Space diversity reception system

In a space diversity reception system, a plurality of receiving circuits, each of which is connected to respective antenna, outputs a first type signal having a level which varies in accordance with the radio frequency signal level input to the receiving circuit. An automatic gain-controlled amplifier amplifies the first type signal so as to output a second type signal having a substantially constant level. A phase control circuit detects a phase difference between the second type signal output from each of the automatic gain-controlled amplifiers and controls the phase difference in each of the first type signals to be null. A combining circuit combines the signals, whose levels are respectively following the input radio frequency signal levels, picked up from each of an inter-stage in the receiving circuits, an output of the combining circuit is the output of the space diversity system. This circuit configuration allows an employment of a less expensive automatic gain-controlled amplifier requiring less severe characteristics. Thus, a circuit for adjusting the input levels to the combining circuit can be avoided, accordingly, a multi-level quadrature amplitude modulation signal having amplitude-modulated component can be received.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a space diversity (referred to hereinafter 
as SD) reception system of a radio signal. More particularly, this 
invention relates a SD reception system which is simple in its 
configuration and requires less severe specifications for its 
automatically gain-controlled intermediate frequency amplifier (referred 
to hereinafter as IF AGC amplifier). 
2. Description of the Related Art 
For receiving a radio signal of a digital radio equipment, etc., SD 
reception systems have been widely employed, where outputs of a first 
receiver for receiving a signal from a first antenna and a second receiver 
for receiving a signal from a second antenna are combined to attain a 
stable receiving signal even when the conditions of the received signals 
by the antennas are fluctuating due to a fading or multipath effect, etc., 
so that the signal is reliably received. 
FIG. 1 shows a prior art SD reception system. The numerals 1 and 11 
respectively denote microwave low-noise pre-amplifiers (MFA); the numerals 
2 and 12 respectively denote microwave variable attenuators (MVA); the 
numerals 3 and 13 respectively denote frequency converters (FC); the 
numerals 4 and 14 respectively denote intermediate-frequency amplifiers 
(referred to hereinafter as IF amplifiers) (IFA); the numerals 5 and 15 
respectively denote automatically gain-controlled (referred to hereinafter 
as AGC) amplifiers; the numerals 6 and 16 respectively denote amplitude 
detectors (DET); the numerals 7 and 17 respectively denote direct-current 
(DC) amplifiers (DCA); the numerals 8 and 18 respectively denote level 
monitor detectors (DET); the numeral 9 and 19 respectively denote 
band-pass filters (BPF) of the IF band; the numeral 21 denotes a microwave 
local frequency oscillator (LO); the numeral 22 denotes a microwave hybrid 
junction (H); the numeral 23 denotes a microwave endless Phase shifter 
(EPS); the numeral 24 denotes a phase control circuit (PHC); the numeral 
25 denotes a phase comparator (PC) of the IF band; the numeral 26 denotes 
a 90.degree. phase shifter (PS) of the IF band; the numerals 
27.sub..about. 29 respectively denote DC amplifiers; the numerals 30 and 
31 respectively denote variable attenuators (IVA) of the IF band; the 
numeral 32 denotes a hybrid junction (H) of the IF band; the numeral 33 
denotes a third AGC amplifier of the IF band; the numeral 34 denotes an 
amplitude detector (DET); and the numeral 35 denotes a DC amplifier (DCA). 
Microwave signals received by a first antenna ANT 1 and a second antenna 
ANT 2 are amplified by pre-amplifiers 1 and 11, respectively; the output 
signal levels therefrom are adjusted by variable attenuators 2 and 12; the 
output signals therefrom are converted into IF signals by frequency 
converters 3 and 13 with the local oscillator frequency; and the IF 
signals are amplified by IF amplifiers 4 and 14, which are of low-noise 
amplifiers. The IF signals output from IF amplifiers 4 and 14 are 
respectively amplified by AGC amplifiers 5 and 15, where detectors 6 and 
16 output DC signals varying in accordance with the output levels of AGC 
amplifiers so as to form feedback loops via DC amplifiers 7 and 17 to 
variable attenuators 2 and 12 as well as AGC IF amplifiers 5 and 15; 
accordingly, outputs of AGC amplifiers 5 and 15 are kept constant even 
when the input levels thereto are fluctuated. Monitoring of the levels of 
the received signals are individually carried out by observing each of the 
AGC voltages, or by detectors 8 and 18 detecting the signal levels at 
inter-stage of AGC amplifiers 5 and 15. 
