GB2203007A

GB2203007A - Oscillator stabilisation

Info

Publication number: GB2203007A
Application number: GB08807160A
Authority: GB
Inventors: Colin Stuart Aitchison
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1987-03-30
Filing date: 1988-03-25
Publication date: 1988-10-05
Also published as: GB8707509D0; GB8807160D0

Abstract

An apparatus for generating electrical oscillations at a predetermined frequency f1, comprises a main oscillator (10) for generating a first signal at said frequency f1; a second oscillator (12) for generating a second signal at a frequency f1/n, where n is a positive integer; a divider (11) coupled to the main oscillator (10) to divide the frequency of the first signal by n to produce a third signal; a phase comparator (13) for comparing the phase of the third signal and the phase of the second signal to produce an error signal indicating a difference in phase therebetween; and means (15) responsive to the error signal and operative to change the frequency of oscillation of the main oscillator (10) to reduce the error signal. Thus the frequency of oscillation of the main oscillator (10) is controlled by the second oscillator (12). The apparatus may also comprise frequency multiplying means (16) coupled to the output of the main oscillator (10) to multiply the frequency f1; and the filter means (17) coupled to the frequency-multiplying means (16) to select a desired frequency of mf1, where m is a positive integer. The apparatus forms the first local oscillator of a double-superheterodyne microwave receiver. <IMAGE>

Description

OSCILLATOR STABILISATION This invention relates to stabilisation of the frequency of an oscillator, such as an oscillator for use in a frequency converter operable at a high frequency, for example in the X-band.

By use of a communications satellite, television signals and data signals can be transmitted over very long distances, using a microwave link. For example, such signals may be transmitted by a transmitting station reflected or relayed by a satellite, and received by a receiving station, or by receivers owned by individual customers. This is the basis of direct broadcasting by satellite" (DBS) systems.

The signals transmitted in such systems generally lie in the X- or Ku-band, and a carrier frequency around 12GHz is commonly used. The received signal is usually fed to a double-superheterodyne receiver, in which the modulated carrier is mixed with a signal from a first local oscillator at around 11GHz, to produce a first intermediate frequency (IF) signal at around 1GHz. The first IF signal is amplified and mixed with a signal from a second local oscillator to produce a second IF signal at around 70MHz. That signal is then amplified and demodulated. It is essential to achieve high stability in the frequency of the first local oscillator.

In known systems stabilisation of the first local oscillator has been effected by positioning a dielectric resonant cavity close to the oscillatory circuit.

However, use of such a device has not proved satisfactory.

A source of trouble in such a system is excessive width of the spectrum of the output of the first local oscillator. In particular, phase noise in the oscillator output can be very troublesome, particularly when phase modulated data signals are being received. Such noise can cause a high data bit error rate in the received demodulated signal, particularly at low bit rates.

According to the invention there is provided apparatus for generating electrical oscillations at a predetermined frequency fl, comprising a main oscillator for generating a first signal at said frequency fl; a second oscillator for generating a second signal at a frequency f1/n, where n is a positive integer; a divider coupled to the main oscillator to divide the frequency of the first signal by n to produce a third signal; a phase comparator for comparing the phase of the third signal and the phase of the second signal to produce an error signal indicating a difference in phase therebetween; and means responsive to the error signal and operative to change the frequency of oscillation of the main oscillator to reduce the error signal; whereby the frequency of oscillation of the main oscillator is controlled by the second oscillator.

Preferably the apparatus also comprises means coupled to the output of the main oscillator to multiply the frequency fl; and filter means coupled to the frequency-multiplying means to select a desired frequency of mf1, where m is a positive integer. Preferably the frequency-multiplying means comprises a step recovery diode and the filter means preferably -comprises a bandpass filter. Preferably the second oscillator is a crystal controlled oscillator.

Thus, when the main oscillator oscillates at a high frequency of the order of gigahertz for which, conventionally, stable oscillators are not available, by controlling the stability of the main oscillator in response to the performance of the second oscillator which operates at a lower frequency for which stable oscillators do exist, it is possible to improve considerably the stability of the high frequency main oscillator. Further, by making a phase comparison between the second and third signals a more strict correspondence is established than would be esatablished by comparing, for example the frequency of these signals.

This further enhances the estability of the high frequency main oscillator.

An example of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block schematic diagram of a typical microwave receiver suitable for use in a DBS system; Fig. 2 is a block schematic diagram of an oscillator circuit in accordance with the invention; and, Fig. 3 shows two oscillograms illustrating an improvement in oscillator output achieved by the invention.

