Telecommunications – Receiver or analog modulated signal frequency converter – Plural receivers
Reexamination Certificate
1997-02-26
2001-07-03
Maung, Nay (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Plural receivers
C455S273000, C455S277100
Reexamination Certificate
active
06256484
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to diversity radio receivers.
In one class of diversity radio receivers, a plurality of antennas are positioned to receive signals transmitted from a single transmitter. The circuitry connecting the antennas to the rest of the diversity receiver reduces fading caused by multipath cancellation of the signals from the single transmitter. In some such receivers a signal indicating the strength of the received signal controls the circuitry in such a manner as to minimize multipath cancellation of signal.
In some prior art diversity receivers of this class the signal from the squelch circuit controls the switching of the receiver from antenna to antenna to obtain the best signal. A phase shift is imposed between the signal received from one antenna and the signal from the other antenna. In other prior art diversity receivers, signals from both antennas are added together and the phase is continually changed in accordance with changes in the sum of the signals. In still another type of diversity receiver of this class, the signals from the antennas are modulated with an average frequency carrier to remove phase shift components and the signals are added together to provide a total signal stronger than the individual signals received by each antenna.
The prior art space diversity receivers have the disadvantage of being relatively complex and thus expensive. Some are complex because the signals are applied from each antenna to a corresponding one of several portions of receivers before being added for final processing. Moreover, some of the circuits for correcting for phase shift, or for switching from antenna to antenna or receiver to receiver or for modulation of signals are complicated and expensive.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a novel method and apparatus for reducing fading from radio systems.
It is a still further object of the invention to provide a simple and inexpensive space diversity radio receiver.
In accordance with the above and further objects of the invention, a space diversity radio system includes at least first and second antennas, antenna switching circuitry, receiver circuitry and a means for detecting a drop in the signal strength received by the receiver circuitry. A one-half wavelength phase shift is provided between the first and second antennas. In the preferred embodiment, the phase shift is obtained by physically spacing the antennas from each other by one-half of a wavelength.
Only one of the first and second antennas is always connected to the first stage of the receiver. The other antenna is connectable to one antenna through a switch. Switching circuitry closes or opens the switch to connect one antenna or a plurality of antennas to the rest of the receiver circuitry when the signal strength drops below a certain threshold. The switching circuitry includes a flip-flop that is switched to connect another antenna to the receiver circuitry when the signal strength drops.
If the signal strength increases as a result of the addition of the antenna, the receiver circuitry remains connected to the antenna, but if it decreases again and remains decreased, the switching circuitry switches back to disconnect the second antenna. In another embodiment, a second threshold is provided that is activated by the squelch signal to cause the switching circuit to connect or disconnect antennas upon a receiving sufficiently low signal strength.
The switching circuit to which the received strength indicator signal is applied conducts it through two paths with opposite polarity output to a NAND circuit. One of the two paths includes a time delay in the form of a one shot multivibrator but the other switches the NAND circuit after only a short propagation delay. Consequently, if the received signal strength indicator signal doesn't improve in a time shorter than the delay time of the one shot multivibrator, the switching circuit will change states again. When the flip-flop is activated by the NAND gate, it applies a potential to a diode switch connecting the two antennas. When a positive signal is applied to the anode of the diode, the signals on the two antennas that are separated by half a wavelength are combined. When there is a negative signal applied to the anode, the diode is blocked and only the signal from the first antenna is applied to the first stage of the radio receiver.
In operation, a transmitted signal is received by each of the first and second antennas along the multipath transmission path between the transmitter and the receiver. When the intensity of a signal from the one antenna or antennas connected to the first stage of the receiver falls, the received signal strength indicating signal begins to drop and causes the antennas to change configurations, and after the delay time, if it is still dropping, a NAND circuit energizes a flip-flop to again switch causing a change in configurations. When the flip-flop changes states, the switch connecting the antennas switches states between its open and closed condition. If the signal does not continue to drop nor remain low, the NAND gate does not open and the flip-flop does not change states. Since the flop-flop has not changed states, the switch does not change states and the antennas remain in their previous groupings. In a second embodiment, if the signal continues to decrease, a lower threshold opens the NAND gate to switch antenna configurations. The required amount of delay time to determine if switching is needed can be adjusted for the location and circumstances.
While the diversity receiver has been described as having two antennas, one-half wavelength apart, other combinations of antennas may be used including, for example, two antennas spaced at other increments than one-half wavelength for other combinations or more than two antennas. Moreover, while a diode switch is preferable such as a PIN diode switch because of its economy, other switches may be utilized such as an FET switch or the like.
From the above description, it can be understood that the space diversity receiver system of this invention has several advantages such as for example: (1) it is relatively inexpensive and simple in construction; (2) it is less subject to false switching; (3) it can be adjusted easily to test for a weak signal for different periods of time before switching; and (4) it is very reliable.
REFERENCES:
patent: 4293955 (1981-10-01), Gehr et al.
patent: 5241701 (1993-08-01), Andoh
patent: 5361404 (1994-11-01), Dent
patent: 5392054 (1995-02-01), Bottomley et al.
patent: 5697075 (1997-12-01), Kim
Basine Robert B.
Conner Charles E.
Carney Vincent L.
Maung Nay
Telex Communications, Inc.
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