AN OSCILLATOR

Abstract: In a differential oscillator means (535, 553, 521, 523, 549 and 551) are provided for making it possible to band switch the oscillator. This is obtained by means of cross connecting two transistors (501 and 503) and connecting inductors (509 and 511) to the transistors (501 and 503). The oscillator can be band switched by means of shortening a circuit comprising of two diodes (521 and 523). The switching time for the oscillator is very short since the oscillating circuit does not comprise any serially connected capacitors. In addition all frequency bands of the oscillator can be made impedance matched by means of adding an additional pair of varactor diodes (529 and 531).

Description: AN OSCILLATOR TECHNICAL FIELD The present invention relates to oscillator circuits. BACKGROUND OF THE INVENTION AND PRIOR ART Voltage controlled oscillators (VCOs) are widely used for purposes where a frequency, which needs to be varied over time, is required. One conventional manner for obtaining such an oscillating circuit is to employ one or two varactor diodes, the capacitance of which can be controlled by means of a variable D.C. voltage. Furthermore, conventional oscillators cannot usually be used for obtaining a frequency varying more than from a lowest frequency to a frequency about twice that frequency, i.e. conventional voltage controlled oscillators can usually only provide output frequencies from a certain frequency f to two times that frequency 2f. However, for many applications such a limited frequency range, i.e. from f to 2f, is insufficient. Such applications can be TV- tuners etc. Thus, there is a need for an oscillator which can generate any frequency ranging over a large bandwidth. This can be obtained by means of band switching a conventional VCO, such as described in the Dutch patent application NL,B 145 422, for example. However, the band switched oscillator as described in the Dutch patent application NL,B 145 422 is mainly intended for TV tuners from the late 60' s, and is not equipped with any differential oscillators. In this Dutch patent application band switching is performed using a switch diode, a capacitor and a serial resistor. The capacitor and the resistor form an RC network which slows down the switching time. The delay imposed on the switching time by the RC network depends on the values on the resistor and the capacitor used. The oscillator as described in NL,B 145 422 suffers from a number of drawbacks, such as long band switching time due to the serial capacitors used in the resonance circuit, which take time to discharge. In applications when it is necessary to switch band quickly it is advantageous to use oscillators not having an RC network which has to be charged or discharged for obtaining the switching. In some applications the requirements on switching time are set to values impossible to achieve when using oscillators comprising an RC network. SUMMARY OF THE INVENTION It is an object of the present invention to provide an oscillator, which has a short band switching time in comparison to prior band switched oscillators. This object and others is obtained by using a differential band switched oscillator circuit, which does not comprise any serially connected capacitors that need to be charged or discharged when switching bands in the switching circuit, and hence having a short band switching time. Thus, the only capacitance in the resonance circuit is the capacitance of serially connected varactor diodes used for varying the oscillator frequency plus stray capacitances. The resistors used in the circuit do not affect the Q-value since they are located in AC zero points With such a differential band switch oscillator, the switching time mainly depends on the time for forwarding and reversing the switch diodes. Thus, the switching time can typically be reduced to less than 1 μs. Across the switching diodes it is possible to have varactor diodes which help the oscillators to cover a very wide range since the reactance area in each band can be controlled to a desired range. DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: - Fig. 1 shows a differential band switch oscillator. - Fig. 2 shows an oscillator similar to the one shown in fig. 1, having two pairs of varactor diodes - Fig. 3 shows an oscillator similar to the one shown in fig. 1, which can be switched between three different frequency bands. - Fig. 4 shows an oscillator similar to the one shown in fig. 3, having two pairs of varactor diodes - Fig 5 shows yet another oscillator. DESCRIPTION OF PREFERRED EMBODIMENTS In fig. 1 a differential band switched oscillator is shown. The oscillator comprises two transistors 101 and 103. The base of each of the two transistors is connected to the collector of the other transistor. The emitters of the two transistors are connected to each other, and a resistor 105 connects the emitters with a point 107 having a given potential, for example -4V. The collectors of the two transistors 101 and 103 are further connected to inductors 121 and 123, respectively, and to varactor diodes 131 and 133, respectively. The other ends of the varactor diodes 131 and 133 are connected to each other and to a variable voltage source which can be connected at a point 125 via a resistor 129. The ends of the inductors 121 and 123, which are not connected to the collectors of the transistors 101 and 103, are connected to diodes 117 and 119, and to ground via inductors 135 and 137. The other ends of the diodes 117 and 119 are connected to a voltage source, which can be connected to a point 109, via a resistor 111. The oscillating circuit in fig. 1 then oscillates as follows. If a positive voltage is applied at the point 109 (+ 5V) , a current flows through the resistor 111. This results in that the path over the diodes 117 and 119 between the points 113 and 115 is shortened. The oscillating circuit then oscillates with a frequency mainly determined by the values on the coils 121 and 123, which are connected in series due to the shortening of the path between the points 113 and 115, and the value on the voltage applied at the point 125 supplying a current to the point 127 via the resistor, which in turn determines the capacitance of the two varactor diodes 131 and 133. If on the other hand a negative voltage (-5V) is supplied to the point 109, no current flows through the path between the points 113 and 115. This results in that