Fig. 1 shows a conventional double-superheterodyne microwave receiver for low rate phase modulated data systems using satellite links. The receiver includes a tuned microwave amplifier 1 which receives from an antenna 2 a modulated X-band signal having a carrier frequency of approximately 12 GHz. The X-band signal is amplified and is fed to a first mixer 3, which also receives a signal from a first local oscillator 4. The signals are mixed, to provide sum and difference frequencies. The difference signal, at a. frequency of approximately 1GHz, i.e. in the L-band, is amplified in a tuned first I.F. amplifier 5. The amplified signal is fed to a second mixer 6 where it is mixed with a signal from a second local oscillator 7 to provide sum and difference signals.The difference signal, at a frequency of approximately 70MHz, is amplified in a tuned second I.F. amplifier 8 and is fed to a demodulator 9.

The modulation signal thereby obtained is fed to further video or audio stages (not shown), as required.

Referring to Fig. 2, the present invention provides a new configuration for the first local oscillator 4.

The oscillator comprises a main oscillator circuit 10 which produces an output at, say, 1GHz. This output is fed to a divide-by-ten chip 11, which therefore produces an output at 100MHz. An auxiliary oscillator 12 oscillates at 100MHz. This oscillator may be crystal-controlled and can be readily stabilised. The output of the divider 11 and the output of the oscillator 12 are fed to a phase comparator 13, which produces an error signal representing any phase difference between the two 100MHz signals. The error signal is fed through a low-pass filter 14 to a frequency-controlling device 15 of the oscillator 10, such as a varactor diode. The frequency of the oscillator 10 is therefore stabilised by the phase-lock loop comprising the components 10-15.

This phase-lock loop is particularly useful in providing stability under conditions of phase noise in the oscillator output such as occur when phase modulated data signals, particularly those having low bit rate are being handled by the receiver.

The thus stabilised 1GHz output from the oscillator 10 is fed to a step recovery diode 16. This has the effect of generating high-order harmonics of the 1GHz signal. A bandpass filter 17, coupled to the output of the diode 16, passes the tenth harmonic and rejects all others, so that the required 10GHz local oscillator signal is fed from its output 18.

An advantage of the present oscillator circuit is indicated in Fig. 3, wherein an oscillogram (a) shows the spectrum of a 10GHz output produced by a conventional circuit mixed down to 1GHz for convenience of inspection, and an oscillogram (b) shows the spectrum of the output produced by the circuit of the present invention. The oscillograms were both produced with identical spectrum analyser settings. It will be seen that the output spectrum of the present invention is very much sharper than that of the conventional circuit and, in particular, the noise is at a much lower level.

Although the oscillator is described above in the context of a complete receiver, clearly it can be used for other applications. For example, it may form part of a "downconverter" which comprises just the X-band amplifier 1, the first local oscillator 4 and the mixer 3, all combined in a unit which is mounted on the antenna 2. Such a unit could then be connected to other equipment via a coaxial cable.

Claims

1. An apparatus for generating electrical oscillations at a predetermined frequency fl, comprising a main oscillator for generating a first signal at said frequency fl; a second oscillator for generating a second signal at a frequency f1/n, where n is a positive integer greater than 1; a divider coupled to the main oscillator to divide the frequency of the first signal by n to produce a third signal; a phase comparator for comparing the phase of the third signal and the phase of the second signal and produce an error signal indicating a difference in phase between them; and means responsive to the error signal and operative to change the frequency of oscillation of the main oscillator to reduce the error signal; whereby the frequency of oscillation of the main oscillator is controlled by that of the second oscillator.

2. An apparatus according to claim 1, in which the second oscillator is a crystal controlled oscillator.

3. An apparatus according to claim 1 or 2, which also comprises means coupled to the output of the main oscillator to multiply the frequency fl; and filter means coupled to the frequency-multiplying means to select a desired frequency of mfl, where m is a positive integer greater than 1.

4. An apparatus according to claim 3, in which the frequency-multiplying means comprises a step recovery diode and the filter means comprises a bandpass filter.

5. An apparatus according to any one of the preceding claims at which the frequency fl of the main oscillator is around 1 GHz.

6. An apparatus according to any one of the preceding claims forming part of a first local oscillator in a double-superheterodyne receiver.

7. An apparatus according to any one of claims 1 to 5 forming part of a shown converter which also includes an amplifier to receive and amplify a signal, and a mixer to mix the amplified received signal with the output of the main oscillator and produce an output of the lower frequency than that of the received signal or the main oscillator.

8. An apparatus substantially as described with reference to Figures 1, 2, and 3b of the accompanying drawings.