Method of eliminating a DC component and filtering device obtained by this method
View Patent ↗A radiotelephony device employing filters and methods for eliminating a DC component from an electronic signal provide for circuitry having a zero corresponding to a cut-off frequency, and circuitry for shifting the zero by an amount corresponding to the absolute value of that cut-off frequency. In such radiotelephony device, the filters and methods, which include integration and operation of gyrators, tend to be (a) suited for incorporation in integrated circuits by virtue of reducing the physical size of the required capacitors and (b) reduce time-dependent inertia.
1. A radiotelephony device, comprising:
an antenna and filter system configured to ensure reception of a radio signal and to deliver on an output an electronic signal representing said signal,
an amplifier connected to the output of the antenna and filter system, configured to deliver on an output an amplified electronic signal,
a mixer having a first input connected to the output of the amplifier, a second input configured to receive a signal that comes from a local oscillator, and an output configured to deliver a signal that has a fixed frequency called intermediate frequency,
a demodulator having an input connected to the output of the mixer and an output configured to deliver a demodulated audio signal, and
a filtering device inserted between the mixer and the demodulator, and having elliptical filters employing the intermediate frequency as a cut-off frequency.
2. A radiotelephony device as claimed in claim 1 , wherein the filtering device comprises:
a first and a second input configured to receive a first and a second input signal respectively, the second input signal being identical to the first but in phase quadrature to the first,
a first and a second output configured to deliver a first and a second output signal respectively, the second output signal being identical to the first but in phase quadrature to the first,
a first elliptical filter including N capacitive elements Ci (for i=1 to N), arranged between the first input and the first output, the filter having a transfer characteristic that offers at least a zero at a predetermined frequency called the cut-off frequency,
a second elliptical filter, identical to the first elliptical filter, arranged between the second input and the second output, and
N gyrators Gi (for i=1 to N), each connected in parallel between two corresponding capacitive elements Ci (for i=1 to N), of the first and second elliptical filters, the gyrators being designed so that the filtering device has a similar transfer characteristic to that of the first and second elliptical filters while featuring a shift whose extent is equal to the absolute value of the cut-off frequency;
wherein the first and second elliptical filters, together with the N gyrators provide a transfer characteristic that is symmetrical with respect to the frequency of the first and second input signals.
3. A radiotelephony device as claimed in claim 2 , wherein each gyrator Gi (for i=1 to N) has a transconductance gi and has a first and a second input configured to receive a first and a second current I 1 i and I 2 i respectively, and a first and a second output configured to deliver the first and second current I 1 i and I 2 i respectively, and said gyrator is configured to develop a first voltage V 1 i between its first input and output, and a second voltage V 2 i between its second input and output, the behavior of each gyrator is defined by the relationships:
I 1 i=(gi)(V 2 i) and I 2 i=(−gi)(V 1 i), and
gi=(2π)(Fc)(Ci),
where Fc is the absolute value of the cut-off frequency.
4. A radiotelephony device as claimed in claim 1 , wherein the filtering device comprises:
a first filter, including N capacitive elements C 1 i (i=1 to N) arranged between a first input terminal configured to receive a first signal from a mixer, the first filter having a transfer characteristic including a zero at a predetermined cut-off frequency;
a second filter, including N capacitive elements C 2 i (i=1 to N), arranged between a second input terminal configured to receive a signal that is phase shifted from the first signal, the second filter having a transfer characteristic including a zero at the predetermined cut-off frequency; and
at least N gyrators Gi (i=1 to N), each connected in parallel between two corresponding capacitive elements C 1 i and C 2 i (i=1 to N), of the first and second filters, the gyrators configured such that the filter device has a transfer characteristic similar to that of the first and second filters but shifted by an amount equal to the absolute value of the cut-off frequency;
wherein the first and second filters, together with the N gyrators provide a transfer characteristic that is symmetrical with respect to the frequency of the first and second input signals.
5. A radiotelephony device as claimed in claim 4 , wherein the first and second filters are elliptical filters.
6. A radiotelephony device as claimed in claim 4 , wherein capacitors C 1 i and C 2 i (i=1 to N) are disposed on an integrated circuit.
7. A radiotelephony device as claimed in claim 6 , wherein the cut-off frequency is equal to the frequency of the first and second input signals.
8. A radiotelephony device as claimed in claim 7 , wherein the cut-off frequency is approximately 100 kHz and the capacitors Ci (i=1 to N) each have a nominal value of one picofarad.