not only the two coils 121 and 123 become part of the oscillating circuit but also the coils 135 and 137 now become connected in series in the oscillating circuit. Thus, in this latter case the oscillating frequency is determined by the total inductance of the four coils 121, 123, 135 and 137 in series with the capacitances of the two varactor diodes 131 and 133. The value of the capacitance of the two varactor diodes is in this case given by the voltage supplied at the point 125. Hence, by changing the switching voltage supplied to the point 109 two different frequency bands can be chosen for the oscillating circuit. The frequency in these different frequency bands can be varied by means of varying the voltage supplied to the point 125. The output signal from the differential oscillator is preferably supplied at either of the points 139 or 141 or both, if two output signals are desired, for example one output signal and one reference signal, which can be used for tuning of the oscillator. The purpose of the resistor 105 is to make the current from the emitters of the transistors 101 and 103 constant. Other conventional methods for keeping the current constant are of course possible to apply. In another preferred embodiment, an extra pair of varactor diodes is connected in parallel with the path between the points 113 and 115. This is shown in fig. 2, which has the same reference numerals as fig. 1, but increased by one hundred. The reason for adding this extra pair of varactor diodes is to make the oscillating circuit impedance matched in both frequency bands . Thus, the two varactor diodes 243 and 245 are controlled by a voltage supplied at the point 247 which feeds a current through the resistor 249 to the point 251. The same voltage is applied to the terminals at the points 225 and 247, respectively, whereby the circuit becomes impedance matched. One additional advantage of the oscillator as described herein is that it very easily can be extended to cover additional frequency bands. How this is achieved is shown in fig. 3 having the same reference numerals as fig. 1 but increased by two hundred. Thus, the oscillator shown in fig. 3 has an additional pair of switching diodes 353 and 355 and an additional pair of coils 357 and 359 added in a ladder structure to the oscillator shown in fig. 1. The path between the points 361 and 363 can like the path between the points 313 and 315, also be shortened by means of applying a voltage to the point 365 resulting in a current through the resistor 367. Hence, by shortening the path between the points 313 and 315 a first frequency band is selected. When another frequency band is desired, the shortening of the path is switched off by means of changing the voltage supplied to the point 309, and the path between the points 361 and 363 is shortened. If a third frequency band is desired, both short paths, i.e. between the points 313 and 315 and between the points 361 and 363 are switched off. This results in that all six coils 321, 323, 335, 337, 357 and 359 become part of the oscillation circuit. It is also understood that the ladder structure as described in conjunction with fig. 3 can be extended to a desired number of steps, and thereby making it possible to obtain an arbitrary number of frequency bands. The oscillator being capable of providing three or more frequency bands can of course also be provided with impedance matching additional varactor diodes as described in conjunction with fig. 2. Such an oscillator arrangement is shown in fig. 4 which has the same reference numeral as fig. 2 and 3 but increased with one and two hundred, respectively. Fig. 5 shows yet another embodiment of an oscillator wherein values of the component also are indicated. Thus, fig. 5 shows two transistors 501 and 503. The base of the transistor 501 is connected to the collector of the transistor 503 and vice versa. The emitters of the two transistors 501 and 503 are both connected to one terminal of a 3.3 kohm resistor 505, the other terminal of which is connected to a constant negative voltage of - 4V at a point 507. The collectors of the two transistors 501 and 503 are further connected to 8.2 nH inductors 509 and 511, respectively, and to varactor diodes 513 and 515, respectively. The other ends of the varactor diodes 513 and 515 are connected to each other and to a variable voltage source 517 via a 10 kohm resistor 519. The variable voltage source 517 can generate a voltage in the range of +0.5V - +25V. The ends of the inductors 509 and 511, which are not connected to the collectors of the transistors 501 and 503, are connected to diodes 521 and 523, to 8.2 nH inductors 525 and 527, and to varactor diodes 529 and 531, respectively. The other ends of the diodes 521 and 523 are connected to each other via a 10 nF capacitor 533, and to a voltage source 535 via 1 kohm resistors 537 and 539. The voltage generator can generate a negative voltage of -5V and a positive voltage of +5V. The other ends of the varactor diodes 529 and 531 are connected to each other, and to the variable voltage source 517 via a 10 kohm resistor 541. The ends of the inductors 525 and 527 which are not connected to the varactor diodes 529 and 531, respectively, are connected to ground via 22 nH inductors 543 and 545, to each other via a 10 nF capacitor 547, and to diodes 549 and 551, respectively. The other ends of the diodes 549 and 551 are connected to a voltage source 553 via 1 kohm resistors 555 and 557, respectively. The oscillator in fig. 5 can be band switched in the corresponding manner as described above in conjunction with fig. 2. Furthermore, as a difference to the oscillator shown in fig. 2, the oscillator in fig. 5 has capacitors 533 and 547 connected between the diodes 521 and 523, and 549 and 551, respectively. This configuration does not form an RC network. The purpose of the capacitors 533 and 547 is to correct for different forward voltage drops for the switch diodes. Such an embodiment can be advantageous when the switch diodes are not located on the same chip. The oscillators as described herein has a number of advantages compared to previously known band switched oscillators. In particular they have a very short switching time, since they do not comprise any serial connected capacitors in the switch circuit that need to be charged or discharged during switching. Furthermore, it is possible to very easily make them impedance matched.

