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Claims  |
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What is claimed is:
1. A distortion generating circuit comprising:
a linear attenuator which has an input impedance substantially coincident
with an impedance of a transmission path for an input signal and an output
impedance substantially coincident with an impedance of a transmission
path for an output signal;
at least one diode which is connected in parallel with the linear
attenuator and which generates a nonlinear distortion component for said
input signal; and
an adder adding output from said linear attenuator and output from said
diode.
2. A distortion generating circuit according to claim 1, further including
a resistor connected with a corresponding one of said at least one diode,
said resistor and said corresponding diode being connected in parallel
with said linear attenuator.
3. A distortion generating circuit according to claim 1, wherein said at
least one diode comprises at least one pair of diodes connected in
mutually opposite polarities with respect to said input signal into said
linear attenuator, said diodes generating odd-order distortion.
4. A distortion generating circuit according to claim 1, wherein said at
least one diode generates even-order distortion substantially of
second-order or higher-order for said input signal of said linear
attenuator otherwise having an even-order distortion after attenuation.
5. A distortion generating circuit according to claim 1, wherein said
linear attenuator is arranged to comprise a plurality of resistors.
6. A distortion generating circuit according to claim 4, wherein said
linear attenuator is a .pi. resistor attenuator.
7. A distortion generating circuit according to claim 4, wherein said
linear attenuator is a T resistor attenuator.
8. A distortion generating circuit according to claim 4, wherein said
linear attenuator includes resistors, some of said resistors in said
linear attenuator being used in a bias circuit for said diode.
9. A distortion generating circuit according to claim 2, wherein said at
least one diode includes two diodes arranged as a pair, bias points for
two of said paired diodes being independently adjustable.
10. A distortion generating circuit according to claim 2, wherein said at
least one diode includes two diodes arranged in a pair and connected in a
direct current manner in series with a bias power supply thereof, bias
points of the diodes being adjustable.
11. A distortion generating circuit according to claim 1, wherein at least
one of said diodes is a pin diode.
12. A distortion generating circuit according to claim 1, wherein at least
one of said diodes is a Schottky diode.
13. A distortion generating circuit according to claim 1, further
comprising a capacitor connected in parallel with said at least one diode.
14. A distortion generating circuit according to claim 1, wherein said at
least one diode includes multiple diodes which are connected in parallel
to each other with respect to the input signal.
15. A distortion generating circuit according to claim 1, wherein said at
least one diode includes multiple diodes which comprise a pin diode and a
Schottky diode connected in a same polarity with respect to the input
signal of said linear attenuator.
16. An optical transmitter comprising:
a distortion generating circuit generating a distortion generating circuit
output by providing an input signal with nonlinear distortion, said
distortion generating circuit comprising:
a linear attenuator which has an input impedance substantially coincident
with an impedance of a transmission path for an input signal and an output
impedance substantially coincident with an impedance of a transmission
path for said distortion generating circuit output,
at least one diode which is connected in parallel with the linear
attenuator and which generates a nonlinear distortion component for said
input signal, and
an adder for adding output from said linear attenuator and said diode; and
a light emitting element driven by said distortion generating circuit
output.
17. An optical transmitter according to claim 16, wherein said at least one
diode comprises:
a first distortion generating circuit including at least one pair of diodes
connected in mutually opposite polarities with respect to said input
signal of said linear attenuator, said first distortion generating circuit
generating a resultant signal by providing said input signal with
odd-order distortion; and
a second distortion generating circuit receiving said resultant signal
output from said first distortion generating circuit, said second
distortion generating circuit generating even-order distortion
substantially of second-order or higher-order for said input signal of
said linear attenuator when said input signal would otherwise have an
even-order distortion after attenuation, wherein
said light emitting element is driven by the signal output from said second
distortion generating circuit.
18. An optical transmitter according to claim 16, wherein said light
emitting element is a semiconductor laser diode.
