Arrangement for demodulating a vestigal sideband signal
View Patent ↗An arrangement is described for demodulating a vestigial sideband signal component contained in an amplitude-modulated frequency signal, and preferably in a vision intermediate-frequency signal derived from a television signal. What distinguishes the invention is that there are provided two parallel, preferably identical, mixing stages ( 8 a, 8 b ) that operate at a phase shift of substantially, and preferably exactly, 90°, of which mixing stages a first one ( 8 a ) produces from the frequency signal a first output signal and a second one ( 8 b ) produces from the frequency signal a second output signal that is phase shifted by substantially, and preferably exactly, 90° relative to the first output signal, a downstream phase-shifter ( 12 ) that causes a phase shift between the first and second output signals from the mixing stages ( 8 a, 8 b ) of substantially, and preferably exactly, a further 90°, an adder ( 14 ) that adds together the output signals from the phase shifter ( 12 ), which output signals have been phase shifted relative to one another by substantially, and preferably exactly, 180°, a local oscillator ( 10 ) that drives the mixing stages with a phase shift of substantially, and preferably exactly, 90°, and a setting device for setting the frequency and phase of the local oscillator ( 10 ), the setting device setting the frequency and phase of the local oscillator ( 10 ) substantially, and preferably exactly, to the frequency and phase of the frequency signal.
1. An arrangement for demodulating a vestigial sideband signal component contained in an amplitude-modulated frequency signal, the demodulating arrangement comprising:
a local oscillator;
two parallel mixing stages to receive the frequency signal, wherein the two parallel mixing stages are driven by the local oscillator to operate at a phase shift of substantially 90°, of which mixing stages a first one produces from the frequency signal a first output signal and a second one produces from the frequency signal a second output signal that is phase shifted by substantially 90° relative to the first output signal;
a downstream phase-shifter that causes a further phase shift between the first and second output signals from the mixing stages of substantially, a further 90° for a total phase difference of about 180° between the first output signal and the second output signal;
an adder that adds together the output signals from the phase-shifter; and
a setting device to receive the frequency signal prior to mixing of the frequency signal by the parallel mixing stages for setting the frequency and phase of the local oscillator substantially to the frequency and phase of the frequency signal.
2. A demodulating arrangement as claimed in claim 1 , wherein the setting device comprises a phase-locked loop circuit that is locked substantially to the phase of the frequency signal.
3. A demodulating arrangement as claimed in claim 1 , wherein the second mixing stage comprises an inverter for inverting the polarity of the second output signal.
4. A demodulating arrangement as claimed in claim 1 , wherein the phase-shifter is formed by a complex filter.
5. A demodulating arrangement as claimed in claim 1 , wherein the phase-shifter comprises a first phase-shifting path for processing the first output signal from the first mixing stage and a second phase-shifting path for processing the second output signal from the second mixing stage.
6. A demodulating arrangement as claimed in claim 5 , wherein each phase-shifting path comprises at least one all-pass filter.
7. A demodulating arrangement as claimed in claim 1 , further comprising an adjustable amplifier connected upstream of the mixing stages to allow the frequency signal to be set to a desired level.
8. A demodulating arrangement as claimed in claim 1 , further comprising a window-filter connected upstream of the mixing stages.
9. A demodulating arrangement as claimed in claim 7 , wherein the window-filter is connected upstream of the amplifier.
10. A demodulating arrangement as claimed in claim 1 for processing a vision intermediate-frequency signal derived from a television signal, within a frequency range of up to approximately 60 MHz.
11. A demodulating arrangement as claimed in claim 1 , wherein the downstream phase-shifter causes a phase difference of substantially 0° between an in-phase path and an in-quadrature path for one vestigial sideband signal component of the frequency signal, and causes a phase difference of substantially 180° between an in-phase path and an in-quadrature path for another vestigial sideband signal component of the frequency signal.
12. A demodulating arrangement as claimed in claim 7 , wherein an input of the setting device is coupled to an output of the adjustable amplifier to directly receive the frequency signal from the adjustable amplifier connected upstream of the mixing stages.