GB2419756A - Tuner suitable for 'Plug and Play' (PnP) applications - Google Patents

Abstract

A single conversion tuner is provided for selecting any one of a plurality of channels for reception from a multi-channel input signal. The tuner comprises a first filter 2 arranged to filter the input signal and provide attenuation of the image frequency. The signal from the output of the first filter 2 passes though an LNA / AGC stage 3. A second filter 4 filters the output of the LNA /AGC stage 3 and provides further image attenuation. A single conversion frequency changer 5, 10 includes an image reject mixer 5 and converts any selected one of the channels to intermediate frequency. Image attenuation is exclusively provided by and distributed among the first and second filters 2, 4 and the mixer 5. The tuner provides image rejection performance and channel flatness similar to that of a double conversion tuner and is compatible with digital terrestrial and cable systems.

Description

The present invention relates to a tuner. Such a tuner is suitable, for

example, for US PnP (plug and play) applications but is equally applicable to applications with similar performance requirements.

PnP is a new standard being adopted in America to offer universal digital and analogue cable and terrestrial reception and as such requires a receiver that is capable of achieving standards of compliance for both distribution media. Traditionally, tuners used in terrestrial applications are of single conversion (SCT) type and for cable of double conversion (DCI) type. Such tuners will not be described in detail since they are well known and documented.

Single conversion is traditionally used in terrestrial since such transmissions are normally characterised by a requirement to receive a desired channel from a sparsely populated spectrum where the desired channel may be significantly weaker than undesired interferers. The SCT is well suited to this since such an architecture employs front end filtering which provides a selectivity to the desired channel before any active stages are exercised, where the active stage will be susceptible to generating interference from the high amplitude undesired on to the lower amplitude desired. The presence of the filtering effectively prevents this from occurring.

A further advantage of the SCT architecture is that, because of the combination of selectivity and low channel utilisation, the active stage does not require a high signal handling capability to prevent interference being generated. This in turn allows the active stages to be designed with a lower Noise Factor - it is well documented that achieving low Noise Factor and high signal handling are not necessarily compatible.

Achieving a low NF in Terrestrial is of benefit since it allows the user to receive weaker signals from, for example, more distant transmitter locations, or for example to operate from a low performance indoor antenna, etc. SCT also has the advantage that it is a relatively low cost solution since it employs very few expensive components such as integrated circuits. It is well known that the consumer television receiver market is extremely cost competitive so that SCT is also suited because of cost reasons.

In a cable environment, the received bandwidth is much more heavily populated. The relative powers of these channels, however, are well controlled and will be substantially of equal amplitude. Cable, however, has a more severe demand for received channel flatness to deliver acceptable performance.

SCT could be applied to cable. However, it is well documented that the SCT, because of the selectivity filters, will struggle to achieve a repeatable flat passband characteristic. This is because, through necessity, the selectivity filters have to track with the desired channel, which will introduce errors from an ideal characteristic, and because the selectivity filter also has to provide an appropriate attenuation to the image channel, which limits the passband bandwidth. This defines the filter quality factor; these filters have to be relatively narrow, which will again introduce further ripple. It is not untypical for such a receiver to have a variable passband flatness of ±3dB.

DCT is more suited to a cable environment because, in a DCT architecture, the image filter is at a fixed frequency and so can be designed to achieve a flat repeatable characteristic. In practise, this may typically be a SAW (surface acoustic wave) filter.

Also, since the received powers are substantially equal, it is easier to manage the interference produced from the undesired channels by applying appropriate signal handling to the active stages.

As described above, the consequence of managing the interference will necessarily lead to a higher Noise Factor. This, however, is not a disadvantage in a cable system since the power delivered to the user can be relatively well controlled by the system operator to be at an acceptable level. Therefore, a very low Noise Figure has no advantage.

DCT is a higher cost solution since it involves more higher cost components such as integrated circuits and SAW filters than the SCT. This, however, is not a disadvantage in the cable arena since cable is typically a subscription market and the added cost can be readily covered.

As can be seen from the above descriptions, the requirements for a PnP receiver are not necessarily compatible, i.e. the capability of receiving both Cable and Terrestrial in the same receiver. There is a further complication with PnP in that the receiver is specified to work with a high image channel desired to undesired ratio. This is driven by the requirement to meet both Cable and digital terrestrial requirements.

Since DCT has been adapted for cable, the fixed frequency image filters have been designed to achieve a high level of image cancellation and already meet these requirements, whereas SCT will struggle to meet the image performance demanded without further compromising channel flatness. In practise, it is debatable whether such a tuner could ever meet the required image performance even if there were no constraint on channel flatness.

