SPECIFICATIONA tunerThis invention relates to a tuner such as might be employed in for example radio, television or direct broadcasting by satellite systems (DBS).
The invention has resulted from a consideration of problem involved in reception of signals from satellites. A band of frequencies of 11.7 - 12.7 GHz has been allocated to such transmissions and normally a double superheterodyne receiver is employed having a first l.F. frequency of 950 - 1350 MHz and a tuner which provides a second l.F. frequency of 600 or 134 or 20 MHz. Such an arrangement requires a local oscillator frequency for the second mixer which is variable between 1550 - 1950 MHz or between 350 - 750 MHz. An oscillator of 15tri0 - 1950 MHz is difficult to produce with the required stability and, in the case of phases lock techniques, this is due to the requirement for dividers operating at this frequency.This frequency can be provided by scaling but complex filtering techniques arq required to provide a suitably clean signal. if an oscillator frequency of 350 750 MHz is used there results an image frequency of 1300 - 2100 MHz the lower frequency of which is approximately one octave away from the l.F. frequency and introduces the requirement of sophisticated l.F. filtering to provide adequate image rejection. The present invention seeks to provide a tuner configuration suitable for application in satellite communications in which at least some of the previously mentioned difficulties are obviated or reduced and which is applicable to a variety of tuning applications.
According to the invention there is provided a tuner, comprising an input for a received signal, a pair of mixers connected in series in a signal path between the input and an output of the tuner, a filter in the signal path between the output of the second mixer and the tuner output and a common local oscillator having an output coupled with an input of each of the mixers. By employing two mixing stages driven by the same oscillator, the oscillator frequency can be substantially reduced.
An additional filter may be provided connected in the signal path between the two mixers.
The tuner may include an amplifier connected in the signal path between the two mixers.
In a refinement of the invention there is provided at least one additional mixer connected in the signal path and having an input coupled with the output of said common local oscillator. An amplifier may be provided in the signal path between each mixer and the next mixer,An amplifier may be provided following the last mixer in the signal path.
In a particularly advantageous form of the invention the local oscillator frequency is derived from a reference frequency by means of a phase lock loop circuit comprising a variable frequency oscillator, for providing the local oscillator frequency, a divider and a phase or frequency comparator which compares the reference frequency with the divided frequency of the variable frequency oscillator. In such an arrangement the divider may have a variable division ratio to permit selection of a required local oscillator frequency mixing with the received signal to provide a signal falling within the pass band of the filter(s). The division ratio of the divider may be controlled by a microprocessor in response to selection of a predetermined channel. The tuner may be incorporated into a receiver in which it is preceded by an aerial and followed by a detector.In such a receiver there may be included in the signal path between the aerial and the tuner a mixer followed by a filter and wherein the mixer is fed from a further local oscillator having a preset frequency.
In order that the invention and its various other preferred features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawings in which:Figure 1 is a block schematic diagram of a satellite communications receiver of conventional type,Figure 2 is a block schematic diagram of a basic satellite communications receiver incorporating a tuner constructed in accordance with the invention,Figure 3 is a block schematic diagram showing a refinement of the tuner in Figure 2,Figure 4 shows the approximate filter characteristic for the filter 13 of Figures 2 and 3,Figure 5 shows the approximate filter characteristic for the filter 31 of Figure 3, andFigure 6 is a block schematic diagram of a phase lock loop local oscillator suitable for use in the embodiments of Figures 2 and 3.
