for generating a current pulse and at least one diode switch which can be controlled by the current pulse. The invention also concerns a method for pulse modulation of a carrier signal by means of at least one diode switch. A current pulse is thereby generated which is used to control the at least one diode switch.
Circuits of the above-mentioned type are used e.g. for pulse modulation of a radio-frequency carrier signal which are used e.g. for distance radar systems in motor vehicles. The diode switch is triggered by the current pulse generated by the pulse generator and switches a radio-frequency (RF) signal for a very short time period (0.1 to 10 ns).
The function of a diode switch is explained in more detail below with reference to a conventional electric circuit shown in
FIG. 2. A
typical diode switch
9
comprises a signal line
1
and a lambda/4 stub lead which branches off therefrom. An oscillator
3
, which provides a radio-frequency carrier signal
4
, is disposed at the input of the signal line
1
. A control voltage source U is connected to the stub lead
2
and the control voltage U is applied to and removed from the stub lead
2
by closing and opening a switch
7
. Repeated opening and closing of the switch
7
pulse-modulates the carrier signal
4
and the pulse-modulated radio-frequency carrier signal
6
is available at an output
5
of the signal line
1
.
When a diode
8
, disposed at the end of the stub lead
2
, is conducting (switch
7
is closed), a signal can pass from the oscillator
3
to the output
5
of the diode switch
9
. When the diode blocks (switch
7
is open) no signal can pass from the oscillator
3
to the output
5
. The lambda/4 stub lead
2
, which is connected to the diode
8
, “inverts” the behavior of the diode
8
. During open-circuit operation (diode
8
blocks) and in case of a short-circuit (diode
8
is conducting), the signal in the stub lead
2
is reflected. This produces destructive overlapping during open-circuit operation (incoming and reflected signal are phase-shifted relative to one another by half a period) after the lambda/4 stub lead
2
. In case of a short-circuit, the signals overlap constructively after the lambda/4 stub lead
2
(incoming and reflected signal are not phase-shifted relative to one another).
Diode
8
is usually a PIN diode. A PIN diode permits switching times (shortest pulse which can be generated at the output
5
) of longer than 10 ns. The very small PIN diode resistance in the switched state is advantageous. To achieve considerably faster switching times, a Schottky diode can be used instead of the PIN diode. This permits switching times in the sub-ns range.
The considerably higher dynamic resistance of the Schottky diode has, however, disturbing effects. The dynamic resistance of the diode determines the insertion attenuation of the diode switch (i.e. the losses). The parasitic or stray quantities, in particular the capacitance, determined the maximum achievable isolation of the diode switch
9
. The minimum achievable performance loss of the diode switch
9
, i.e. with conducting diode
8
, is determined by the dynamic resistance of the diode
8
. The capacitance also permits small current flow to ground in the blocking state of the diode
8
. The isolation of the diode switch
9
can be doubled by using a second diode connected in parallel to the first diode
8
. However, this also doubles the minimum achievable power loss for the switched diode switch
9
.
The circuit shown in
FIG. 2
has the additional disadvantage that the envelope
6
a
of the pulse-modulated radio-frequency carrier signal
6
present at the output
5
of the diode switch
9
does not have a clean, square behavior. Rather, the dependence of the envelope is exponential (e
−t
). When using the known circuit e.g. in a distance radar system, the position resolution obtained can be higher the shorter the emitted pulses. The shorter pulses, however, also contain reduced signal energy which renders the signal particularly susceptible to disturbances. For this reason, it is important, in particular with very short pulses, that the emitted signal, i.e. the pulse-modulated signal
6
present at the output
5
of the diode switch
9
has a clean, preferably square envelope
6
a.
In view of these considerations, it is the object of the present invention to provide a circuit and a method for modulating a carrier signal with as short pulses as possible having a clean, square envelope.
SUMMARY OF THE INVENTION
To solve this object, the invention proposes, on the basis of the electrical circuit of the above-mentioned type, that the circuit has at least one chargeable capacitive element whose discharge current controls the at least one diode switch.
The capacitive element is formed e.g. as a capacitor. It can be one individual capacitor or a series connection, a parallel connection or a combination of series and parallel connections of several capacitors. The inventive electric circuit permits modulation of a carrier signal with very short pulses having a particularly clean, square envelope. Moreover, the inventive circuit can be realized at particularly low costs.
The at least one capacitive element is initially charged through a charging resistance. Thereafter, the at least one capacitive element can be discharged at any desired point in time. The discharge current thereby controls the at least one diode switch of the electric circuit. Rapid discharge of the capacitive element produces very short current pulses. A discharge current thereby flows only until the at least one capacitive element has been discharged. The charging resistance is thereby dimensioned such that after discharging, the or each capacitive element can nearly completely recharge prior to the next current pulse.
An advantageous further development of the present invention proposes that the circuit has several diode switches with stub leads connected in parallel. Each stub lead branch has one diode, to ensure that the discharge current flows in one direction only. The maximum achievable isolation of the diode switch can be considerably increased through parallel connection of several diodes.
In another advantageous further development of the present invention, the circuit comprises at least one semiconductor switch connected in series with the/each diode switch, wherein the charged capacitive element can be discharged by a control pulse via the/each semiconductor switch and the/each diode switch after controlling the at least one semiconductor switch. The at least one capacitive element is thus discharged by controlling a semiconductor switch with a control pulse. The control pulse merely serves for triggering the discharge process. It may even last longer than the current pulse. As soon as the capacitive element is discharged, the current pulse also terminates, irrespective of whether or not the control pulse is still present on the semiconductor switch.
The/each semiconductor switch is preferably formed from a transistor and, advantageously, from a field effect transistor (FET). An FET has nearly ohmic behavior in the switched state.
In a preferred embodiment of the present invention, the circuit comprises two semiconductors, wherein the control pulse can be applied to a control contact of one semiconductor switch and a variable direct voltage is applied to a control contact of the other semiconductor switch. Application of the control pulse to the one semiconductor switch initiates the discharging process of the capacitive element. The minimum resistance in the conducting state of the one semiconductor switch can be adjusted through the variable direct voltage which is applied to the other semiconductor switch. The larger the resistance
Chang Joseph
Pascal Robert
Valeo Schalter und Sensoren GmbH
Vincent Paul
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