19. An optical receiver comprising:
a light receiving element receiving light and generating an electric signal
based on said received light; and
a distortion generating circuit receiving said electric signal output from
said light receiving element and generating a distortion generating
circuit output by providing said electric signal with nonlinear
distortion, said distortion generating circuit comprising:
a linear attenuator which has an input impedance substantially coincident
with an impedance of a transmission path for said received electric signal
and an output impedance substantially coincident with an impedance of a
transmission path for said distortion generating circuit output,
at least one diode which is connected in parallel with the linear
attenuator and which generates a nonlinear distortion component for said
received electric signal, and
an adder for adding output from said linear attenuator and said diode.
20. An optical receiver according to claim 19, comprising:
a light receiving element receiving light and generating an electric signal
based on said received light;
a first distortion generating circuit receiving said electric signal output
from said light receiving element and generating a first distortion
generating circuit output by providing said received electric signal with
odd-order distortion, said first distortion generating circuit comprising
at least one pair of diodes that are connected in mutually opposite
polarities with respect to said received electric signal, said diodes
generating said odd-order distortion; and
a second distortion generating circuit receiving said first distortion
generating circuit output and providing the signal with even-order
distortion, said second distortion generating circuit generating said
even-order distortion substantially of second-order or higher-order for
said electric signal when said electric signal would otherwise have an
even-order distortion after attenuation.
21. An optical transmitter for outputting modulated light from unmodulated
input light, comprising:
a distortion generating circuit receiving a modulation signal and
generating a distortion generating circuit output by providing said
modulation signal with nonlinear distortion, said distortion generating
circuit comprising:
a linear attenuator which has an input impedance substantially coincident
with an impedance of a transmission path for said received modulation
signal and an output impedance substantially coincident with an impedance
of a transmission path for said distortion generating circuit output,
at least one diode which is connected in parallel with the linear
attenuator and which generates a nonlinear distortion component for said
received modulation signal, and
an adder for adding output from said linear attenuator and said diode; and
an optical modulator for generating modulated light based on said
unmodulated input light and said distortion generating circuit output.
22. An optical transmitter according to claim 21, wherein said at least one
diode comprises:
a first distortion generating circuit including at least one pair of diodes
connected in mutually opposite polarities with respect to said modulation
signal received by said linear attenuator, said first distortion
generating circuit generating a resultant signal by providing said
received modulation signal with odd-order distortion; and
a second distortion generating circuit receiving said resultant signal
output from said first distortion generating circuit and providing the
signal with even-order distortion, said second distortion generating
circuit generating said even-order distortion substantially of
second-order or higher-order for said modulation signal received by said
linear attenuator when said modulation signal would otherwise have
even-order distortion after attenuation, wherein
said optical modulator generates modulated light based on output from said
second distortion generating circuit.
23. A low-distortion amplifier comprising:
a distortion generating circuit generating a distortion generating circuit
output by providing an input signal with nonlinear distortion, said
distortion generating circuit comprising:
a linear attenuator which has an input impedance substantially coincident
with an impedance of a transmission path for an input signal and an output
impedance substantially coincident with an impedance of a transmission
path for said distortion generating circuit output,
at least one diode which is connected in parallel with the linear
attenuator and which generates a nonlinear distortion component for said
input signal, and
an adder for adding output from said linear attenuator and said diode; and
an amplifier for amplifying said distortion generating circuit output.
24. A low-distortion amplifier according to claim 23, comprising:
a first distortion generating circuit including at least one pair of diodes
connected in mutually opposite polarities with respect to said input
signal of said linear attenuator, said first distortion generating circuit
generating a resultant signal by providing said input signal with
odd-order distortion; and
a second distortion generating circuit receiving said resultant signal
output from said first distortion generating circuit, said second
distortion generating circuit generating even-order distortion
substantially of second-order or higher-order for said input signal of
said linear attenuator when said input signal would otherwise have an
even-order distortion after attenuation, wherein
said amplifier amplifying output from said second distortion generating
circuit. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion generating circuit which
provides distortion for canceling nonlinearity of input/output
characteristics of a high-frequency signal output stage (for example a
nonlinear device such as a nonlinear amplifier as an output stage in
transmitter or a semiconductor laser) to linearize an analog signal output
therefrom. More particularly, the invention relates to a distortion
generating circuit suitable for multiple analog image transmission such as
CATV.
