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BACKGROUND OF THE INVENTION
This invention relates to an amplitude modulation circuit for a transmitter
in which overmodulation, as well as production of unnecessary spurious
signals, is suppressed by means of an automatic level control (ALC)
circuit.
In general, modulation circuits for transmitters are required to be capable
of modulating a carrier to a sufficient degree with minor distortion in
order to improve the signal to noise ratio (S/N ratio) and the
articulation of call. Above all, when used with a radio transmitter, the
modulation circuit must be minimally subject to production of spurious
signals which may interfere with other frequency bands. As a modulation
circuit in accordance with these requirements there has conventionally
been used the following composition. That is, an input signal for
modulation or audio-frequency signal (AF signal) for modulating the
modulation circuit is supplied to a modulation unit through a limiter
circuit or compression circuit. The limiter circuit or compression circuit
has a function to prevent automatically overmodulation exceeding a
modulation degree of 100%. That is, these circuits are so designed as to
augment the mean transmission output by restraining overmodulation while
securing sufficiently high mean modulation degree by supplying a
high-level input signal for modulation to the modulation unit, thereby
improving the S/N ratio.
The limiter circuit out of the aforesaid two circuits, however, has a
defect to cause distortion of the modulation input signal. On the other
hand, the compression circuit will hardly distort the input signal for
modulation. Transmitters (transceivers) employing such compression circuit
have already been proposed. Where a sufficiently high modulation degree
was required, these transmitters could not satisfactorily restrain the
production of spurious signals despite the use of such compression
circuit, let alone the use of the limiter circuit.
The reason may be explained as follows. That is, the modulation circuit as
an analog circuit is necessarily subject to non-linearity. Therefore, when
the modulation input signal is supplied to the modulation circuit, the
modulation circuit produces a harmonic distortion attributable to the
non-linearity. The modulated wave or transmitted radio wave involving the
harmonic distortion includes spurious components attributable to the
harmonic distortion, besides the frequency components of the carrier. That
is, the level of spurious signals is chiefly attributable to the
non-linearity of the modulation circuit used with the transmitter.
Generally, the higher the modulation degree as well as the frequency of
the modulation signal, the more remarkable the non-linearity is.
Therefore, in the transmitter provided with the prior art modulation
circuit, the modulation degree with respect to the whole range of the
frequency band of the AF signal used as the modulation input signal is
restricted to a somewhat lower level in order to restrain the production
of spurious signals. Thus, the prior art transmitter is so devised that
any spurious signals exceeding a level prescribed by the specifications of
the transmitter will not be caused outside the occupied band thereof if a
modulation input signal having many higher range frequency components
within the audio-frequency band is supplied to the modulation circuit.
Further, in the transmitter including the conventional modulation circuit,
a voltage controlled oscillator (VCO) in a frequency synthesizer circuit
to produce the carrier and a radio-frequency amplifying circuit to amplify
the carrier are securely shielded in order to avoid the modulation
distortion attributable to unfavorable connections among the circuits
within the transmitter.
The defects of the transmitter employing the prior art modulation circuit,
as described above, may be summarized as follows. Since the production of
spurious signals must be restrained, the performance primarily required
for a transmitter, i.e. high mean modulation degree, cannot be secured.
Moreover, in order to restrain the production of spurious signals, the
transmitter requires well-selected, high-performance circuit elements as
well as a number of components for shielding. Furthermore, in order to
make the most of the performance of the transmitter within the range of
modulation degree limited so as to bring the production of spurious
signals in compliance with the specifications, many adjusting processes
will be required for the transmitter. Accordingly, such type of
transmitter cannot help being highly expensive.
SUMMARY OF THE INVENTION
An object of this invention is to provide an amplitude modulation circuit
for a transmitter capable of reducing overmodulation, modulation
distortion, and production of spurious signals after securing a high mean
modulation degree.
