Patent Application
20080074178
The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein:
The low pass filter shown at
The basic idea of the present invention is to modify the filter characteristic of a 2nd order low pass filter constituted by the single coil or inductor Lp and the capacitor C. This modification is achieved by:
sensing the current Ip flowing through the coil Lp by measuring the voltage across resistor Rsense;
amplifying and filtering of this current Ip by means of the amplifier A; and
feeding an amplified and filtered current Is at the output of the amplifier A back to a secondary winding Ls of transformer T, consisting of the coils Lp and Ls.
The operation of the single-ended low pass filter of
The current Ip is sensed across Rsense and is amplified in amplifier A, resulting in a current Is.
The current Is is fed back to the second coil or inductor Ls, which forms the secondary winding of the transformer T, of which inductor Lp is the primary winding.
The current Is is given by the following formula:
I
S
=A·I
P
The current Is in the coil Ls influences the magnetic field in the core of the transformer T in such a way that the resulting coil Lr, between the nodes 1 and 2, can be described by the following formula:
L
R
=L
P
+n·A·L
S
Where n is the winding ratio of transformer T, which is given by the following formula (nP is the number of windings of Lp and ns is the number of windings of Ls):
The filter characteristic (in function of frequency ω) without Ls and without feedback (filter characteristic for 2nd order filter composed of coil Lp and C) is given by:
The filter characteristic (in function of frequency ω) with Ls and feedback via amplifier A present, is given by:
As such, the coil Lr, that defines the filter characteristic of the total filter, can be adapted by adjusting Ls and the characteristics of the amplifier A.
Depending on the amplification in amplifier A and the winding ratio of transformer T, different adaptations of the coil are possible:
for −1<(n·A)<0, i.e. current Is fed back to the coil Ls in opposite phase to the current Ip in the coil Lp, the resulting coil Lr is smaller than the original coil Lp. This is called the “suppressed” coil mode. The condition (n·A)≦−1 has to be avoided as this will make the circuit instable;
for (n·A)>0, i.e. current Is fed back to the coil Ls in phase with the current Ip in the coil Lp, the resulting coil Lr is larger than the original coil Lp. This is called the enhanced coil mode; and
Also complex values of the amplification of amplifier A (Ar+j·Ai) are possible, representing a possible phase shift of the current Is and Ip of 0° to 360°. Ar is the real part of amplification A, Ai is the imaginary part of amplification A.
In order to realize a multiple (higher) order low pass filter, still based on a single coil and capacitor, the amplifier A preferably comprises the cascade coupling (not shown) of a frequency dependent amplifier, say A(ω) and a transconductance amplifier, say gm. The transconductance amplifier translates the output voltage of the frequency dependent amplifier into a corresponding current.
By making the amplification A, from current Ip to current Is, frequency dependent, the resulting coil Lr also becomes frequency dependent. As such, a multiple order filter is generated. The order of the low pass filter is dependent on the order of the filter in the so-created feedback loop.
The block diagram of a higher order low pass filter is given at
a current sense circuit or sense amplifier CS having inputs connected across the impedance Rsense and an output coupled to the input of the amplifier A, and more particularly of the frequency dependent amplifier A(ω) thereof. The frequency independent sense amplifier CS translates the current Ip into a corresponding voltage at the sense amplifier output; and
the amplifier A comprising the frequency dependent amplifier A(ω) and the gm amplifier (transconductance amplifier) which translates the output voltage of amplifier A(ω) into a corresponding current Is.
When the amplifier A represents a high pass filter, giving coil enhancement at high frequencies, of order OA. The total order OT of the filter is given by:
O
T=2+OA
The resulting coil Lr changes its value of frequency with order OA, the order of 2 is inherently present in the low pass filter consisting of coil Lp and capacitor C.
It is to be noted that besides using a high pass filter for amplifier A, other filters are possible, like band pass filter, to obtain the required frequency response of the filter.
Ai is the amplification of A(ω) at frequency ωi, with i=0, 1, 2, 3
The frequency characteristic of the total filter, based on resulting coil Lr and capacitor C, follows a 3rd order characteristic, as the frequency characteristic moves from the 2nd order characteristic based on L0C to the 2nd order characteristic based L3C as the frequency increases from ω0 to ω3.
The same applies for higher order and band pass filter characteristics of amplifier A(ω).
The above-described principle is applied for the implementation of the low pass filter in the splitter function of an xDSL telecommunication system. A block-diagram of a low pass filter used in xDSL is given at
In more detail, the balanced or differential low pass filter of
The first coil Lp1, the second coil Lp2, the third coil Ls1 and the fourth coil Ls2 are magnetically coupled and are located on a same core of the transformer T1.
In a preferred embodiment, as shown at
Similarly to the single-ended low pass filter of
The principle of the above-described filter allows for an implementation (e.g. by integration) that makes the filter characteristics of the filter enhancement means of
At the Central Office CO side, the splitter function in an xDSL environment allows frequency multiplexing of the low frequency band (POTS or ISDN) and the high frequency band (xDSL like ADSL, VDSL) on the same telephone line TL. Therefore, the xDSL splitter consists of a high pass filter (most often placed on the xDSL line termination board) and a low pass filter (most often placed on separate boards).
At the Customer Premises CPE, also a splitter function, consisting of a low and high pass filter, is present for frequency multiplexing of the POTS/ISDN signals towards the POTS/ISDN terminal LT and the xDSL signals towards the xDSL modem.
This topology of xDSL and POTS/ISDN frequency multiplexing is shown at
LT=Line Termination;
LPF=Low Pass Filter;
HPF=High Pass Filter;
CPE=Customer Premises Equipment;
CP=Customer Premises;
TL=Telecommunication Line;
CO=Central Office;
POTS=Plain Old Telephone Services; and
ISDN=Integrated Services Digital network.
Owing to the different implementations described above, it is possible to achieve the following advantages:
small size (only 1 coil) compared to passive splitter implementations;
less costly, especially on bulky components (coils, high voltage capacitors);
insensitive to over-voltages and over-currents on the telephone line (high Ohmic current sense network and galvanic isolation via the Transformer T1);
no crossover distortion (in zero-crossing of the wires of the telephone line) as often encountered in other active splitter implementations;
no DC voltage drop, as is present on other active splitter implementations; and
flexible programming of the filter characteristics.
A final remark is that embodiments of the present invention are described above in terms of functional blocks. From the functional description of these blocks, given above, it will be apparent for a person skilled in the art of designing electronic devices how embodiments of these blocks can be manufactured with well-known electronic components. A detailed architecture of the contents of the functional blocks hence is not given.
While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is merely made by way of example and not as a limitation on the scope of the invention, as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06291502.0 | Sep 2006 | EP | regional |