Each of the IF signals is co-phased with each other as follows. Outputs of 
AGC IF amplifiers 5 and 15 are input to narrow-band bandpass filters 9 and 
19 so as to allow carrier components of the IF signals input thereto to 
pass. Output of bandpass filter 9 is input directly to phase comparator 
25, while output of another bandpass filter 19 is input via a 90.degree. 
phase shifter 26 to phase comparator 26, where the phases of the IF 
signals input thereto are compared so as to output the phase difference 
therebetween. Thus detected phase difference is applied via phase 
controller 24 to endless phase shifter 23 so as to adjust phase of the 
local oscillator signal to be input from hybrid junction 22 to frequency 
converter 13, while output of local oscillator 21 is directly input from 
hybrid junction 22 to frequency converter 3, so that the phases of the two 
IF signals output from two AGC IF amplifiers 5 and 15 are always kept in 
phase with each other. 
Furthermore, on the other hand, the outputs of AGC IF amplifiers 5 and 15 
to be combined by hybrid junction 32 are attenuated by first and second 
variable attenuator 30 and 31, respectively, to adjust the signal levels 
as described below. Differential amplifier 27 detects the difference of 
the AGC voltages respectively generated for two AGC IF amplifiers 5 and 
15; and the detected difference is amplified by DC amplifiers 28 and 29, 
one of which outputs an opposite polarity signal of the other. Outputs of 
DC amplifiers 28 and 29 control attenuation of first and second variable 
attenuators 30 and 31, respectively, so that the IF signal levels input to 
hybrid junction 32 are corresponding to the microwave signal levels 
received by the first and second antennas, ANT 1 and ANT 2, respectively. 
In other words, when the microwave signal level received by first antenna 
ANT 1 is higher than that of the second antenna ANT 2, the attenuation by 
first variable attenuator 30 is adjusted to be less than that of second 
variable attenuator 31. This is because, if the outputs of the same level 
from two AGC IF amplifiers 5 and 15 are combined by hybrid junction 32, 
the distortion or low SIN ratio carried in the signal of the lower level 
is included in the combined signal by fifty-fifty share. Therefore, the 
inputs to the hybrid junction 32 must be adjusted so that the inputs to 
the hybrid junction follow the input levels of respective antennas. 
The combined IF signal output from hybrid junction 32 is amplified by third 
AGC IF amplifier 33. Detector 34 detects signal level of third AGC IF 
amplifier 33 so as to output a DC signal varying in accordance with the 
output signal level of third AGC IF amplifier 33. The output DC signal is 
fed back via a DC amplifier 35 to third AGC IF amplifier 33, so that the 
output level of third AGC IF amplifier 33 is stabilized constant. 
In the prior art SD reception system shown in FIG. 1, there are the 
following problems. That is, as many as three AGC IF amplifiers 5, 15 and 
33 are required; furthermore, the specifications required in these 
amplifiers are severe because they are employed in the main signal route, 
where linearity characteristics, phase characteristics as well as 
saturation characteristics are strictly required. Moreover, the IF signals 
once amplified by AGC IF amplifiers 5 and 15 must be attenuated at the 
expense of the additional variable attenuators 30 and 31 and control 
circuits 27.sub..about. 29 therefor. 
SUMMARY OF THE INVENTION 
It is a general object of the invention, therefore to provide a less 
expensive space diversity reception system, where the amplifiers provided 
for its phase control circuit requires less severe characteristics, as 
well as no particular control circuit is required for adjusting signal 
levels of the two receiver circuits to be combined. 
In a space diversity reception system, according to the present invention, 
a plurality of receiving circuits, each of which is connected to 
respective antenna, outputs a first type signal having a level which 
varies in accordance with the radio frequency signal level input to the 
receiving circuit, and further includes an automatically gain-controlled 
(AGC) amplifier for amplifying the first type signal so as to output a 
second type signal having a substantially constant level. The space 
diversity reception system further has a phase control circuit for 
detecting a phase difference between the second type signals output from 
each of the AGC amplifiers and for controlling the phase difference in the 
two first type signals to be null, and a combining circuit for combining 
the signals, whose levels are respectively following the input radio 
frequency signal levels, picked up from each of an inter-stage in the 
receiving circuits. Output of the combining circuit is the output of the 
SD system. This circuit configuration allows an employment of a less 
expensive AGC amplifier requiring less severe characteristics and deletion 
of the circuit for adjusting the input levels to the combining circuit. 