2. Related Background Art
Conventionally, negative feedback is used to improve the linearity of an
amplifier in the low frequency region. However, conventional systems do
not normally use the negative feedback to improve the linearity of the
amplifier in the high frequency region because of rotation of phase caused
by a signal delay. Thus, when high linearity is required in a broad
region, the linearity is normally improved using a circuit having
input/output characteristics which cancel the nonlinearity (i.e., a
distortion generating circuit). A distortion generating circuit is
provided before an output stage or a semiconductor laser of a transmitter.
It has a nonlinear element such as a diode or a transistor. Based on the
circuit's nonlinearity, distortion is generated, that distortion being
used to linearize an object circuit having nonlinearity. Specific examples
are described for example in Japanese Utility Model Publication No.
2-30192 and ELECTRONICS LETTERS Vol. 28 No. 20, 1992, pp 1875-1876. The
circuits as described in these references are arranged with two diodes, as
nonlinear elements, which are connected opposite to each other with
respect to a signal and which are operated as nonlinear elements at a
predetermined bias point.
The multiple analog image transmission such as the CATV requires transfer
characteristics of very low level distortion to obtain a high quality
image. The distortion correcting circuit in multiple analog image
transmission needs to have the following characteristics: (1) Independent
correction of second order and third-order distortions components, which
could cause a problem in analog image transmission; and (2) correction of
the distortion while also compensating a frequency dependency of a
nonlinear device, because wide band transmission is required. There are
distortion correcting circuits developed for correcting the nonlinearity
of a transmission device, which could be a cause to produce distortion in
analog signal. For example, U.S. Pat. No. 4,992,754, U.S. Pat. No.
5,132,639 and U.S. Pat. No. 5,119,392 describe such circuits.
FIG. 1 diagrammatically shows a circuit as described in U.S. Pat. No.
4,992,754 and U.S. Pat. No. 5,132,639. This circuit is constructed in a
so-called parallel circuit arrangement, where a signal to be supplied to a
circuit 901 is split by a distributer 910 into two paths. A nonlinear
device 915 is provided in one path, and a delay circuit (delay line) 925
is provided in the other path. Furthermore, two signals are combined in a
coupler 911. This circuit has such an advantage that distortion components
can have frequency characteristics without influencing a base signal and
the frequency dependency of distortion of the nonlinear device can also be
compensated.
SUMMARY OF THE INVENTION
A distortion generating circuit of the present invention is a distortion
generating circuit for canceling nonlinearity of input/output
characteristics of a high-frequency signal output stage. It is provided
with a linear attenuator having matched input and output impedances and at
least one diode connected in parallel with the linear attenuator. The
diode provides a component in the input signal input to the linear
attenuator with nonlinear distortion. A sum of an output from the linear
attenuator and an output from the diode is then output.
The at least one diode comprises at least a pair of diodes connected in
mutually opposite polarities with respect to the input signal into the
linear attenuator. The distortion generating circuit can supply an output
obtained by giving an odd-order distortion component to the input signal.
It is here preferred that the paired diodes are adjustable in bias point
independently of each other, or that they are connected in series with a
bias power supply and are adjustable in bias point thereof at a time.
Also, the at least one diode may comprise one or more diodes for generating
an even-order, substantially second-order or higher-order, distortion
component for the input signal into the linear attenuator. The distortion
generating circuit can supply an output obtained by giving the odd-order
distortion to the input signal.
It is practical that the linear attenuator is constructed as to comprise a
plurality of resistors. Part of the resistors in the linear attenuator can
be used as part of bias circuits for the first and second diodes.
The bias points of the diodes may be independently adjustable or they may
be simultaneously connected in a DC manner in series to the bias power
source and adjustable
An optical transmitter may be constructed of a distortion generating
circuit of the present invention for giving a nonlinear distortion
component to an input signal, and a light emitting element driven by an
output signal from the distortion generating circuit. The transmitter may
be so arranged as to have a first distortion generating circuit of the
present invention supplying an output obtained by giving an odd-order
distortion component to an input signal, a second distortion generating
circuit of the present invention for receiving a signal output from the
first distortion generating circuit and supplying an output obtained by
giving an even-order distortion component to the signal, and a light
emitting element driven by the signal output from the second distortion
generating circuit.