In order to attain the above object, the modulation circuit according to
the invention comprises a gain controlled amplifier circuit for amplifying
an input audio-frequency signal to provide a modulation signal, the
amplification degree of the circuit being negative-feedback-controlled by
an automatic level control signal; a filter circuit for filtering the
high-pass frequency components out of the modulation signal so as
relatively to augment the amplitude of the higher range frequency
components of the modulation signal; a rectifier circuit for rectifying
the output signal of the filter circuit to provide a DC component
proportional to the amplitude of the modulation signal; an eliminator
circuit for eliminating ripples from the output signal of the rectifier
circuit to provide the automatic level control signal converted into a
direct current; a modulation unit for amplitude-modulating an input
carrier to produce a modulated radio-frequency signal for transmission,
the modulation degree of the unit being controlled by the modulation
signal; and a frequency synthesizer circuit for synthesizing a
radio-frequency signal equivalent to the transmission frequency to provide
the carrier.
In the transmitter with the above construction, uniform modulation
throughout the frequency band of the modulation signal may be achieved
when the modulation signal level is relatively low. Meanwhile, even though
the modulation signal level is raised, any overmodulation will not be
caused and a sufficiently high mean modulation degree will be provided if
the main amplitude components of the modulation signal are distributed in
the intermediate range of the frequency band. On the other hand, if the
main amplitude components of the modulation signal are distributed in the
higher range of the frequency band, then the upper limit of the modulation
degree will be restricted to a moderately lower level so that the level of
the unnecessary spurious signals as produced becomes lower than the level
prescribed by the specification. Thus, the transmitter provided with the
modulation circuit of the invention, subject to less modulation distortion
and production of spurious signals, may securely prevent overmodulation,
thereby producing radio waves with sufficiently high mean modulation
degree with respect to the intermediate range, i.e. the essential band of
the spectrum distribution of a human voice.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the fundamental composition of a
modulation circuit according to this invention;
FIG. 2 is a block diagram showing a further definite example of the
composition in accordance with the composition of FIG. 1;
FIG. 3 is a circuit diagram showing an embodiment of the modulation circuit
of the invention in accordance with the composition of FIG. 2;
FIG. 4 is a graph showing an example of the frequency response of a filter
circuit included in the circuit of FIG. 3;
FIGS. 5 and 6 are graphs showing the modulation degree characteristics of
the modulation circuit of the invention where the filter circuit with the
characteristics as shown in FIG. 4 is used;
FIGS. 7 to 15 show examples of the circuit applicable to the filter circuit
for the modulation circuit of the invention;
FIG. 16 is a graph showing an example of the frequency response where a
band-rejection filter is used for the filter circuit according to the
invention;
FIGS. 17 and 18 are graphs showing the modulation degree characteristics of
the modulation circuit of the invention where the filter circuit with the
characteristic as shown in FIG. 16 is used;
FIGS. 19 to 21 show examples of the circuit applicable to a rectifier
circuit for the modulation circuit of the invention;
FIGS. 22 to 27 show examples of the circuit applicable to a gain controlled
amplifier circuit for the modulation circuit of the invention;
FIGS. 28 to 30 are block diagrams showing alternative embodiments of the
modulation circuit according to the invention; and
FIG. 31 is a circuit diagram showing a further definite composition in
accordance with the composition of FIG. 30.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Illustrative embodiments of this invention will now be described with
reference to FIGS. 1 to 31, in which like reference numerals refer to the
same parts throughout the several views. FIG. 1 is a block diagram showing
the fundamental composition of an amplitude modulation circuit for a
transmitter according to this invention. That is, an audio-frequency
signal given to an input terminal 10 is applied to the input of a gain
controlled amplifier circuit (GCA) 12. Part of a modulation signal
produced from the GCA 12 is applied to the input of a filter circuit 14.
The filter circuit 14 is provided for giving the selectivity for frequency
to the modulation degree suppressing property of the modulation circuit.