The above-mentioned features and advantages of the present invention, 
together with other objects and advantages, which will become apparent, 
will be more fully described hereinafter, with reference being made to the 
accompanying drawings which form a part hereof, wherein like numerals 
refer to like parts throughout.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 shows a block diagram of a first preferred embodiment of the present 
invention. Microwave signals received by first and second antenna ANT 1 
and ANT 2 are respectively amplified by microwave low-noise pre-amplifiers 
1 and 11, outputs from which are attenuated respectively by variable 
attenuators 2 and 12 for some degree according to an AGC voltages fed back 
from outputs of AGC IF amplifiers 5A and 15A, as explained later in 
detail. Outputs from variable attenuators 2 and 12 are frequency-converted 
by frequency converter 3 and 13 with local frequency signals input thereto 
from a local oscillator 21 via hybrid junction 22, so as to be 
respectively output as IF signals. Details of the local frequency signals 
are described later on. Outputs from frequency converter 3 and 13 are 
amplified by low-noise IF pre-amplifiers 4 and 14, respectively. Degree of 
the AGC fed back to the variable attenuators 2 and 12 are only for 
preventing a distortion or a noise caused from too week a signal or too 
strong a signal input to frequency converters 3 and 13; therefore, the 
output signal levels from IF amplifiers 4 and 14 are varying in accordance 
with the levels of the microwave signals received by the antennas, 
respectively, as shown in FIG. 6. Variable attenuators 2 and 12 are of 
widely known circuit typically employing PIN diodes to which the AGC 
voltage is applied. 
IF signals output from IF amplifiers 4 and 14 are further amplified by AGC 
IF amplifiers 5A and 15A, where their output levels are automatically 
gain-controlled respectively, as shown in FIG. 6. For the AGC 
amplification, detectors 6 and 16 detect average levels of the outputs of 
AGC IF amplifiers 5A and 15A, so as to output signals in accordance with 
the output levels of the AGC IF amplifiers 5A and 15A, respectively. DC 
amplifiers 7 and 17 amplify the output of detectors 6 and 16, outputs of 
which are negatively fed back to variable attenuators 2 and 12 and AGC IF 
amplifiers 5A and 15A to control their gains, respectively. Though no 
drawing is shown in the figure, there may be provided a reference voltage 
source to which the output of the detector 6 (or 16) is compared, where 
the detected difference is employed as the AGC voltage. This feedback 
circuit provides more constant output level from the AGC IF amplifiers. 
Then, AGC loops, i.e. feedback loops, are established so that the output 
levels of AGC IF amplifiers 5A and 15A are respectively kept substantially 
constant. Thus, the circuits from pre-amplifier 1 through AGC IF 
amplifiers 5A constitute a first receiver circuit for first antenna ANT 1, 
as well as the circuits from pre-amplifier 11 through AGC IF amplifiers 
15A constitute a second receiver circuit for second antenna ANT 1. 
Microwave signal levels received by the receiver circuits are individually 
monitored by observing each of the AGC voltages independently for each 
receiver circuit. 
Out of the outputs of AGC IF amplifiers 5A and 15A of the constant level, 
carrier frequency spectrums are extracted by narrow bandpass filters 9 and 
19, respectively. Output of bandpass filter 9 of the first receiver 
circuit is directly input to a phase comparator 25, while output of 
bandpass filter 19 of the second receiver circuit is input via a 
90.degree. phase shifter 26 to phase comparator 25, where phases of the 
two carrier spectrums are compared. Phase comparator 25 is formed of an 
analog multiplier circuit of the 90.degree.-shifted two signals as well 
known, where the signals to be compared must be of equal level in order to 
achieve accurate phase-comparison. The phase difference output from phase 
comparator 25 is applied via phase controller 24 to endless phase shifter 
23 to adjust phase of the local oscillator signal to be input from hybrid 
junction 22 to frequency converter 13 of the second receiving circuit, 
while output of local oscillator 21 is directly input via hybrid junction 
22 to frequency converter 3 of the first receiving circuit, so that the 
phases of the two IF signals output from two frequency converters 3 and 13 
are always kept co-phase with each other. 