Claims: CLAIMS 1. An oscillator having means for band switching the oscillator, characterized in that the band switching means comprises a path of two serially connected diodes, which path can be shortened by means of applying a voltage between the diodes. 2. A band switched oscillator, characterized in that the only capacitance in the oscillating circuit of the oscillator is the capacitance of serially connected varactor diode (s) . 3. A band switched oscillator according to claim 2, characterized in that the oscillator comprises two cross connected transistors 4. An oscillator, characterized by - two transistors (101, 103; 201, 203; 301, 303; 401, 403; 501, 503), the base of each of the two transistors ((101, 103; 201, 203; 301, 303; 401, 403; 501, 503) being connected to the collector of the other transistor, and the emitters of the two transistors being connected to each other, - two inductors (121, 123; 221, 223; 321, 323; 421, 423; 509, 511) connected to and to a collector of the transistors (101 and 103) each, and to ground via inductors (135, 137; 235, 237; 335, 357, 337, 359; 435, 457, 437, 459; 525, 543, 527, 545), - two varactor diodes (131, 133; 231, 233; 331, 333; 431, 433; 513, 515) connected to each other and each connected to one end of the inductors (121, 123; 221, 223; 321, 323; 421, 423; 509, 511) , respectively, and - two diodes (117, 119; 217, 219; 317, 319; 417, 419; 521, 523) connected to each other and to the other ends of the inductors (121, 123; 221, 223; 321, 323; 421, 423; 509, 511), respectively. 5. An oscillator according to claim 4, characterized by - a resistor (105; 205; 305; 405; 505) interconnecting the emitters with a fixed potential (107; 207; 307; 407; 507), for obtaining a constant current from the emitters of the transistors (101, 103; 201, 203; 301, 303; 401, 403; 501, 503). 6. An oscillator according to any of claims 4 or 5, characterized by - a resistor (129; 229; 329; 429; 519) connected in one end to a point between the varactor diodes (131, 133; 231, 233; 331, 333; 431, 433; 513, 515), and the other end of which can be connected to a variable voltage supply. 7. An oscillator according to any of claims 4 - 6, characterized by - a resistor (111; 211; 311; 411; 537, 539) connected in one end to a point between the diodes (117, 119; 217, 219; 317, 319; 417, 419; 521, 523), and the other end of which can be connected to a voltage supply. 8. An oscillator according to any of claims 4 - 7, characterized by - a capacitor (533) connected between the diodes (117, 119; 217, 219; 317, 319; 417, 419; 521, 523). 9. An oscillator, characterized by two cross connected transistors, a separate inductor connected to each transistor, whereby the oscillating frequency being essentially determined by the inductances of the inductors. 10. An oscillator according to claim 9, characterized by varactor diodes interconnecting the separate inductors, and a variable voltage source attached between the varactor diodes, by means of which the oscillating frequency of the oscillator can be adjusted. 11. An oscillator according to claim 9 or 10, characterized by switching means for connecting inductors having different inductances to the transistors, for making the oscillator oscillate at different frequencies. 12. An oscillator according to any of claim 8 - 11, characterized in that each inductor comprises a first inductor and a second inductor connected in series with each other, the interconnections between a first inductor and a second inductor being connected to voltage source having two levels, a first level when only the inductances of the first inductors essentially determine the oscillating frequency and a second level when the sum of the inductances of the first and second inductors essentially determine the oscillating frequency.

Inventors: BLADH MATS, BLADH, MATS
Applicants: BLADH MATS, ERICSSON TELEFON AB L M, BLADH, MATS, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1997
Main classification: H03B5
IPC codes: H03B1, H03B5