The invention relates to a transformer circuit arrangement which is designed, in particular, for the transmission of signals in message transmission systems or communication systems, such as, for example, xDSL systems.
The devices referred to as transformers in message transmission systems are essential passive electrical components which fulfil a wide variety of tasks such as, for example, electrical decoupling, the transforming of voltages/currents, or the changing of impedance values. Due to their large spatial requirements, their high price in comparison with other passive components, their non-linear behaviour, their losses, and the absence of integration capability, the use of such transformers is, however, to be avoided as far as possible. In many cases, it has not hitherto been possible for the transformer to be replaced by a device of equal value, with the result that it cannot be done away with.
In xDSL systems (xe2x80x9cDigital Subscriber Linexe2x80x9d), for example, the transformer determines the performance capacity of these systems. What is required is a high degree of linearity of the transformer over the entire transmission range, with, at the same time, minimum manufacturing costs. These two requirements are, however, difficult to fulfil simultaneously. In practice, the actual properties of a transformer therefore derive from a compromise, which is to be decided on according to the particular application situation.
An additional problem is incurred by the bandwidth of the transformer. In many applications, a large bandwidth is required. In conjunction with high linearity, however, this is associated with higher costs. Accordingly, with the use of a transformer, the problem arises of optimisation of the parameters of bandwidth, linearity, and manufacturing costs. Inasmuch as no compromises are possible with the bandwidth or the linearity, the comparatively high costs for the transformer, incurred in particular due to its core material as well as by its mechanical layout, were hitherto unavoidable.
Reductions are possible with regard to the bandwidth, but these are not optimum. With SHDSL systems (xe2x80x9cSingle-Pair-High-Bit-Rate Digital Subscriber Linexe2x80x9d), the required bandwidth of the transformer is, for example, proportional to the variable signal bandwidth of the system or the transceiver. The optimum lower limit frequency of the transformer for a maximum data rate or range width of the system is approximately proportional to the bandwidth of the system. While too low a lower limit frequency of the system can be adjusted by means of a digital highpass filter in increments to the signal bandwidth of the system, i.e. can be increased, a lower limit frequency which is too high cannot be compensated for subsequently. Efforts must therefore be made to arrange the lower limit frequency of the transformer to suit the smallest bandwidth of the system which is to be used, and the upper limit frequency of the transformer to correspond to the greatest bandwidth which is to be used. If this is not possible, a compromise which frequently satisfies is to lay out the transformer only for the largest bandwidth to be used, whereby reductions in the performance at the smallest bandwidths to be used are taken into account.
With xDSL systems, in general a substantial linearity of the transformer for a maximum data rate or range is in most cases only required on such channels as have only low crosstalk, or none at all. The use of xDSL systems is in many cases subject to strictly formulated rules (known as xe2x80x9cDeployment Rulesxe2x80x9d), which are intended to ensure functional performance even with the greatest fault incidence to be assumed (xe2x80x9cworst casexe2x80x9d). Accordingly, if a system is only designed for the greatest fault incidence to be assumed, such as, for example, the presence of severe cross-talk interference, the requirements for linearity are reduced, because the interference caused by the non-linearity in the presence of interference caused by cross-talk noise does not occur. A transformer designed in this way is in this case not well-suited for transmission on channels with low cross-talk.
A further possibility consists of offering different equipment fitting variants on a line card, for example, with in each case a transformer for specific applications. A disadvantage of this procedure is, inter alia, the additional re-equipping with components required in addition to the transformer, since the dimensions of the circuit as a whole is frequently dependent on the properties of the transformer used in each individual case. In addition to this, extensive test series are in most cases required for such systems, which are incurred anew for each circuit variant.
The re-equipping of components referred to, however, excludes a desirable switchover in terms of software between different operational frequency ranges or transformers, with the result that only one electronic or electromechanical switchover comes into consideration. The use of electronic switches is problematic, however, since highly linear electronic switches are necessary because of the high demands for linearity, which are associated with high costs. In addition, switchover by means of relays is also possible. This, however, represents an additional component to be accommodated on a line card per channel, so that less space remains for the equipping of as many channels as possible.
The object of the invention is to provide an economical transformer circuit arrangement with which signals can be transmitted with a large frequency bandwidth and with high linearity.
This object is achieved by a transformer circuit arrangement according to claim 1. Subclaims refer to preferred embodiments.