To date manufacturers have approached PnP by two routes. The first and simplest is to use separate receiver modules, i.e. one optimised for terrestrial and a second optimised for cable. This obviously has a severe impact on cost since the receiver function is duplicated and it must be remembered that PnP will be a very high volume consumer application with associated cost implications.

The second route is to adapt a signal receiver architecture and optimise this for one of the distribution media only, i.e. using an SCT or DCT, and accepting that this will give relatively acceptable performance for one distribution medium and merely be operational and not "standards conformant" with the other. For example, if double conversion is adapted, the NF will be inferior and the receiver will not be able to receive as far distant signals or a more expensive antenna may be required.

According to the invention, there is provided a tuner as defined in the appended claim 1.

Embodiments of the invention are defined in the other appended claims.

It is thus possible, for example, to provide a new integrated circuit and associated receiver architecture that allows a manufacturer to meet all of the market expectations in a single receiver design.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of a tuner constituting a first embodiment of the invention; and Figure 2 is a block schematic diagram of a tuner constituting a second embodiment of the invention.

Like reference numerals refer to like parts in the drawings.

The input signal (1) is connected to a first filter stage (2). This filter may be of single element construction and may be of bandpass characteristic. In an alternate embodiment, this may have a low pass characteristic. In the bandpass embodiment, the centre frequency of this filter is arranged to track with the centre of the desired channel through the controlling means (12).

The output of the first filter is connected to a LNA/AGC (low noise amplifier/automatic gain control) stage (3), which provides a first system variable gain. This stage typically has a low NF. The output of this stage is coupled to a second filter stage (4). This filter may be of dual element construction and may be of bandpass characteristic. In an alternate embodiment, this may have a low pass characteristic. In the bandpass embodiment, the centre frequency of this filter is arranged to track with the centre of the desired channel through the control means (12). The combination of filter stages I and 2 provides selectively to the desired channel, i.e. filters out some or all undesired channels, and also provides a first image cancellation.

The output of the second filter (4) is connected to an image reject mixer stage (5). This stage downconverts the desired channel to an IF (intermediate frequency) whilst providing a second image cancellation. The combination of the filter and mixer image rejection provides the overall system requirements, which will be in excess of that achievable by filters or an image reject mixer alone.

In a preferred embodiment, the IF is a conventional TV IF, for example 47. 25 MHz picture carrier in the case of US systems. In an alternate embodiment, the IF may be at a low IF frequency. The image reject mixer (5) may also contain band limit filtering at the IF output frequency.

The commutating signals for the mixer are provided by the local oscillator (10), whose frequency is controlled by the PLL frequency synthesiser (11). The local oscillator frequency is set by the common controlling means (12). The first and second filter stages together with the local oscillator are arranged through a combination of design and/or manufacturing alignment to respond similarly to the controlling means (12) such that all stages track in sympathy with the desired channel.

The image reject mixer is then coupled to a third filter stage (7). This functions as the channel filter and may typically be of bandpass characteristic. In addition, a buffered output (6) may be provided for connecting, for example, to a separate IF demodulation stage. The channel filter output, which contains substantially only the desired channel, is coupled to a final variable amplifier stage (8). This stage provides further system gain to present desired output (9).

Dependent upon the desired tuning range, the stages (2) through (4) and all or part of the image reject mixer (5) may be duplicated across two or more sub-bands. In an alternate embodiment, the buffered output (6) may be coupled to a fourth channel filter (not shown) and then to the input of the variable gain stage (8).

The at least one image reject mixer stage, the channel amplifier, the at least one local oscillator, (with the exception of the sustaining network) and PLL synthesiser are readily integratable on a common substrate such as an integrated circuit. Alternately, the above functions plus any combination of the remaining stages may be integrated on a common substrate, such as an integrated circuit or MCM (multi-chip module).

Another embodiment is shown in Figure 2. Blocks (1) through (11) serve the same function as described above except that the first and second filter stages and the local oscillator are no longer controlled by a common controlling means and therefore require no mechanical alignment.

In this alternate embodiment, tracking of the first and second filter stages together with the local oscillators is achieved through a controlling means stage (13). This stage generates independent controlling signals for each of the stages. These may be generated through use of, for example, an electronic look-up' table generated during production alignment or by, for example, a dynamic algorithm which automatically optimises the alignment of some or all stages in operation.

In this embodiment, the at least one image reject mixer stage, the channel amplifier, the at least one local oscillator, (with the exception of the sustaining network), PLL synthesiser and controlling means are readily integratable on a common substrate such as an integrated circuit. Similar to the first embodiment, the above functions plus any combination of the remaining stages may be integrated on a common substrate, such as an integrated circuit or MCM.