Thus co-phased IF signals output from IF amplifiers 4 and 14 are, on the 
other hand, are input to hybrid junction 32, where the two IF signals are 
combined, i.e. added. Output of hybrid junction 32 is amplified by a third 
IF amplifiers 33. Detector 34 detects the out-.out of third IF amplifier 
33 so as to output a signal which corresponds to the average output level 
thereof. Output of detector 34 is amplified by a DC amplifier 34, output 
of which is negatively fed back to third IF amplifier 33 so that the 
output of third IF amplifier 33 is automatically gain-controlled to be 
substantially constant even when the input signal level thereto 
fluctuates. 
FIGS. 3 show signal levels at each stage of amplifications and attenuations 
in each receiving circuit of the FIG. 2 circuit configuration. Abscissa of 
FIG. 3(B) is represented by the FIG. 3(A) blocks. In FIG. 3(B), 
fluctuations of the received signal levels are indicated by the solid 
lines and the dotted lines for the first antenna and for the second 
antenna, respectively. It is observed there that the output levels of AGC 
IF amplifiers 5A and 15A are always kept constant as predetermined by the 
control of the attenuations in variable attenuators 2 and 12 for the 
microwave signals as well as by the control of gain of AGC IF amplifiers 
5A and 15A, even when the input signal levels to the pre-amplifiers 1 and 
11 are fluctuating. On the other hand, it is also observed that the signal 
levels at the outputs of IF amplifiers 4 and 14, from which the signals 
are taken out so as to be combined, are following the signal levels input 
to the pre-amplifiers 1 and 11. In other words, when the signal level from 
first antenna is higher than that of the second antenna the output level 
of IF amplifier 4 of the first receiver circuit is higher than that of the 
second receiver circuit. Thus, the signal levels to be combined are 
corresponding to the levels of the signals received by respective antenna. 
Accordingly, variable attenuators 30 and 31 and their control circuit 
(indicated by dotted lines in FIG. 1), each employed in prior art circuit 
configuration, are no more necessary. Output of the combining circuit 32 
is the IF signal output of the SD system. 
Among microwave signals having fluctuating signal levels due to fading or 
multipath effect, etc., low level signals are generally inferior in the 
distortion or S/N ratio, etc,. However, according to the present 
invention, a first type IF signal of higher level being dominant in the 
output of the SD system reduces the effect of the unfavorable low level 
microwave signal, without paying for an expensive AGC IF amplifiers 
requiring severe specifications and for complicated control circuit to 
attenuate the once amplified IF signals. 
In the FIG. 2 circuit configuration, the amplifiers 1, 11, 4, 14 and 33 in 
the main signal routes are required to be as high grade the 
characteristics as the FIG. 1 prior art circuit configuration; however, 
AGC IF amplifiers 5A and 15A in the present invention being merely for 
phase control outside the main signal route do not require such high grade 
characteristics for the linearity and bandwidth, etc. as those of the main 
signal route. Therefore, the FIG. 2 configuration contributes to reduce 
the power consumption of the amplifiers, the size of the circuits and 
accordingly the cost. 
FIG. 4 shows further detail of the FIG. 2 embodiment of the present 
invention. The same parts as in FIG. 2 circuit are denoted with the same 
numerals. The numerals 41 and 51 denote bandpass filters (BPF) for 
allowing the signals of IF bandwidth to pass. The numerals 43, 53, 45, 55, 
46 and 48 denote IF amplifiers (IFA) for amplifying the IF signal band. 
The numerals 44, 54 and 47 denote variable attenuators (IVA) for 
attenuating the IF signal band. Thus, in FIG. 4 configuration, AGC IF 
amplifiers 5A and 15A are respectively constituted with: IF bandpass 
filter 41 and 51 provided at the out,,outs of the low-noise IF 
pre-amplifiers 4 and 14, IF amplifiers 43 and 53, variable attenuators 44 
and 54 controlled by the AGC signals out-puts from DC amplifiers 7 and 17, 
and IF amplifiers 45 and 55. Each of the AGC signals is also employed for 
independently monitoring the signal levels received by each antenna. The 
third AGC IF amplifier 33 for amplifying the combined IF signal is 
constituted of IF amplifier 46, variable attenuator 47 and IF amplifier 
48, where the attenuation at variable attenuator 46 is controlled by AGC 
signal output from DC amplifier 35 so as to keep the output level of IF 
amplifiers 33 constant. 