The output signal of the filter circuit 14 is applied to the input of a
rectifier circuit 16, where it is rectified. A pulsating DC signal
produced from the rectifier circuit 16 is supplied to an eliminator
circuit 18. The eliminator circuit 18 is intended for providing a
substantially entire DC signal by removing the ripple of the supplied
pulsating DC signal. The DC signal produced from the eliminator circuit 18
is delivered to the GCA 12 as an automatic level control (ALC) signal. The
gain of the GCA 12 is subject to negative feedback control by the ALC
signal. Namely, the gain of the GCA 12 is inversely proportional to the
voltage V.sub.ALC of the ALC signal.
Thus, the GCA 12 forms an ALC circuit 20 including in its negative feedback
loop the filter circuit 14, rectifier circuit 16 and eliminator circuit 18
connected in series. That is, when the amplitude of the AF signal supplied
to the ALC circuit 20 is narrow, the gain of the GCA 12 is large. On the
other hand, when the amplitude of the supplied AF signal is wide, the gain
of the GCA 12 is small. Here it is to be noted that the transmission
frequency response of the GCA 12 would never change with the level of the
voltage V.sub.ALC. The gain is the only characteristic of the GCA 12 that
may be changed according to the voltage V.sub.ALC.
Meanwhile, included in the negative feedback loop of the ALC circuit 20 is
the filter circuit 14. Let us assume that the filter circuit 14 is a
high-pass filter, for example. Thereupon, if the amplitude of the output
signal or modulation signal of the GCA 12 is supposed to be constant, the
voltage V.sub.ALC increases as the frequency of the signal supplied to the
ALC circuit 20 becomes higher. In other words, although the mean level of
the AF signal supplied to the ALC circuit 20 throughout the frequency band
is the same, the gain of the GCA 12 will be relatively increased if the
main amplitude (spectrum) components is distributed in the middle or lower
range of the frequency band, whereas the gain of the GCA 12 will be
relatively reduced if the main amplitude components are distributed in the
higher range of the band. More plainly, it may be explained as follows.
That is, even though with the same sound volume supplied, the modulation
signal level is raised if a cello sound is applied to the input, while the
modulation signal level is lowered if a violin sound is applied to the
input.
The output or modulation signal of the ALC circuit 20 with the
aforementioned construction is supplied to an amplitude modulation unit
22. This amplitude modulation unit 22 is further provided with a carrier
delivered from a frequency synthesizer 24. In the amplitude modulation
unit 22, the carrier is modulated by means of the modulation signal, and
converted into a modulated radio-frequency signal (RF signal) for
transmission, which is led to an antenna (not shown) and radiated in the
air.
Generally speaking, the operation of thus constructed amplitude modulation
circuit is as follows. That is, if the amplitude of the input AF signal is
narrow, the modulation will be conducted to a depth in proportion to the
AF signal level without regard to the frequency distribution of the
amplitude. Meanwhile, if an AF signal with a wide amplitude which is
mainly formed of frequency components attenuated by the filter circuit 14
is applied to the input, then a deep modulation in proportion to the AF
signal level will be achieved. There will not, however, be achieved any
over-modulation exceeding a degree of 100% because of the modulated signal
level suppression by the ALC circuit 20. On the other hand, if an AF
signal with a wide amplitude which is mainly formed of frequency
components not attenuated by the filter circuit 14 is applied to the
input, then the modulation will be conducted to a shallower degree than
the depth proportional to the AF signal level. Let us suppose that the
frequency characteristic of the filter circuit 14 is such that the
amplitude response to frequency components at 400 Hz is 3 dB lower than
that at 2.5 KHz. Here, if the modulation circuit is so adjusted that at
most 100% modulation may be conducted for a sine-wave input at 10 mV (rms)
and 400 Hz, the modulation degree for the sine-wave input at 10 mV (rms)
and 2.5 KHz will be restricted to the maximum degree of approximately 70%.
Naturally, the maximum modulation degree for the input at 400 Hz may be
adjusted at 100% or any level lower than 100%.
