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United States Patent 
Dolman et al.
 Feb. 7, 1984
 OSCILLATOR CONTROL CIRCUIT
 Inventors: Rodney A. Dolman, Lynchburg;
Gregory N. Mears, Forest, both of
 Assignee: General Electric Company,
 Appl. No.: 350,981
 Filed: Feb. 22,1982
 Int. C1.3 H04B 1/44; H04B 1/50
 U.S. CI 455/78; 455/87
 Field of Search 455/78, 79, 86, 87,
 References Cited
U.S. PATENT DOCUMENTS
3,153,194 10/1964 Orwin 455/87
3,300,720 1/1967 Kowols 455/86
4,095,183 6/1978 Kakigi 455/78
FOREIGN PATENT DOCUMENTS 613217 11/1948 United Kingdom .
An oscillator is controlled so that it produces the correct frequency in the transmitting condition, and so that it produces a slightly different frequency in the receiving condition. This is achieved by a circuit which supplies a first control voltage to the oscillator for transmitting and which supplies a second and different control voltage to the oscillator for receiving. This second and different control voltage causes the oscillator frequency in the receiving condition to be different so that there is no interference to or improper operation of a repeater system. The circuit also prevents any modulation signals from reaching the oscillator in the receiving condition, thus preventing false signals or other problems.
2 Claims, 3 Drawing Figures
U.S. Patent Feb. 7, 1984 Sheet 1 of 2 4,430,756
OSCILLATOR CONTROL CIRCUIT
BACKGROUND OF THE INVENTION
Our invention relates to an oscillator control circuit, ^ and particularly to a circuit for controlling the frequency of a single oscillator that supplies signals to a radio transmitter and to a radio receiver that operate with a radio repeater or similar installation. 10
In some radio communication systems, a station will transmit on a first frequency and receive on a. second frequency. Thus, for example, a mobile radio station on a vehicle might transmit to a base station or repeater on a frequency of 800 MHz and receive on a frequency of 55 845 MHz. The repeater of course would receive on the frequency of 800 MHz and transmit on the frequency of 845 MHz. In the interests of economy and simplification, the mobile station preferably would have only one stable oscillator which produces a 800 MHz signal for 20 transmitting, and which mixes or heterodynes the 800 MHz signal with the received 845 MHz signal to produce an intermediate frequency of 45 MHz which is detected for utilization. However, in the receiving condition, some amount of the 800 MHz oscillator signal is 25 radiated, so that if the mobile station is close to the base station or repeater, this 800 MHz signal will capture or control the repeater even though the mobile station does not to intend to do so. This of course prevents other mobile stations from using the repeater on that 30 frequency or channel.
In addition, the single oscillator must not or should not be modulated in the receiving condition, as such modulation causes noise or other problems in the receiving condition, and may cause false alerting in a tone ^ controlled system.
SUMMARY OF THE INVENTION
Our invention provides an improved arrangement for controlling an oscillator so that it produces the correct frequency in the transmitting condition, and so that it produces a slightly different frequency in the receiving condition. This is achieved by a circuit which supplies a first control voltage for transmitting and which supplies 4J a second and different control voltage for receiving. This second and different control voltage causes the oscillator frequency in the receiving condition to be different so that there is no interference to or improper operation of a repeater system. Our arrangement also 50 prevents any modulation signals from reaching the oscillator in the receiving condition, thus preventing false signals or other problems.
BRIEF DESCRIPTION OF THE DRAWING
The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of our invention, as well as advantages, may be better understood from the following description given in connection with the ac- go companying drawing in which:
FIG. 1 shows a pictorial view of a communication or repeater system which can utilize our invention;
FIG. 2 shows a block diagram of a radio transmitter and receiver utilizing our improved oscillator control 65 circuit; and
FIG. 3 shows a schematic diagram of a preferred embodiment of our improved oscillator control circuit.
DESCRIPTION OF THE PREFERRED
FIG. 1 shows a pictorial view of a radio repeater or communications system with which our invention is advantageously used. Persons skilled in the art will appreciate, of course, that our invention can also be used in other radio systems or arrangements. In such a system, a base station or repeater provides radio communications to and from a number of vehicular or mobile units. Such a system may have a number of spaced channels. As shown by way of example for one such channel, the repeater transmits on a frequency of 845 MHz to the mobile unit. The mobile unit transmits on a frequency of 800 MHz to the repeater. If the mobile unit has a single stable oscillator of 800 MHz, even though the mobile unit is in the receive condition, this 800 MHz signal may, when the mobile unit is close to the repeater, capture or lock up the repeater. As a result, the channel can not be used by any other mobile unit, and is wasted. At the very least, this is an undesirable condition.
To overcome this problem, we provide an improved control circuit that changes the frequency of the stable oscillator slightly from its nominal or normal transmitting frequency (800 MHz in the example) so that in the receiving condition, the repeater is not captured by any radiated signal. With reference to FIG. 2, our control circuit is represented by a block 12. Input or information signals to be transmitted are supplied by a microphone 10 and amplified if need be by an amplifier 11. The output of the amplifier 11 is connected to a switch SI (which may be a solid state device) for selective application to our control circuit 12 when the switch SI is actuated for transmitting. Our control circuit 12 is controlled by a switch S2 which is supplied by a suitable source of positive direct current voltage when the switch S2 is unactuated for receiving. The output of our circuit 12 is connected to a voltage controlled crystal oscillator 13. The output of the oscillator 13 may, if desirable or necessary, be applied to a multiplier 14 which multiplies the fundamental oscillator frequency up to the desired frequency, which in the assumed example is 800 MHz. Subsequently, when reference is made to the oscillator frequency, that frequency may be the actual oscillator frequency multiplied to a higher frequency. The output of the multiplier 14 is applied to a switch S3 which has an unactuated or receive terminal R and an actuated or transmit terminal T. The transmit terminal T of the switch S3 is connected to a suitable filter and power amplifier 20 whose output is connected to a transmit terminal T of a switch S4. The switch S4 is connected to the antenna 24 of the mobile unit. The receive terminal R of the switch S3 and the receive terminal R of the switch S4 are connected to the inputs of a mixer 21 which produces the difference of the frequency supplied by the oscillator 10 and the frequency of the received signal. Thus, for the assumed example, the oscillator frequency of 800 MHz and the received frequency of 845 MHz produces an intermediate frequency of 45 MHz. This intermediate frequency is amplified and detected in the block 22 and audio or other output signals supplied to a loud speaker 23. As indicated by the dashed line, the switches SI, S2, S3, S4 may be actuated at the same time, such as by a push-totalk button. When so actuated, the switches are moved from their lower position (as shown) to their upper position.