Oscillators – Solid state active element oscillator – Transistors
Patent
1990-03-07
1991-04-23
Grimm, Siegfried H.
Oscillators
Solid state active element oscillator
Transistors
331 45, 331 60, 331 75, 331159, H03B 536
Patent
active
050103083
DESCRIPTION:
BRIEF SUMMARY
This invention relates to oscillators particularly reference oscillators for use in phase locked loops.
There is a requirement for oscillators capable of providing a clean, well balanced stable output frequency for use in phase locked loops to control, for example, the frequency of some other local oscillator the output of which is mixed with a received signal in a tuner to provide an appropriate intermediate frequency.
Conventionally, a crystal oscillator has been used to provide a reference oscillator due to the inherent stability of the crystal. A simple circuit is shown in FIG. 1 in which the crystal 10 and a tuning capacitor 11 are connected between the emitters of a pair of transistors 12, 13 to the respective bases of which an input signal is differentially fed. The output of such a circuit, taken from the collectors of the transistors 12, 13 is shown in FIG. 2. The output comprises asymmetric signals and is rich in harmonics. If such a reference oscillator, operating at, for example, 4 MHz, is for use in a television tuner having an intermediate frequency of, for example, 39 MHz and a bandwidth of 6 MHz, the 9th, 10th and 11th harmonic of the output of the reference oscillator may appear on the I. F. signal and cause patterning on the video display.
FIG. 3 shows a better circuit for a reference oscillator. The circuit provides a linear sinusoidal output. In this form, the crystal 10' is connected between the collector of one transistor T.sub.1 of a differential pair T.sub.1 and T.sub.2 and the base of the transistor T.sub.2. Bond pads 14, 15 permit external components such as the crystal 10' and capacitor 11' to be connected to the circuit. The output of such an oscillator, again taken from the collectors is shown in FIG. 4. It is well balanced and has much lower radiation levels but is of low gain at the oscillation frequency. However, present in a digital TV tuner are data signals. These are square wave signals at a frequency of approximately 100 KHz and have very fast rise times giving rise to radiation which appears as spikes on the output of the oscillator. Such spikes 16 and 17 are shown in FIG. 4. If the spikes (spikes 16) occur on the linear portions of the output, problems arise in a divider normally provided downstream of the reference oscillator. Spikes 17 have little effect. The spikes 16 cause misdivision, the phase locked loop becomes astable and hence control of the loop by the reference oscillator becomes difficult.
It is an object of the present invention to provide an oscillator wherein the aforesaid disadvantages are minimized or obviated.
According to the present invention, there is provided a crystal oscillator comprising a pair of differentially arranged transistors to the bases of which an input signal may be differentially applied, a crystal and a capacitor being connectable between the collector of a first of the transistors and the base of a second of the transistors, the oscillator outputs being provided differentially on the collectors of the first and second transistors, and a feedback loop established by an output of one of the transistors serving to drive a further transistor connected as a load in the collector of the other of the transistors.
The invention will be described further, by way of example, with reference to the accompanying drawings, in which:
FIGS. 1 to 4, as hereinbefore described, relate to known oscillators;
FIG. 5 is a circuit diagram of an oscillator constructed in accordance with the present invention; and
FIG. 6 is a graphical representation of the output of the oscillator shown in FIG. 5.
Referring to FIGS. 5 and 6, an oscillator in accordance with the present invention comprises a differential pair of NPN transistors T.sub.1, T.sub.2 having external components, a crystal 10 and a capacitor 11 (shown in broken lines), coupled via bond pads 14, 15 between the collector of transistor T.sub.2 and the base of transistor T.sub.1.
An input signal is provided to the bases of transistors T.sub.1 and T.sub.2. The emitters of the transistors T.sub.1 and T
REFERENCES:
patent: 3824491 (1974-07-01), Treadway
patent: 4128817 (1978-12-01), Gomi
patent: 4234858 (1980-11-01), Gomi
patent: 4587500 (1986-05-01), Tanaka et al.
Evans, "Crystal Oscillator", IBM Technical Disclosure Bulletin, vol. 9, No. 2, Jul. 1966.
Grimm Siegfried H.
Plessey Overseas Limited
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