Crystal Oscillator Having Switchably Disabled Harmonic-Tuned Output Circuit

The invention relates to an oscillator comprising a one-stage amplifier and a quartz crystal resonator which operates in series resonance to an LC oscillating circuit.

In order to produce high frequencies with the aid of quartz crystal oscillator circuits, it is known to use harmonic quartz crystals which oscillate at a specific mechanical harmonic of the oscillating quartz crystal in oscillator circuits. As quartz crystals are very difficult to produce for very high frequencies and consequently are expensive, in specific circumstances it can be more economical to use a harmonic quartz crystal with half the desired frequency and to double this frequency for example with the aid of a following multiplier stage.

A plurality of series connected stages which are each provided with an amplifier have the disadvantage, however, that this impedes an exact tuning of the oscillating circuit to the frequency which is to be produced.

If oscillators provided with oscillating quartz crystals are used for example to drive acousto-optical deflecting devices in non-mechanical high-speed printers, it is neccessary for such oscillators to be switched on and off very rapidly. Short switching times on the order of approximately 100 ns cannot be achieved, however, in known oscillator circuits having only one stage such as disclosed in Halbleiterschaltungstechnik by Tietze and Schenk, 2nd Edition, 1971, page 371.

An object of the invention is to make available a quartz crystal oscillator having a one-stage amplifier which facilitates extremely rapid on and off switching.

This object is realized in accordance with the invention by an oscillating circuit which is turned to a harmonic of the oscillator frequency and which is connected via a filter device to an output of an amplifier stage and with a switching device which short-circuits the oscillating circuit as and when required.

In a particularly advantageous embodiment the switching device consists of a first diode path of series-connected switching diodes and a following, second diode path with parallel-connected switching diodes, one of the parallel diodes being connected to the oscillating circuit and the other of which is connected to ground potential. A switching arrangement is provided which precedes the diode paths in the blocking direction of the diodes and short-circuits the oscillating circuit as and when required by feeding the diode paths with an operating voltage.

The invention advantageously allows the production of an oscillator of very high frequency in a single stage which can nevertheless be switched extremely rapidly. The output voltage which corresponds to double the oscillator frequency is switched on and off rapidly by short-circuiting the output oscillating circuit.

This is facilitated in that the switching speed is determined only by the relatively low quality of the output circuit, whereas the quartz crystal itself continues to oscillate uninterrupted in spite of the short-circuited output circuit and consequently build-up processes which would otherwise be required can be dispensed with.

The drawing is a schematic illustration of the circuit arrangement corresponding to the invention.

The oscillator which is shown in the drawing and which serves to produce HF oscillations in the frequency range from 200-250 Mhz and an on/off time of less than 100 ns consists of a harmonics quartz crystal 1 with an oscillating frequency of approximately 100-120 Mhz. This quartz crystal resonator operates in series resonance with a parallel oscillating circuit which is coupled to the base of a transistor 2 and which consists of a coil 3 and a capacitor 4. The oscillating circuit connected to the base of the transistor is not tuned to the oscillator frequency, but somewhat lower. As a result the 180.degree. phase rotation between collector voltage and base voltage which is required for feedback is achieved. Just above its series resonance the quartz crystal in fact exhibits an inductive impedance and the base oscillating circuit operates capacitively above its parallel resonance.

The collector of the transistor 2 is connected to a small capacitor 5 which acts as a high-pass filter and couples a parallel oscillating circuit which is connected following the capacitor 5. This parallel oscillating circuit is tuned to the second harmonic of the oscillator frequency and consists of a capacitor 6 and a coil 7. This separation of multiplier and actual oscillator facilitates a very rapid switching on and off of the overall oscillator. This switching on and off is carried out with the aid of a switching device which short-circuits the oscillating circuit 6, 7 as and when required.

The switching device comprises a first diode path of series-connected switching diodes 8 and 9 and a second diode path which is arranged between the oscillating circuit and the first diode path with parallel-connected switching diodes 10 and 11. Here a diode 10 is coupled to ground, whereas the other diode 11 is connected to the oscillating circuit. The diodes are preceded in the blocking direction by a voltage divider which is composed of the resistors 12 and 13, and on the one hand is connected to an operating voltage source 14 and on the other hand to a switch 15. The switch 15 has been illustrated only by way of example. A logic switching element which is controlled in dependence upon level can also be provided, for example, in place of the switch.

The oscillating circuit 6 and 7 is short-circuited, and consequently via a resistor 16 so is the output 17 across the series connection of the diodes 10 and 11, and in fact whenever the diode 10 is conductive on account of the current with which it is impressed via the resistors 12 and 13. This is the case whenever the switch 15 is open and thus the potential 14 is present through the resistors 12, 13 and the diodes 8, 9, and 10. For such time as the switch 15 is closed and thus resistor 12 is connected to zero potential, the oscillating circuit 6, 7 is not attenuated.

The transistor current and thus the high frequency output voltage can be modified with the aid of resistors 18 and 19 arranged in the emitter circuit of the transistor 2. The tapping, illustrated in the Figure, of the coil 3 and thus of the base oscillating circuit, improves the adaptation of the collector resistance across the transistor 2 to the base resistance. This enables the regulating range of the output amplitude to be increased. The operating voltage source 14 is also connected to resistors 20, 21 and 22 and to capacitors 23 and 24 and, in the emitter circuit, a capacitor 25.

Although various minor modifications may be suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent warranted hereon, all such embodiments as reasonably and properly come within the scope of my contribution to the art.