In the drawings, Figure 1 illustrates a conventional satellite communications receiver comprising a dish antenna 10 coupled with the input of a mixer 11 which is provided with a fixed frequency local oscillator 12. The output of the mixer 11 is coupled via a band pass filter 13 to a tuner 14. The tuner comprises a mixer 15, which receives the output of the filter 13, a second local oscillator 16 feeding the mixer 15 and a band pass filter 17 coupled between the output of the mixer 15 and the output of the tuner 14. The output of the tuner 14 is coupled to a detector 18.Unless the receiver is intended for receipt of a single transmission frequency, the local oscillator 16 is variable in frequency to tune the receiver to a predetermined range of incoming signals so as to provide a mix with the signal inputto the tuner and provides a signal falling within the pass band of the filter 17. In such an arrangement FIF = / FIN - Fosc/ where FIF is the centre frequency of the filter 17, FIN is the signal frequency at the input to the tuner 14 and Fosc is the frequency of the local oscillator 16. As previously indicated it is difficult to provide a suitable oscillator 16.
Figure 2 illustrates a basic tuner in accordance with the invention incorporated in a receiver configuration similar to Figure 1. The same reference numbers are given to identical circuit blocks. In this case the tuner 20 has an input coupled with the output of the filter 13 and provides a signal path routed via two mixers 21 and 22 and a band pass filter 17 to the tuner output which is connected to the detector 18. The two mixers are provided with a common local oscillator 23 which, in view of the double mixing action, need be only half the frequency of the oscillaror 16 employed in Figure 1. Such an oscillator is more easily provided. In such an arrangement FIF = IFIN - 2 Fose A The arrangement of Figure 2 is susceptible of refinement as shown in Figure 3.To provide improved sensitivity and/or selectivity an amplifier 30 andlor a band pass filter 31 is/are provided in the signal path between the two mixers 21 and 22.
In the arrangement described in Figure 3, which includes a filter 31, relatively simple filtering techniques are required. An indiction of the requirements of filters 13 and 31 are shown in Figures 4 and 5 respectively. As can be seen in Figure 4, filter 13 requires a pass band of from 950-1350 MHz, requires 50dB of attenuation at the nearest image frequency 2150 MHz and requires a 30dB attenuation trap at 600 MHz the frequency of the final IF. As can be seen from Figure 5 the filter 31 requires a pass band of 175-375 MHz and 50dB attenuation by the nearest image frequency of 1375 MHz which is ratio of 3.6 relative to the top pass band frequency and requires attenuation of only 14dB/octave.
In the embodiments described the oscillator 23 may be a fixed frequency if the receiver is to receive only one transmitted frequency or may be variable to select different transmitted frequencies. The oscillator is preferably formed as a phase locked loop oscillator which is locked to a stable reference frequency. A schematic diagram of a suitable oscillator is shown in Figure 6.
In Figure 6 a variable frequency oscillator 40 is phase locked to a reference oscillator 41 by means of a phase lock loop including a divider 42 and phase or frequency comparator 43. By selecting the appropriate division ratio n, the local oscillator frequency can be made to oscillate at fref x n, where fref is the frequency of the reference oscillator 41, and is phase locked to the reference oscillator frequency. The division ratio is preferably selectable by means of a microprocessor 44 which operates with a predetermined algorithm in response to selection of a particular channel by the channel selector 45. In practice, as the local oscillator control range required is small, the division ratio is normally constituted by a constant factor plus a variable controlled by the microprocessor. The output from the oscillator 40 provides the inputs to the mixers of the tuner.
Although the embodiments described employ two mixers it will be appreciated that three or more mixers fed from a common local oscillator can be employed without departing from the spirit of this invention. In such arrangements an amplifier and/or filter may be provided between each mixer and the next mixer in the signal path.
Although the invention has been described in connection with satellite transmission systems it is also applicable to other tuning applications e.g.
conventional radio and television receivers.
Advantages which may be provided by the system are:1) Local oscillator frequency reduced by a factor of 2 or more.
2) Local oscillarortuning range reduced by a factor of or more.
3) improved image and band filtering.
4) Improved conversion gain results from lowerfrequency mixing and lower drive levels to themixers required.
5) Overall gain improvement by amplificationbetween mixers in the tuner also improves noise figure.
6) Suitable for implementation in integrated circuit or hybrid form.