An optical transmitter may be constructed of a distortion generating
circuit of the present invention for receiving a modulation signal and
providing the modulation signal with nonlinear distortion to output a
resultant signal, and an optical modulator for modulating input light by
the signal output from the distortion generating circuit to output
modulated light. An optical transmitter can be so arranged as to have a
first distortion generating circuit of the present invention for receiving
a modulation signal and providing the modulation signal with odd-order
distortion to output a resultant signal, a second distortion generating
circuit of the present invention for receiving the signal output from said
first distortion generating circuit and providing the signal with
even-order distortion to output a resultant signal, and an optical
modulator for modulating input light by the signal output from the second
distortion generating circuit to output modulated light.
An optical receiver may be constructed of a light receiving element for
receiving light to convert it into an electric signal, and a distortion
generating circuit of the present invention for receiving the electric
signal output from the light receiving element to give a nonlinear
distortion component thereto. Here, it can be so arranged as to have a
light receiving element for receiving light to convert it into an electric
signal, a first distortion generating circuit of the present invention for
receiving the electric signal output from the light receiving element and
supplying an output obtained by giving an odd-order distortion component
thereto, and a second distortion generating circuit of the present
invention for receiving a signal output from the first distortion
generating circuit and supplying an output obtained by giving an
even-order distortion component to the signal.
A low-distortion amplifier may be constructed of a distortion generating
circuit of the present invention for giving a nonlinear distortion
component to an input signal, and an amplifier for receiving a signal
output from the distortion generating circuit and amplifying it. Here, it
can be so arranged as to have a first distortion generating circuit of the
present invention for supplying an output obtained by giving an odd-order
distortion component to an input signal, a second distortion generating
circuit of the present invention for receiving a signal output from the
first distortion generating circuit and supplying an output obtained by
giving an even-order distortion component thereto, and an amplifier for
receiving a signal output from the second distortion generating circuit to
amplify it.
In the distortion generating circuit of the present invention, an input
signal from the outside is split into two signals, which are supplied to
the linear attenuator and to the diode, respectively. A signal passing
through the diode is given a distortion component according to a bias
point. The signal with distortion passing through the diode is added to
the signal passing through the linear attenuator. The sum signal is output
from the circuit. After the split input signal is let to pass through the
linear attenuator, it is added to the signal from the diode and the sum
signal is output, as described. This arrangement can prevent unnecessary
intermodulation (cross modulation) distortion from appearing in the diode
with input of large amplitude.
If the first and second diodes are connected in opposite polarities with
respect to the input signal, they have nonlinear characteristics depending
upon a bias point thereof, which are equivalently opposite in polarity to
each other with respect to the input signal. Signals passing through the
diodes are provided with respective distortion components depending upon
the respective bias points of the first and second diodes, so that output
signals have distortion components depending upon the respective nonlinear
characteristics of the first and second diodes, which permits flexible
distortion correction.
In case the linear attenuator is one composed of resistors, it is very
excellent in return loss and frequency characteristics, and good impedance
matching can be effected in a broad band. In case part of the resistors is
arranged as part of the bias circuits for the first and second diodes, the
circuit can be more simplified.
Also, if the bias points of diodes are adjustable independently of each
other, the nonlinear characteristics of the first and second diodes can be
independently adjusted. Then, the nonlinear characteristics of the diodes
can be adjusted to be symmetric or asymmetric with respect to an input
signal, which enhances the flexibility for provision of distortion. If
they can be adjusted at the same time, the adjustment can be simpler.
In case at least one of the diodes is a pin diode, the minority carrier
contributes to the operation of a junction diode (especially of the pin
diode) used for band switch the like, whereby phase distortion becomes
dominant in the generated distortion. Then, the high-frequency-range
distortion becomes greater than the low-frequency-range distortion, so
that the present distortion correcting circuit can correct distortion with
frequency dependency.
If the circuit further has a capacitor connected in parallel with the
diode, a part of the signal passing through the diode goes into the
capacitor, through which high-frequency components pass. This can suppress
high-frequency-range distortion, giving more flexibility for correction of
distortion with frequency dependency.