As has been described above, in this modulation circuit, the upper limit of
the modulation degree is restricted more as the frequency components of
the amplitude of the input AF signal are distributed more in the band not
attenuated by the filter circuit 14. Therefore, if the filter circuit 14
is a high-pass filter or band-rejection filter for the intermediate range,
for example, the higher the main components of the AF input, the more the
upper limit of the modulation degree is restricted. That is, especially
suppressed is the large modulation with respect to the AF input of high
frequency components that is the main cause of the production of spurious
signals, so that the transmitter including this modulation circuit is
subject to production of less spurious signals. Moreover, since fully deep
modulation may be effected with respect to the intermediate range which is
the main frequency band of the AF signal, the average transmission power
of the transmitter can be increased to a sufficiently high-level. Further,
overmodulation exceeding 100% may securely be avoided, thus reducing the
modulation distortion.
Thus, according to the transmitter including the modulation circuit of the
invention, substantially distortion-free radio communication can be
achieved with improved S/N ratio as well as with reduced spurious
disturbance.
FIG. 2 shows an example of practical composition rearranged from the
fundamental composition as shown in FIG. 1. That is, the AF signal applied
to the input terminal 10 is supplied to a voltage controlled attenuator
(VCA) 12.sub.1. The output signal of the VCA 12.sub.1 is applied to the
input of an audio-frequency amplifier 12.sub.2, whose output signal is
used as a modulation signal. Part of the modulation signal is supplied to
either the filter circuit 14 or a coupler circuit 28 by means of a
changeover switch 26. When the switch 26 is on the side of the filter
circuit 14, the part of the modulation signal is converted into an ALC
signal by means of the filter circuit 14, rectifier circuit 16 and
eliminator circuit 18. The attenuation of the VCA 12.sub.1 may be varied
according to the DC voltage level V.sub.ALC of the ALC signal. Thus, there
is formed an ALC circuit 20.sub.1. It may easily be understood that the
ALC circuit 20.sub.1 of FIG. 2 is substantially the same as the ALC
circuit 20 of FIG. 1 by identifying the series circuit of the VCA 12.sub.1
and audio-frequency amplifier 12.sub.2 of FIG. 2 with the GCA 12 of FIG.
1.
Meanwhile, when the switch 26 selects the coupler circuit 28, the filter
circuit 14 is replaced by the coupler circuit 28. In this case, the
modulation circuit including the ALC circuit operates in the same manner
as the modulation circuit employing the prior art compression circuit.
That is, in the composition of FIG. 2, the transmission performance may be
switched optionally by selecting the coupler circuit 28 when the
modulation is required to be conducted deep into the higher range
components of the AF input in spite of an increase in production of
spurious signals and selecting the filter circuit 14 when the production
of spurious signals is to be checked.
The modulated signal of the modulation circuit used for a carrier which is
synthesized by a frequency synthesizer 24 and amplified to a predetermined
level by a radio-frequency amplifier 30. This carrier is
amplitude-modulated in the modulation unit 22 by means of the modulation
signal from the audio-frequency amplifier 12.sub.2. The modulated RF
signal modulated by the modulation unit 22 is led to an antenna 34 through
an output filter circuit 32, and radiated in the air.
FIG. 3 shows a further definite example of circuit in accordance with the
composition as shown in FIG. 2. In FIG. 3, however, the switch 26 and
coupler circuit 28 are omitted, whereas a speaker circuit is added instead
thereof. In FIG. 3, the AF signal applied to the input terminal 10 is
supplied to the audio-frequency amplifier 12.sub.2 through a resistor
R.sub.1. Separately connected between the input terminal of the amplifier
12.sub.2 and the ground line are the collector and emitter terminals of a
PNP transistor Q.sub.1 for control and a compensation capacitor C.sub.1.