In combination with the above embodiments, the image reject mixer may be implemented with a high intermodulation intercept capability and I dB compression. In combination with the above embodiments, the first filter stage may be preceded by an LNA with two or more outputs, one of which is coupled to the first filter stage.

It is thus possible to provide a cost-effective solution, which meets all of the above defined requirements within a single receiver and which achieves: 1) an improved image rejection similar to DCT and compatible with digital terrestrial and cable system requirements whilst offering; 2) a similar selectivity in strong undesired weak desired environments to SCT for terrestrial; 3) channel flatness similar to DCT for cable environment; 4) interference generation similar to DCT in a cable environment; 5) NF similar to SCT for terrestrial

Claims (25)

1. CLAIMS: I) A single-conversion tuner for selecting any one of a plurality

   of channels for reception from a multi-channel input signal, comprising a first filter arranged to filter the input signal so as to provide a first image attenuation, a second filter arranged to filter the signal at the output of the first filter so as to provide a second image attenuation, and a single-conversion frequency changer arranged to convert any selected one of the channels to an intermediate frequency and comprising an image reject mixer arranged to provide a third image attenuation, whereby image attenuation is exclusively provided by and distributed among the first and second filters and the mixer.
2. 2) A tuner as claimed in claim I, in which the composite image rejection provided by the first and second filters and the mixer is at least 60 dB.
3. 3) A tuner as claimed in claim 2, in which the composite image rejection is at least dB.
4. 4) A tuner as claimed in any one of the preceding claims, comprising a first variable gain stage between the first and second filters.
5. 5) A tuner as claimed in any one of the preceding claims, in which the first and second filters are arranged to track the frequency of the selected channel.
6. 6) A tuner as claimed in claim 5, in which the first filter is a bandpass filter.
7. 7) A tuner as claimed in claim 5 or 6, in which the first filter is single-tuned.
8. 8) A tuner as claimed in any one of the claims 5 to 7, in which the second filter is a bandpass filter.
9. 9) A tuner as claimed in any one of claims 5 to 8, in which the second filter is double-tuned.
10. 10) A tuner as claimed in any one of the preceding claims, in which the frequency changer comprises a variable frequency oscillator arranged to supply commutating signals to the mixer and having a frequency control input.
11. 11) A tuner as claimed in claim 10 when dependent on claim 5, in which the first and second filters have tuning control inputs connected with the frequency control input to receive a common tuning signal.
12. 12) A tuner as claimed in claim 10 when dependent on claim 5, comprising a controller for controlling the variable frequency oscillator to select any one of the channels and for adjusting the first and second filters independently for reception of the selected channel.
13. 13) A tuner as claimed in any one of the preceding claims, in which mixer comprises band-limit output filtering.
14. 14) A tuner as claimed in any one of the preceding claims, comprising a plurality of signal paths between an input of the tuner and the frequency changer covering different frequency ranges, a first of the signal paths containing the first and second filters and the or each other of the signal paths containing a further first filter and a further second filter.
15. 15) A tuner as claimed in any one of claims 1 to 13, comprising a plurality of parallel signal paths covering different frequency ranges, a first of which contains the first and second filters and the mixer and the or each other of which contains a further first filter, a further second filter and a further image reject mixer.
16. 16) A tuner as claimed in any one of the preceding claims, comprising a third filter for filtering the output signal of the mixer.
17. 17) A tuner as claimed in claim 16, in which the third filter is a bandpass filter.
18. 18) A tuner as claimed in claim 16, in which the third filter is a low pass filter.
19. 19) A tuner as claimed in any one of claims 16 to 18, comprising a second variable gain stage connected to the output of the third filter.
20. 20) A tuner as claimed in any one of claims 16 to 19, comprising a fourth filter for filtering the output signal of the mixer and a third variable gain stage connected to the output of the fourth filter.
21. 21) A tuner as claimed in claim 19, comprising a fourth filter disposed in parallel with the third filter between the mixer and the second variable gain stage.
22. 22) A tuner as claimed in any one of the preceding claims, having a frequency range from substantially 50 MHz to substantially 1000 MHz.
23. 23) A tuner as claimed in any one of the preceding claims, in which the cumulative passband of the first and second filters has a variation of less than substantially 1dB in the channel bandwidth.
24. 24) A tuner as claimed in any one of the preceding claims, at least part of which is formed on a common substrate.
25. 25) A tuner as claimed in claim 24, in which the common substrate comprises a monolithic integrated circuit.