Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
2000-09-05
2003-02-11
Ham, Seungsook (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S175000
Reexamination Certificate
active
06518859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of filters and notably, but not exclusively, to filters in association with modulators employed in the field of radio and television signal broadcast. In this type of application, the filters concerned by the invention are placed between the modulator circuits and the power circuit that drives the aerial.
2. Prior Art
Efforts in this field are towards totally digital transmission systems for both television and radio. Compared to analog transmission systems, digital technology allows a much denser occupation of the spectrum and a greater immunity to noise and interference problems.
For hertzian wave broadcasting, present day digital audio and video broadcast (respectively DAB and DVB) development programs aim to use the UHF IV and V carrier frequency bands.
The modulation technique envisaged, which is known in itself, is coded orthogonal frequency division multiplexing (COFDM). This protocol is used notably in European standards.
Such a form of modulation is well known, being described among others in patent documents EP-A-0 902 574 and WO-A-98 11698. Only the basic concepts shall be recalled here, with reference to FIG.
1
.
This simplified diagram shows the functional units that serve to elaborate a phase quadrature modulated signal from two input signals I and Q. These two signals convey modulated information and have a 90° phase difference with respect to each other. The two signals are fed to inputs of respective mixers
2
,
4
which also receive signals from a 0°/90° dephaser at the frequency F
o
=sin w
o
t. The two respective mixers
2
,
4
thus supply a digital signal which is fed to respective inputs of an adder circuit
6
. The output I(binary) of that circuit is supplied to the input of a digital-to-analog converter
8
to form the modulated signal I(a) to be transmitted.
This signal I(a) is generally a signal that carries a large number of carriers, for example 6800 carriers on a 7.61 MHz band, as shown in FIG.
2
. This signal has a central frequency termed Fnum positioned at a frequency on the order of 18 MHz.
In order to provide power amplification to this signal, it is first necessary to transpose the frequency Fnum to a higher frequency in the UHF band.
To do this, the technique presently used involves a two-stage transposition, as shown schematically in FIG.
3
. The different points of the circuit shown in
FIG. 2
are identified by the letters(a) to (d); the signals at these corresponding points are depicted in
FIG. 4
, which is a graph showing the frequency along the X-axis and the signal level along the Y-axis.
The signal I(a) with a central frequency Fnum is processed by a classical heterodyne circuit
10
with two transposition stages. The input signal (a) passes through a first mixer circuit
12
where it is mixed with a fixed frequency signal Foll having a higher frequency than Fnum. This mixer circuit
12
produces at the output (b) two spectrums S
1
and S
2
(
FIG. 4
) corresponding respectively to the difference and the sum of the mixed frequencies.
These two spectrums are separated by a first bandpass type filter
14
whose output transmits only the spectrum S
2
of the upper mixed frequency (C). Because the two spectrums are very close in frequency, this separation calls for a highly selective filter. To this end, a surface acoustic wave device (SAW) is normally used. This spectrum is then produced at the input of a second mixer
16
which also receives as an input a mixing frequency Fol
2
having a frequency higher than Fol
1
. As with the first mixer circuit
12
, this second mixer circuit
16
produces two spectrums S
3
and S
4
corresponding respectively to the difference and the sum of the frequencies in the spectrum delivered by the first filter
14
and the frequency Fol
2
.
The frequencies of signals Fol
1
and Fol
2
are chosen such that the upper frequency spectrum S
4
of filter
16
corresponds to the chosen frequency band (that is the UHF IV and V bands in the example considered). This spectrum S
4
is conserved by eliminating the other using a second filter
18
.
In the state of the art, this second filter is fixed in frequency. In other words, it selects just one frequency—or narrow band of frequencies—by eliminating all the others. This filter is therefore chosen so as to be tuned to the desired output frequency.
Normally, as the transmitter is of the fixed frequency type, the filter
18
is selected so as to pass the range of frequencies around the carrier corresponding to the transmission channel of the UHF band. It is thus necessary to provide a different fixed filter
18
for each transmission channel.
SUMMARY OF THE INVENTION WITH OBJECTS
An object of the present invention is to provide a bandpass filter of variable frequency so that it can adapt to different channels, notably in the 400 MHz to 1 GHz frequency band.
In the example considered, such a filter can be implemented as a replacement for the fixed filter
18
to provide flexibility to the circuit
10
with respect to the different channels which can be used.
To this end, a first object of the present invention is to provide a bandpass filter with adjustable central frequency and operative in the UHF band, characterized in that it comprises a series of cells coupled to each other by coupling capacitors, each cell forming a resonant circuit composed of at least one inductor connected in parallel with at least one variable capacitor.
Advantageously, the coupling capacitors are also variable capacitors.
According to a particularly remarkable characteristic of the invention, the filter can be made substantially symmetrical between its signal input and its signal output.
In a preferred embodiment the cells are four in number.
Preferably, each variable capacitor forming the resonant circuits and each variable coupling capacitor is in the form of at least one electrically controllable variable capacitor.
In this case, it is possible to provide that each electrically controllable variable capacitor is formed by at least one voltage controlled variable capacitance diode.
Preferably, each variable coupling capacitor is formed by a pair of variable capacitor diodes connected head to head.
In order to provide an optimization of the input and output impedance matching characteristics, the filter can comprise an input connected to an intermediate tapping of the inductor of the first cell of the series of cells and an output connected to an intermediate tapping of the inductor of the last cell of the series of cells.
Advantageously, the inductors of the first cell and the last cell have a value different from that of the inductor(s) of the intermediate cell(s), the difference in value enabling to employ a same variable control voltage for controlling on the one hand the electrically controllable capacitors of the resonant circuits formed by the first and last cells and on the other hand the electrically controllable capacitor(s) of the resonant circuit(s) formed by the intermediate cell(s).
In this case, the inductors of the first cell and the last cell preferably have an inductance value greater than that of the inductors of the intermediate cell(s)
For an easier implementation of the filter, it is possible to provide that the electrically controllable variable capacitors respectively forming the coupling between the first cell and the cell adjacent to the latter and the coupling between the last cell and the cell adjacent to the latter have a same capacitance value for a same capacitance control voltage over a determined range of control voltages.
Preferably, each inductor is in the form of a microstrip deposited on an insulating substrate.
A second object of the present invention is to provide a bandpass filtering circuit with adjustable central frequency operational in the UHF band, characterized in that it comprises a filter such as described above and voltage supply means for controlling the central frequency.
Advantageously, the voltage supply means produces a first vol
Ham Seungsook
Itis Corporation
Tarolli, Sundheim, Covell Tummino & Szabo L.L.P.
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