That is, the VCA 12.sub.1 is formed of a voltage divider circuit including
the resistor R.sub.1, transistor Q.sub.1 and capacitor C.sub.1. While the
attenuation ratio of the VCA 12.sub.1 is low when the base current of the
transistor Q.sub.1 is little, it increases as the base current of the
transistor Q.sub.1 is increased. Namely, the transistor Q.sub.1 is a
variable resistance or rheostat controlled by the base current. The
compensation capacitor C.sub.1 is connected mainly for the purpose of
reducing an operations distortion due to the non-linear voltage variation
in the output capacity C.sub.0b of the transistor Q.sub.1. Thus, the
modulation distortion, as well as the spurious emission, may be reduced.
However, the modulation frequency characteristic will be adversely
affected if the capacity of the capacitor C.sub.1 is excessively
increased, so that it is not advisable to restrain the production of
spurious signals by increasing the capacity of the capacitor C.sub.1.
Further, the amplifier 12.sub.2 is supplied from a first power source
+V.sub.B1. The VCA 12.sub.1 and amplifier 12.sub.2 form the GCA 12.
The output terminal of the amplifier 12.sub.2 is connected to the primary
tap (usually, center tap) of an output transformer T.sub.1 through an
output capacitor C.sub.2, while the secondary side of the transformer
T.sub.1 is connected with a speaker 38 through a switch 36. That is, when
the switch 36 is closed at time of transmission, the transmitted sound is
monitored. When the switch 36 is closed at reception, sound play-back of
the received radio wave is performed. One end on the primary side of the
transformer T.sub.1 is supplied from a second power source, while the
other end on the primary side of the transformer T.sub.1 is connected to
the anode of a switch diode D.sub.1. Whereas the diode D.sub.1 conducts
when its anode potential is higher than the cathode potential, it is cut
off when the anode potential is lower than the cathode potential.
Therefore, the diode D.sub.1 functions as a switch. The diode D.sub.1 is
so biased as to be cut off at reception and to conduct at transmission.
The cathode of the diode D.sub.1 is connected to a first terminal a of the
filter circuit 14 through a capacitor C.sub.3. A second input terminal b
of the filter circuit 14 is connected with the second power source
+V.sub.B2. Connected between the first input terminal a and a first output
terminal c is a parallel circuit of a capacitor C.sub.4 and a resistor
R.sub.2. Further, the second input terminal and a second output terminal d
of the filter circuit 14 are directly connected with each other, and a
resistor R.sub.3 is connected in a circuit arranged between the first
output terminal c and the second output terminal d. Thus, the filter
circuit 14 forms a kind of high-pass filter. Connected between the first
output terminal c and the second output terminal d are the anode and
cathode of a rectifying diode D.sub.2. The anode of the diode D.sub.2 is
connected with the cathode of a rectifying diode D.sub.3. The rectifier
circuit 16 formed of the diodes D.sub.2 and D.sub.3 is a half-wave voltage
doubler rectifier circuit to provide a negative rectifying output. The
anode of the diode D.sub.3 is connected to the junction of the respective
one-side ends of resistors R.sub.4 and R.sub.5. The other end of the
resistor R.sub.4 is connected to the ground line. The resistor R.sub.4 is
connected in parallel with an elimination capacitor C.sub.5. Connected
between the other end of the resistor R.sub.5 and the ground line is a
filter capacitor C.sub.6. The resistors R.sub.4 and R.sub.5 and the
capacitors C.sub.5 and C.sub.6 form the eliminator circuit 18. Depending
on the selection of the time constants of these capacitors and resistors
forming the eliminator circuit 18, the ALC operation may be conducted in
response to either the peak value of the AF signal input, or the mean
value of the AF signal input, or the intermediate value between the peak
and mean values. It is to be understood that the ALC operation may be
conducted in accordance with RMS value detection. The output terminal of
the eliminator circuit 18, i.e. the junction of the resistor R.sub.5 and
capacitor C.sub.6, is connected to the base of the control transistor
Q.sub.1 of the VCA 12.sub.1. Thus formed is the ALC circuit 20.
