Diversity receiver

The diversity receiver including plural antennas and one receiver comprises a detector means to detect a signal voltage responsive to an antenna output from an IF amplifier of the receiver and a comparator which is supplied with the signal voltage separated in two courses, one including a fixed bias applying circuit and the other including a time constant circuit, so as to select one of the antennas which is in a good receiving condition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a diversity receiver including plural antennas 
and one receiver so as to switch the antennas in response to their 
receiving conditions, and thereby suited for use in a car. 
2. Description of the Prior Art 
An FM receiver mounted in a car or other moving bodies is usually used 
while the body moves. Therefore, the direction of its antenna always 
largely changes as compared to a house-use FM receiver, and is apt to 
disaccord with directions of antennas of broadcasting stations. As the 
result, it is difficult to block influences by multipath noises. 
The receiver of this type is mostly used in a weak electric field because 
electric field strength for the receiver always varies as the car goes on 
from highways to mountainous places, or from valleys to valleys between 
tall buildings. Due to this, the receiver must deal with noises 
intermittently generated within the receiver itself during reception of 
weak input signals. 
One of conventional methods to overcome the above-mentioned problem is to 
change the receiver from stereophonic mode to monoral mode upon reception 
of weak input signals, or to attenuate high frequency band wave in 
addition to the foregoing mode changeover. By this method, however, since 
S/N (signal-to-noise ratio) improvement is done within the receiver to the 
input signal received by a single antenna, there is accordingly a limit in 
said improvement. 
Another method is to provide two antennas and two tuners, thereby 
constructing a so-called diversity receiver so as to switch and select the 
antennas and the tuners in response to the receiving conditions. Although 
this method is more effective in said S/N improvement than the foregoing 
method, provision of two tuners causes increase of production cost and 
space. 
OBJECT OF THE INVENTION 
It is therefore an object of the present invention to overcome the 
above-mentioned drawbacks involved in the prior art, and to provide a 
diversity receiver including plural antennas and a single receiver and 
capable of maintaining always good receiving condition of the receiver. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a signal voltage responsive to an 
input signal received by one of plural antennas and detected by a single 
receiver is separated into two routes for comparison of the separated 
voltages so that, the antennas are switched in response to the result of 
the comparison, and one of them which is in a good receiving condition is 
always selected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in detail by way of preferred 
embodiments referring to the drawings. 
FIG. 1 is a block diagram of an embodiment of the diversity receiver 
according to this invention. Reference numerals 1a and 1b refer to 
antennas, 2 to an antenna selecting circuit, 3 to an RF amplifier, 4 to an 
IF amplifier, 5 to a demodulating circuit, 6 to an audio amplifier, 7 to a 
speaker, 8 to a detector, 8' to a judging circuit, and 9 to a control 
circuit, respectively. The detector 8 detects, from IF signals of the IF 
amplifier 4, a signal voltage responsive to an output of the antenna 1a or 
1b. The judging circuit 8' detects, from the signal voltage, a dc voltage 
proportional to the electric field of a signal received by the antenna 1a 
or 1b, or detects a dc voltage obtained by comparison and computation of 
the electric field and the multipath level, and judges whether the 
receiving condition is good or not. The control circuit 9 makes the 
antenna selecting circuit excute or withhold its antenna switching action 
in response to a signal voltage from the judging circuit 8' so that the 
receiver is always connected to one of the antennas in a good receiving 
condition. 
FIG. 2 shows the judging circuit 8' which performs comparison and 
computation of the detection voltage detected by the detector 8 and 
generates a resultant output to actuate the control circuit 9. The 
detection voltage is separated into two courses (paths), namely, course A 
including a resistor R.sub.1 and a variable resistor R.sub.2, and course B 
including a time constant circuit consisting of resistors R.sub.3, R.sub.4 
and a capacitor C. The separated voltages are applied to a (-) terminal 
and a (+) terminal of the comparator, respectively, for comparison and 
computation thereby. In this case, while the (+) terminal is applied with 
the separated voltage after voltage division by the resistors R.sub.1 and 
R.sub.2, the (-) terminal is applied with the separated voltage after 
voltage division by the resistors R.sub.3 and R.sub.4. However, since the 
charging/discharging capacitor C is included in the course B, the 
separated voltage to the (-) terminal is affected by the time constant 
circuit. The (+) terminal is designed to normally receive a lower 
separated voltage than that to the (-) terminal. 