The transformer circuit arrangement according to the invention has a first transformer with at least two inputs or input connections respectively, and two outputs or output connections respectively, and a first frequency response with a first lower limit frequency and a first upper limit frequency, as well as a second transformer with at least two inputs or input connections respectively, and two outputs or output connections respectively, and a second frequency response with a second lower limit frequency and a second upper limit frequency. The first lower limit frequency is smaller than the second lower limit frequency, and the second upper limit frequency is greater than the first upper limit frequency. In addition, the second lower limit frequency is preferably not greater or smaller by a factor of 10 than the first upper limit frequency. The transformer circuit arrangement according to the invention has a frequency behaviour with bandpass character with a lower overall limit frequency and an upper overall limit frequency. In this situation, the lower overall limit frequency is smaller than the first upper limit frequency of the first transformer and the second lower limit frequency of the second transformer, and the upper overall limit frequency is greater than the second lower limit frequency of the second transformer and the first upper limit frequency of the first transformer.
The first and/or the second transformer can have an individual transformation ratio in each case, an individual main inductance, and an individual scatter inductance. Advantageously, the transformation ratio of the first transformer is equal to the transformation ratio of the second transformer. As a result, the transformers can be arranged symmetrically to one another in respect of their frequency response.
The first and second transformers can be designed as quadripole units with in each case two inputs and two outputs. The inputs of the transformers can be connected together in parallel or series. Likewise, the outputs of the transformers can be connected in parallel or series. According to one advantageous embodiment, either the inputs are connected in parallel and the outputs as series, or the inputs as series and the outputs in parallel. As a result of this, an addition of the frequency responses of the individual transformers can be achieved, to form one overall total frequency response of the transformer circuit arrangement according to the invention, so that, in an advantageous embodiment, a bandwidth can be derived which consists of the total of the bandwidths of the first and second transformers.
Preferably, the first transformer has a first capacitor which is connected in parallel upstream or downstream. Likewise, the second transformer can have a second capacitor, which is connected in series upstream or downstream, so that, for example, the individual transformers cannot be short-circuited in their connection for their individual transformer frequency ranges.
According to an advantageous embodiment, one or more transformers have a frequency response with Butterworth behaviour, such as Butterworth behaviour of the second degree. Accordingly, a maximum flat path of the frequency response is derived over the entire bandwidth, so that the maximum flat path can be obtained over the entire bandwidth of the transformer circuit arrangement according to the invention. In addition, the Butterworth frequency response has, advantageously, a low flank slope, as a result of which an increased sensitivity in the overall frequency response of the transformer circuit arrangement according to the invention can be avoided, in relation, for example, to component tolerances and additional phase distortions.
One or more limit frequencies can be selected in such a way that the frequency response of the individual insertion loss (value of the transformation function) at this frequency or frequencies respectively has an attenuation of 6 dB. Advantageously, the first upper limit frequency of the first transformer is equal to the second lower limit frequency of the second transformer. The lower overall limit frequency can be equal to the first lower limit frequency of the first transformer and/or the upper overall limit frequency can be equal to the second upper limit frequency of the second transformer.
According to a further embodiment, the first transformer can have an additional inductance, which is connected in series upstream or downstream to it or to its scatter inductance. This is particularly advantageous if a desired scatter inductance for the transformer cannot be achieved with sufficient precision during manufacture. In this case, for example, a small scatter inductance can be manufactured which can be disregarded in relation to the additional inductance which is to be connected, so that close to the desired overall scatter inductance of the transformer is derived. The scatter inductance of the transformer and the additional inductance pertaining to it can, however, be dimensioned in such a way that in total they produce the desired overall scatter inductance.
The transformer circuit arrangement according to the invention is designed in particular for the transmission of signals in message transmission systems, such as, for example, xDSL systems. With this, one large signal bandwidth or several smaller different bandwidths can be transmitted, in multiplex format, in an economical manner. Thanks to the use of two economically priced transformers of small bandwidth, an even more economical and space-saving device or circuit arrangement can be created for the transmission of signals with, simultaneously, large bandwidth and high linearity, which in overall terms behaves like one individual transformer, as a result of which a switchover in terms of software between different signal frequency ranges is possible. An electronic or electromechanical switchover, and the use of component fitting variants, are not necessary.
With the transformer circuit arrangement according to the invention, it is therefore possible for systems to be created with a large bandwidth dynamic. It is conceivable, for example, for an xDSL Multistandard line card to be created, with which SHDSL signals can be transmitted in the frequency range from, for example, 5 kHz to 500 kHz, and VDSL signals (xe2x80x9cVery High Speed Digital Subscriber Linexe2x80x9d) in the frequency range from 500 kHz to 10 MHz in frequency multiplex. In this situation, the first upper 6 dB limit frequency of the first transformer and the second lower 6 dB limit frequency of the second transformer can lie at 500 kHz. There are, however, also applications conceivable in systems with a single large signal bandwidth in the order of, for example, 5 kHz to 10 MHz.