If the diodes comprise a pin diode and a Schottky diode, the distortion is
uniform in a broad band, because the majority carrier is dominant in the
operation of Schottky barrier diode. Therefore, the correction of
distortion with frequency dependency can be effected with more flexibility
by using both the pin diode having large distortion in the high frequency
region and the Schottky barrier diode having uniform distortion in a wide
band.
Also, in case the distortion generating circuit of the present invention is
used in combination with a light emitting element, a light receiving
element, an optical modulator, or an amplifier, an output signal can be
produced while reducing distortion generated by an input signal path or an
employed active element in an optical transmitter, an optical receiver or
a low-distortion amplifier.
The distortion generating circuit of the present invention can well provide
desired distortion, because a signal of input with large amplitude is
distributed to the attenuator and to the nonlinear element to prevent
unnecessary cross modulation distortion from occurring. Especially, it can
be achieved in a simple circuit arrangement, so that an excellent
operation can be assured.
Also, a signal with reduced distortion can be attained by employing the
distortion generating circuit of the present invention in an arrangement
of optical transmitter, optical receiver or amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a scheme of a conventional example;
FIG. 2 is a circuit diagram to show an example of arrangement of a
distortion generating circuit in a first embodiment;
FIG. 3 is a circuit diagram to show an example of the arrangement of
distortion generating circuit in the first embodiment;
FIG. 4 is a diagrammatic illustration to show an operation of the
distortion generating circuit in the first embodiment;
FIG. 5 is a drawing to show an example of arrangement of measuring system
for distortion characteristics of the distortion generating circuit in the
first embodiment;
FIG. 6 is a drawing to show an example of third-order distortion
measurement;
FIG. 7 is a circuit diagram to show an example of arrangement of distortion
generating circuit in the first embodiment;
FIG. 8 is a circuit diagram to show an example of arrangement of distortion
generating circuit in a second embodiment;
FIG. 9 is a drawing to show frequency characteristics of the distortion
generating circuit shown in FIG. 8;
FIG. 10 is a drawing to show frequency characteristics of the distortion
generating circuit shown in FIG. 8;
FIGS. 11A, 11B and 11C are circuit illustrations to show examples of
arrangement of distortion generating circuit in the second embodiment;
FIG. 12 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 13 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 14 is a drawing to show current dependency of CSO of the distortion
generating circuit shown in FIG. 12;.
FIG. 15 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 16 is a drawing to show current dependency of CSO of the distortion
generating circuit shown in FIG. 15;
FIG. 17 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 18 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 19 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIGS. 20, 21 and 22 are drawings to show current dependency of CSO, CTB,
and XM of the distortion generating circuit shown in FIG. 18;
FIG. 23 is a concrete circuit diagram of the distortion generating circuit
shown in FIG. 12;
FIG. 24 and FIG. 25 are drawings to show current dependency of CSO, CTB,
and XM of the distortion generating circuit shown in FIG. 23;
FIG. 26 is a circuit diagram to show an example of arrangement of
distortion generating circuit in the second embodiment;
FIG. 27 is a drawing to show CSO of the distortion generating circuit shown
in FIG. 26;
FIG. 28 is a drawing to show an example of arrangement of CATV system using
the distortion generating circuit of the present invention;
FIG. 29 is a drawing to show an example of arrangement of optical
transmitter using the distortion generating circuit of the present
invention;
FIG. 30 is a drawing to show an example of arrangement of optical receiver
using the distortion generating circuit of the present invention;
FIG. 31 is a drawing to show an example of arrangement of optical
transmitter using the distortion generating circuit of the present
invention; and
FIG. 32 is a drawing to show an example of arrangement of low-distortion
amplifier using the distortion generating circuit of the present
invention.
FIG. 33 is a circuit diagram to show an example of arrangement of
distortion generating circuit of the present invention;
FIG. 34 is a circuit diagram to show an example of arrangement of
distortion generating circuit of the present invention;
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The first embodiment of the present invention is first described with
reference to the accompanying drawings. FIG. 2 shows an example of a
distortion generating circuit in the first embodiment. This distortion
generating circuit is characterized in that two diodes are interposed to
produce distortion, in opposite polarities to each other and in parallel
to a resistor attenuator which is superior in return loss and frequency
characteristics and which is matched in impedances of input and output.