The modulation signal taken out from the junction of the cathode of the
diode D.sub.1 and the capacitor C.sub.3 is convoluted by the direct
current from the second power source +V.sub.B2, and supplied to the power
supply input terminal of the modulation unit 22. If the supply voltage
+V.sub.B2 is 13.9 V, the convoluted modulation signal is practically
available at approximately .+-.6.3 V, for example, with the modulation
degree of 100%. That is, the modulation unit 22 is a collector modulation
type amplitude modulator in which the amplitude modulation is achieved at
the supply voltage level. The modulation unit 22 is supplied with a
carrier as a modulated wave that has been synthesized by the frequency
synthesizer 24 and adjusted to a proper level by the radio-frequency
amplifier 30. Further, the output signal of the modulation unit 22 is
supplied to the antenna 34 through the output filter circuit 32. In the
circuit of the modulation unit 22, the coils L.sub.31 and L.sub.32 and the
capacitors C.sub.31 and C.sub.32 are intended for carrier excitation.
Moreover, the capacitor C.sub.33 is used for impedance matching, while the
resistors R.sub.31 and R.sub.32 are used for bias resistances.
Let us suppose that the filter circuit 14, among other circuits as
aforementioned, is formed of CR with the constants given as follows:
R.sub.2 =15 K.OMEGA., R.sub.3 =10 K.OMEGA. and C.sub.4 =0.0068 .mu.F.
Whereas the transfer frequency response of the filter circuit 14 somewhat
varies with the impedances of the circuits connected with the input and
output terminals, the frequency response of the filter circuit 14 formed
of the CR with the above constants varies substantially as shown in FIG.
4. The operation of the ALC circuit including the filter circuit 14 with
such characteristic may be described as follows.
The AF signal applied to the input terminal 10, after attenuated by the VCA
12.sub.1, is amplified into a modulation signal by the amplifier 12.sub.2.
The diode D.sub.1 is allowed to conduct while the modulation circuit is
engaged in the transmitting operation. Accordingly, the modulation signal
offered by the amplifier 12.sub.2 is applied to the input terminals a and
b of the filter circuit 14 through the capacitor C.sub.2, transformer
T.sub.1, diode D.sub.1 and capacitor C.sub.3. The modulation signal
subjected to frequency selection by the filter circuit 14 is converted
into a negative pulsating direct current by the rectifier circuit 16. This
negative pulsating direct current is converted into a substantially
ripple-free direct current, i.e. negative ALC signal, by the eliminator
circuit 18. The negative ALC signal is given as a negative feedback signal
to the base of the control transistor Q.sub.1 of the VCA 12.sub.1.
Thereupon, the attenuation by the VCA 12.sub.1 increases, and the
amplitude of the modulation signal is automatically adjusted to a fixed
level. That is, the ALC circuit may be so adjusted that the modulation
circuit may not perform overmodulation exceeding a degree of 100% for a
400 Hz sine-wave AF signal. Let it be supposed that a 2.5 KHz sine-wave AF
signal with the same amplitude as that of the 400 Hz AF signal at 100%
modulation is applied to the input of the modulation circuit adjusted in
the aforesaid manner. Hereupon, as may be clear from the characteristic
curve of FIG. 4, the level of the ALC signal supplied to the control
transistor Q.sub.1 of the VCA 12.sub.1 increases by 3 dB. The attenuation
at the VCA 12.sub.1 is substantially in inverse proportion to the level of
the ALC signal. Therefore, if the 2.5 KHz single tone is applied to the
input as aforesaid, the amplitude of the output or modulation signal of
the amplifier 12.sub.2 decreases by nearly 3 dB. Thus, the maximum
modulation at 2.5 KHz may be restricted to about 70%. Accordingly, with
the frequency of the AF input signal taken as a parameter, the relation
between the AF input signal and modulation degree of the modulation
circuit may be given by the curves as shown in FIG. 5.