FIG. 5 shows a second preferred embodiment of the present invention. The 
same parts as in the FIG. 4 first embodiment are denoted with the same 
numerals. The FIG. 5 configuration is further provided with narrow 
bandpass filters 69 and 79 in front of IF amplifiers 43 and 53, for 
allowing only the carrier component of the IF signals to pass 
therethrough, so that noise components in the IF signals are reduced in 
AGC IF amplifiers 5A and 15A. Detectors 49 and 59 detect the outputs from 
inter-stage IF amplifiers 43 and 53 for independently monitoring the 
signal levels of the two receiver circuits. These level monitors also 
enjoy the reduced IF noises, for achieving more accurate monitoring. 
FIG. 7(A) shows typical relations of signal levels output from the AGC IF 
amplifiers 5A and 15A, signal levels for monitor detection and signal 
levels to be combined, versus signal level input to the receiver circuit, 
for the case where the amplifier is composed of a single stage of 
pre-amplifier, a first variable attenuator, a frequency converter, and 
five IF amplifiers and four variable attenuators, as shown in FIG. 6. The 
IF signal to be combined is output from the first IF amplifier, as well as 
the monitor signal is output from the fourth IF amplifier. It is seen in 
the figure that, while the AGC IF amplifiers is outputting a constant 
level signal for the phase comparison for wide dynamic range of the input 
signal, the signal to be combined and the signal to be monitored are 
corresponding the input signal level to the receiver. 
FIGS. 7(A) and 7(B) show typical relation of difference of IF signal levels 
to be combined versus difference of microwave signal levels input to the 
receiver circuits in the case where are employed a single stage of the 
variable attenuators in the pre-amplifier and four or five stages of 
variable attenuators in IF amplifiers for establishing AGC feedback loops, 
according to the circuit configuration of the invention. It is observed 
that IF signals which correspond to the difference of the signal levels 
input to each receiver circuit are taken out of each receiver circuit 
where the signal level difference is a little compressed because the 
signals are taken out of the stage after merely the first one of the four 
or five variable attenuators in total. 
In addition to the effects of the invention as already described above, 
there are advantageous effects in the invention as follows. The circuit 
configuration of the invention is advantageous particularly for receiving 
a multi-level QAM (Quadrature Amplitude Modulation) signal, because the 
QAM signal carries multi-channel signals by means of the phase- as well as 
amplitude-modulation. Accordingly, the amplifiers and the converters must 
satisfy strict requirements for the linearity in the dynamic range, the 
phase characteristics, etc. These strict specifications can be achieved in 
the invention circuit paying less cost, less space and less power 
consumption. In some prior art circuit configuration where the two 
receiver circuits are automatically gain-controlled commonly by a common 
AGC voltage so that the amplification factor of each receiver circuit is 
equal to each other for outputting the signals to be combined in 
accordance with the received RF signal levels, the independent monitoring 
of the input signal level of each receiver circuit must be given up. 
However, in the circuit configuration of the invention, received signal 
level of each receiver circuit can be monitored simply and accurately. 
Though in the above preferred embodiments the phase control of the two 
receiving circuits is carried out by detecting the phase difference of the 
output signals of the AGC IF amplifiers, it is apparent that the phase 
difference may be detected from other signals than IF, i.e., may be 
detected from the radio frequency signal. 
Though in the above preferred embodiments the phase control of the two 
receiving circuits is carried out by controlling the phase of the local 
frequency signal input to the frequency converter, it is apparent that the 
phase control may carried out by a phase shifter provided in the receiving 
circuit. 
The many features and advantages of the invention are apparent from the 
detailed specification and thus, it is intended by the appended claims to 
cover all such features and advantages of the system which fall within the 
true spirit and scope of the invention. Further, since numerous 
modifications and changes may readily occur to those skilled in the art, 
it is not desired to limit the invention to the exact construction and 
operation shown and described, and accordingly, all suitable modifications 
and equivalents may be resorted to, falling within the scope of the 
invention.