The attenuator is a .pi. attenuator composed of resistors R30, R32, R10 and
R20, which is well matched in impedances as being input impedance Zi and
output impedance Zo both of 75.OMEGA. and which is nearly constant in
attenuation factor and in input/output impedances in a broad frequency
region.
A diode D11 is connected to the attenuator in parallel and in an AC manner
with respect to an input signal from an input terminal IN, a bias circuit
for diode D11 is composed of resistors R10, R12 and R30, and a bias point
is adjusted by a bias voltage +B. The diode is biased in a DC manner at a
point where the current-voltage characteristics show nonlinearity, and a
distortion component according to the bias point is given to a signal
passing through the diode D11. Another diode D21 is connected to the
attenuator in parallel and in an AC manner with respect to the input
signal, and is connected to the diode D11 in an opposite polarity to that
of the diode D11. A bias circuit for diode D21 is composed of resistors
R20, R22 and R30 and a bias point thereof is adjusted by a bias voltage
-B. The resistors R10, R20, R30 also function as part of the attenuator,
whereby the circuit is simplified. Incidentally, both bias currents of the
diodes D11, D21 flow through-the resistor R30. However, mutual influence
can be negligible, because the resistance is low and the bias currents are
small.
Capacitors C11, C21, C31, C32, C12, C22 cut a direct current but let an AC
component pass therethrough. Signals passing through the diode D11, the
diode D21 and the attenuator are combined with each other and a combined
signal is output from an output terminal OUT to a next-stage circuit.
FIG. 3 shows an example of arrangement in which only a single power supply
is provided as a bias power source to the diodes D11, D21. The arrangement
of FIG. 2 has such an advantage that the diodes D11, D21 can be adjusted
in bias point independently of each other. In contrast, an arrangement of
FIG. 3 has such an advantage that if the diodes D11, D21 are matched in
characteristics the bias points thereof can be adjusted at a balanced
state to ensure operation at a same bias point. The two arrangements are
nearly identical except for the above point, both being arranged in a
suitable manner for miniaturization.
FIG. 4 is an equivalent circuit to summarize the operation of the
distortion generating circuits as shown in FIG. 2 and in FIG. 3. The input
signal into the input terminal IN is distributed to the diode D11, the
diode D21 and the attenuator 110. The diodes D11, D21 provide the signals
with respective distortion components depending upon the respective bias
points. These diodes D11, D21 perform a so-called push-pull operation,
because they are connected as opposite in polarity to each other. The
signals passing through the diodes are added to the signal passing through
the attenuator 110, and a sum signal is supplied to a next-stage circuit.
In FIG. 4, (a) shows a diagrammatic sketch of current-voltage
characteristics on the diode D11 side, higher-order distortion components
of that diode of differing relative to the bias point of the diode. The
same is the case on the diode D21 side, but the transfer characteristic is
different because of the opposite polarity in connection, at sketched as
(b), clearly showing the difference. The signals from the diodes D11, D21
and the attenuator 110 are added to each other and a sum signal is output.
This means that the characteristics of (a) and (b) in FIG. 4 are added to
the linear characteristic of attenuator 110. Thus, the input/output
characteristics of the distortion generating circuit in FIG. 2 can be
diagrammatically shown by the characteristic of (c) in FIG. 4. Since the
diodes each have a relatively large impedance and are arranged with the
resistors R12, R22, respectively, the majority of base wave components are
those passing through the attenuator 110. As described, the higher-order
distortion components are given to the input signal and the distortion
components may vary depending upon a change in bias points of the diodes.
In case of the arrangement in FIG. 2, the diodes D11, D21 are arranged to
be adjusted in bias point independently of each other, so that the
characteristics of (a) and (b) in FIG. 4 can be different from each other
depending upon a difference of bias point. Thus, the characteristic of (c)
in FIG. 4 will change depending upon the bias points of the diodes D11,
D21. In case of the arrangement in FIG. 3, assuming the properties of
diodes D11, D21 are identical, the characteristics of (a) and (b) in FIG.