FIG. 5 is a graph illustrating a case in which the AF signal level is
selected as a variable, while if the AF signal level is fixed at a value
indicated by "Lr" in FIG. 5, for example and the frequency of the AF
signal is taken as the variable, a curve of FIG. 6 may be obtained. That
is, the maximum modulation degree is reduced as the frequency of the AF
input signal increases. Here it is surely to be noted that FIG. 6 shows by
no means the frequency characteristic of the normal modulation degree, but
shows the frequency characteristic of the maximum modulation degree.
Therefore, the modulation circuit would not operate in such a manner as
relatively to reduce the high-frequency range of the input AF signal.
Although a kind of high-pass filter is used for the filter circuit 14 in
the embodiment of FIG. 3, a band-pass filter or band-rejection filter may
also be used for the filter circuit 14. What is essential is only that the
filter be one which may relatively enlarge the high-frequency range
including few large-amplitude components and causing spurious signals as
compared with the intermediate range which is the main band of the
amplitude distribution of a human voice (AF signal input). FIGS. 7 to 15
show examples of the circuit available for the filter circuit 14. Among
the circuits of these drawings that of FIG. 11 functions as a kind of
high-pass filter, whereas that of FIG. 9 functions as a kind of band-pass
filter. On the other hand, the circuits of FIGS. 7, 8, 10 and 12 to 15 are
kinds of band-rejection filter. Those of FIGS. 7 and 8 are LC-type passive
filters, while those of FIGS. 10, 14 and 15 are CR-type passive filters.
FIG. 12 shows an active filter including an LC circuit in the negative
feedback loop of a phase-inverted amplifier. Further, FIG. 13 shows an
example of the band-rejection filter taking advantage of the frequency
selectivity of a bootstrap circuit formed of a phase-noninverted amplifier
A.sub.13, capacitor C.sub.132 and resistors R.sub.132 and R.sub.133, and a
capacitor C.sub.131. Available for the amplifier A.sub.13 is an emitter
follower circuit with the amplification degree of +1. Such bootstrap
circuit is the one used as a semiconductor inductance.
By way of example, let use assume a case where the circuit of FIG. 7 among
other band-rejection filters as aforesaid is applied to the filter circuit
14 of FIG. 3. If R.sub.71 =1 K.cndot., L.sub.7 =33 mH, C.sub.7 =1 .mu.f
and R.sub.72 =180 .OMEGA. in FIG. 7, then FIGS. 16 and 17 may be obtained
like the case with FIGS. 4 to 6. That is, if the band-rejection filter is
used for the filter circuit 14, the maximum modulation degree with respect
to the lower range of the AF input, as well as to the higher range, may be
restrained. On the other hand, if the band-pass filter is used for the
filter circuit 14, then the maximum modulation degree with respect to the
frequency band (higher range) made relatively larger by means of the
filter will specially be suppressed.
Although the half-wave voltage doubler rectifier circuit is used for the
rectifier circuit 16 in the embodiment of FIG. 3, it is to be understood
that any other suitable circuits may be also available for this purpose.
Whereas a simple half-wave rectifier circuit with a single diode may be
used for the rectifier circuit 16, there may be employed such a full-wave
voltage doubler rectifier circuit as shown in FIG. 19 when the input and
output sides of the rectifier circuit are entirely separate from each
other for the sake of circuit design, for example. Alternatively, there
may be also used a multistage rectifier (Cockcroft-Walton circuit) as
shown in FIG. 20. Further, also available is a linear rectifier circuit
with a diode in the negative feedback loop of an amplifier A.sub.21, as
shown in FIG. 21.
As for the GCA 12, it may be selected from those shown in FIGS. 22 to 27.
Among the circuits in these drawings those of FIGS. 22 to 24 employ e.g.
an N-channel junction FET as a control transistor, controlling the
internal resistance of the FET by means of the ALC signal voltage
V.sub.ALC. On the other hand, the circuits of FIGS. 25 to 27 employ
instead of the control transistor a photo coupler including a
light-emission diode or lamp.