When the car runs carrying the receiver including said comparator 10 for a 
certain distance, output voltages with waveforms as shown by (a), (b) and 
(c) in FIG. 3 are obtained from the respective terminals of the comparator 
10. The waveform (a) corresponds to the separated output voltage applied 
to the (-) terminal via the route A and shows that this voltage varies in 
accordance with change of receiving condition. The waveform (b) 
corresponds to the separated output voltage applied to the (+) terminal 
via the route B, and shows that variation of this voltage is moderated due 
to the time constant circuit with combination of the resistor R.sub.3 and 
the capacitor C or with the combination of the resistor R.sub.4 and the 
capacitor C. The waveform (c) corresponds to the output supplied from the 
comparator 10 as the result of comparison and computation of the both 
separated voltages shown by the waveforms (a) and (b). The comparator 10 
supplies an H (high level) signal, for example, when the detector voltage 
lowers down the average level due to electric field variation caused by a 
bad receiving condition, and supplies an L (low level) signal when the 
detector voltage becomes higher than the average level, thereby allowing 
judgement that the receiving condition is good when the L signal is 
supplied, and bad when the H signal is supplied. 
The output signal from the comparator 10 is applied to the control circuit 
9. The control circuit 9 is operative only when the comparator 10 supplies 
the H level signal (when the receiving condition is bad) so as to actuate 
the antenna switching circuit 2. 
FIG. 4 shows properties of the input voltages Vi at the (-) and (+) 
terminals of the comparator 10 (ordinate) with respect to the antenna 
input signal Vo (abscissa). 
By adjusting the variable resistor R.sub.2 provided in the course A, the 
(-) terminal voltage may be increased or decreased as shown by the two 
head arrow in FIG. 4. This property teaches that antenna selection 
sensitivity is extremely good when the antenna input Vi is weak, and is 
bad when the antenna input Vi is sufficiently strong (changeover of the 
antennas is not effected unless the electric field strength extremely 
lowers). 
FIG. 5 shows another construction of the judging circuit 8'. D is a diode 
and R.sub.5 is a resistor connected in series to the variable resistor 
R.sub.2 to supply the (-) terminal with a forward fixed bias derived from 
a source voltage VCC. The diode D performs dc blocking and voltage level 
shifting of the signal from the detector 8. 
With this arrangement of the judging circuit 8', if the input impedance of 
the judging circuit 8' is sufficiently high upon reception of a weak 
input, its equivalent circuit is shown by FIG. 6, and the voltages V.sup.- 
and V.sup.+ at the (-) and (+) terminals of the comparator 10 are 
expressed by the following equations: 
##EQU1## 
FIG. 7 shows properties of the voltages at the respective terminals of the 
comparator 10 in the judging circuit 8' with respect to the antenna input 
signal. By supplying the (-) terminal of the comparator 10 with a forward 
fixed bias as described in the above, antenna selection sensitivity 
especially upon weak input reception can be lowered as compared to the 
property of FIG. 4. Due to this, since the antenna changeover is not 
effected even if the electric field strength varies more or less, too 
frequent antenna switching actions are prevented while the car passes 
through a weak signal region such as tunnel, for example. 
FIG. 8 shows still further properties of the voltages at the (-) and (+) 
terminals of the comparator 10. The comparator input voltages can be 
shifted by use of forward voltage drop of the diode D provided at the 
input end of the judging circuit 8'. The variation properties can be 
modified by increasing and adjusting the number of the diodes D. This 
Figure shows that the antenna switching sensitivity is dull upon weak 
input reception but sharp upon sufficient increase of the input. 
Whichever point the antenna switching action is effected or not can be 
selected as desired by adjustment of the variable resistor R.sub.2. 
When the resistance of the resistor R.sub.5 is larger than those of the 
other resistors so as to supply the (-) terminal of the comparator 10 with 
a slight forward fixed bias, it is effective in performance of its 
purpose. 
As described in the above, the receiver of this invention, including plural 
antennas and one receiver and designed to separate a signal voltage 
detected by the receiver into two courses to compare the separated 
voltages so as to switch the antennas in response to the output resulted 
from the comparison, effectively reduces multipath noises and improves S/N 
ratio with only one receiver, thereby leading to reduction of production 
cost and space of the diversity receiver. 
Especially when a fixed bias applying circuit is provided in one of the two 
courses, the output as the result of the comparison responsive to the 
input to the receiver can be freely determined so as to prevent too 
frequent changeover between the antennas, thereby reducing noises caused 
by the changeover. Further, the separated voltages can be adjusted so as 
to sharpen the antenna switching sensitivity when the input is 
sufficiently large.