4 are symmetric with each other. The characteristic of (c) in FIG. 4
changes the shape with a change in bias point but is kept in point
symmetry. Namely, the arrangement of FIG. 3 is effective to give only an
odd-order component.
FIG. 5 shows a measurement system for measuring a third-order distortion
component for a circuit 101 equivalent in structure to the circuit in FIG.
3. The measurement system is so arranged that a multi-channel signal
generator (MSG) 932 supplies a signal of 80 ch and 38 dBmV, the circuit
101 gives distortion to the signal, the signal passes through an
attenuator 935 and a band pass filter 936, and intermodulation (cross
modulation) distortion is measured by a spectrum analyzer 938. The
attenuator 935 is used to suppress a distortion due to reflection from the
band pass filter so as to keep it constant at 10 dB (the band pass filter
totally reflects components other than those in the passing region). FIG.
6 shows a measurement result of third-order distortion component at 547.25
MHz, from which it is clear that the intermodulation third-order
distortion CTB changes with adjustment of bias point of the distortion
generating circuit.
As described above, the diodes of opposite polarities are interposed in
parallel with the attenuator 110 and the majority of fundamental wave
components are those passing through the attenuator 110, so that the
frequency characteristics of the nonlinear attenuator are mainly
determined by the attenuator 110. Since the attenuator 110 is composed of
the resistors, it can have flat frequency characteristics in a broad
region and can be excellent in impedance matching. Thus, the return loss
can be made small and the characteristics of the distortion generating
circuit can be greatly improved. Also, an insertion loss is not influenced
greatly by the bias points of diodes D11, D21, but is determined by an
attenuation amount of the attenuator 110. Thus, the attenuation amount and
the input and output impedances of attenuator 110 can be determined
easily.
Further, with large amplitude input, unnecessary harmonic distortion can be
prevented from being generated, because only a part of input signal passes
through the diodes and the resistors R12, R22. Also, the attenuation
amount of the attenuator 110 can be adjusted to match with an amount of
the generated distortion and the amplitude of signal to be driven. In this
case, constants of the attenuator 110 can be easily determined depending
upon these conditions, so that the flexibility and the degree of freedom
of circuit designing are both very high.
The above circuit arrangements can improve the characteristics in respect
of the insertion loss, the frequency characteristics and a large signal
operation, which have been desired to be improved for the conventional
circuits. Especially, a high-frequency output can be obtained with high
power and linearity in communications requiring a broad band, for example
in the frequency multiplexing communication or CATV, enabling
long-distance transmission with less relays.
Various modifications can be possible for the first embodiment.
For example, another circuit arrangement may be as shown in FIG. 7 with the
basic operation shown in FIG. 4, so that the attenuator 110 may be not
only of the .pi. type as shown in FIG. 2 or 3 but also of a T type. In
FIG. 7, L11, L12 denote choke coils for blocking a signal. Also, these may
be so arranged to adjust the bias points at the same time as shown in FIG.
3. The attenuator is constructed of resistors in order to make the
frequency characteristics flat in this modification, but an attenuator may
be constructed of a combination of capacitors and coils to have a
frequency dependency in another modification.
Next described are the second embodiment of the present invention and
modifications thereof.
FIG. 8 shows a most fundamental constitutional example of the second
embodiment according to the present invention. This circuit in FIG. 8 is
characterized in that the circuit is composed of an attenuator composed of
resistors R71, R72, R73, and a diode D71 (e.g., pin diode) connected in an
AC manner in parallel to the attenuator with respect to an input signal.
Capacitors C70, C71, C72 are provided for blocking a direct current and
are adjusted to have a sufficiently small impedance to an input signal. A
resistor R74 is provided for adjusting an operating point (intercept
point) of diode D71 to an input signal, and the intercept point is set by
a bias from the outside. The circuit generates distortion of desired
amplitude to effect distortion correction (wherein the resistors R71, R73
serve as a bias circuit for the diode D71).
Supposing, as a specific example of constants of this circuit, the diode
D71 is 1SS241 (manufactured by TOSHIBA), the resistors R71, R73 each have
a resistance of 620.OMEGA., the resistor R72 a resistance of 18.OMEGA.,
the resistor R74 a resistance of 47.OMEGA., and the capacitors C70, C71,
C72 each a capacitance of 0.01 .mu.F, a resultant distortion correcting
circuit can be constructed with input/output impedance Z0 of 75.OMEGA..