FIG. 28 shows a modification of the modulation circuit as shown in FIG. 2.
That is, the ALC circuit 20.sub.2 of FIG. 28 includes a first filter
circuit 14.sub.1 and a second filter circuit 14.sub.2, these filter
circuits being optionally selected by means of the switch 26. If the
characteristics of the first and second filter circuits 14.sub.1 and
14.sub.2 are varied from each other, different ALC operations may be
optionally selected according to the kinds of AF input.
FIG. 29 is another modification of the circuit of FIG. 1 (or FIG. 2). Thus,
by connecting entirely independent two systems of ALC circuit in series,
the modulation circuit may be given the following function, for example.
That is, a first ALC circuit 20.sub.3 includes a band-pass filter for a
specific frequency f.sub.1 which may effectively restrain the maximum
modulation degree when the AF input of the frequency component f.sub.1 is
supplied at a high level. On the other hand, a second ALC circuit 20.sub.4
includes a high-pass filter to allow a specified frequency f.sub.2 or
higher to pass, effectively restraining the maximum modulation degree when
the AF input of the frequency component f.sub.2 or higher is supplied at a
high level. Not changing the frequency characteristic of the AF input
signal, both the first and second ALC circuit 20.sub.3 and 20.sub.4 may
operate quite independently according to the characteristics of their own
filter circuits.
FIG. 30 shows still another modification of the circuit of FIG. 1 (or FIG.
2). The ALC circuit 20.sub.5 as shown in FIG. 30 includes the filter
circuits 14.sub.1 and 14.sub.2. A wide variety of ALC characteristics may
be obtained by combining these filter circuits at need with the various
filter circuits as shown in FIGS. 3 and 7 to 15, or by employing filter
elements with varied constants for similar filter circuits.
FIG. 31 shows a further definite example of the circuit in accordance with
the composition of FIG. 30. In this composition the circuit as shown in
FIG. 23 is used for the GCA 12. Used for the first filter circuit 14.sub.1
is a bridged T-type band-rejection filter as shown in FIG. 14. For the
second filter circuit 14.sub.2 there is employed a high-pass filter as
shown in FIG. 11. A half-wave linear rectifier circuit as shown in FIG. 21
is used for the rectifier circuit 16, including the amplifier A.sub.32 of
the second filter circuit 14.sub.2. Further, the eliminator circuit 18
includes a General-use .pi.-shaped CR low-pass filter. Here the function
of a diode D.sub.312 connected in parallel with a resistance R.sub.320
will be described. When the eliminator circuit 18 is supplied with a
negative voltage from the rectifier circuit 16, the diode D.sub.312 is
forwardly biased unless a capacitor C.sub.315 is fully charged. Thereupon,
the capacitor C.sub.315 is charged quickly, and the ALC signal voltage
-V.sub.ALC may increase to a predetermined level in a short time. On the
other hand, when the charge on the capacitor C.sub.315 is discharged, the
diode D.sub.312 is inversely biased. Thus, the time constant of discharge
of the charge on the capacitor C.sub.315 increases, so that the reduction
in the ALC signal voltage -V.sub.ALC becomes slower. That is, by using
such eliminator circuit, the ALC operation may be quickly started and
slowly stopped, thereby reducing the unnaturalness caused by the ALC
operation, i.e., the unevenness of the level of the transmitted AF signal.
Thus, according to transmitter provided with the amplitude modulation
circuit of this invention, the overmodulation may securely be avoided, and
the modulation distortion, as well as the production of spurious signals,
may substantially be reduced. Accordingly, the troublesome measure to
counter the spurious emission that has conventionally been taken with
difficulties may be simplified, thus reducing the cost of production.
Although a specific circuit has been illustrated and described herein, it
is not intended that the invention be limited to the elements and circuit
arrangements disclosed herein. One skilled in the art will recognize the
particular elements or subcircuits may be used without departing from the
spirit of the invention.
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