FIG. 9 and FIG. 10 show frequency characteristics of the distortion
correcting circuit in this arrangement. FIGS. 11A-11C show examples of
arrangement of distortion generating circuit using a K attenuator or a T
attenuator. In FIGS. 11A-11C, Z.sub.0 is a characteristic impedance and A
is an attenuation factor. In FIG. 11A, R.sub.1 =Z.sub.0 *(1-A)/(1+A) and
R.sub.2 =Z.sub.0 *(1-A.sup.2)/2A. In FIG. 11B, R.sub.1 =Z.sub.0
*(1+A)/(1-A) and R.sub.2 Z.sub.0 *2A/(1-A.sup.2). In FIG. 11C, R=Z.sub.0,
R.sub.1 =Z.sub.0 *(1-A)/(A) and R.sub.2 =Z.sub.0 *A/(1-A).
The distortion generated by this circuit can be determined by a level of
input signal, the bias and the resistor R74. Therefore, there could be a
case in which a proper intercept point cannot be adjusted in case of a
signal level being high. In that case, the adjustment can be made possible
by connecting two diodes D71, D72 in series as shown in FIG. 12. As the
level of input signal becomes higher, more diodes should be connected in
series. Also, if the level of input signal is low, the adjustment can be
possible by connecting diodes D71, D72 in parallel as shown in FIG. 13. As
the level of input signal becomes lower, more diodes should be connected
in parallel.
Also, the distortion correcting circuit using only one diode as shown in
FIG. 8 has such advantages that it is cheap, excellent in operation
stability, and simple in circuit arrangement. However, since the nonlinear
characteristics depend upon the V-I characteristics of diode, a
third-order distortion component depending upon the V-I characteristics
will be simultaneously produced with correction of a second-order
distortion component, which could cause a case in which a large
second-order distortion component cannot be corrected. Specifically, in
case nonlinear distortion of a semiconductor laser which has a larger
second-order distortion component than a third-order distortion component
is corrected, the third-order distortion could be degraded with correction
of the second-order distortion, thus limiting a correctable amount of
second-order distortion.
This case can be dealt with by the arrangement as shown in FIG. 13 in which
same diodes are connected in parallel so that each diode produces a
not-too-large distortion component. Thus, generation of unnecessary
third-order distortion can be prevented by decreasing an amount of
second-order distortion generated in each diode. In more detail, an amount
of third-order distortion is decreased by 2 dB if an amount of
second-order distortion generated from the general properties of amplifier
or the like is decreased by 1 dB. In this arrangement, the second-order
distortion generated by the circuit in FIG. 13 is a sum of those of the
two diodes, which makes the third-order distortion relatively smaller
while obtaining a necessary amount of second-order distortion.
FIG. 14 illustrates the diode current dependency of CSO (ISS241) by showing
second-order distortion (CSO) produced with a change in bias for a
relatively large level of input signal of 35 dBmV. The measurement was
conducted using the circuit as shown in FIG. 12. A resistance of the
resistor R74 was 75.OMEGA. (and the other constants are the same as those
in FIG. 8). The input signal was a signal of 80 ch in the range of 55.25
MHz to 547 MHz similarly as in the measurement in FIG. 5. Increasing the
bias current through the diodes from 0 .mu.A, the level of second-order
distortion was measured (in dBc) for maximum frequency of 547.25 MHz
(upper line designating data points with boxes) and for minimum frequency
of 55.25 MHz (lower curve designating data points with circles). The
measured input RF level was 35 dBmV for 80 channels. As apparent from this
result, the generated distortion becomes larger with an increase in
frequency.
A reason why the distortion is generated depending upon the frequency is
that in case of a junction diode used in band switch, especially in case
of a pin diode, the minority carrier contributes to the operation thereof
and therefore the phase distortion becomes dominant, whereby
high-frequency-range distortion becomes larger than low-frequency-range
distortion. Since amplifiers or semiconductor lasers generally have larger
distortion in the high frequency range, the distortion correcting circuits
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