Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Patent
1995-08-17
1996-12-10
Faile, Andrew
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
455306, 455307, 455339, H04B 106
Patent
active
055840639
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an I.F. filter arrangement for FM receiving signals.
An I.F. filter arrangement having a narrow band I.F. filter with varying centre frequencies is known from WO 88/08223, equivalent to EP 0 358 649.
The I.F. filter is disposed between the tuner and the demodulator and its centre frequency can be tracked dependent on the demodulated FM signal by means of capacitative diodes.
The desired advantage which this device should offer as opposed to conventional fixed filters is, on the one hand, a significantly increased selection with respect to neighbouring channel interferences and, on the other hand, a measurable improvement of the signal-to-noise ratio as a result of the considerable reduced bandwidth and the deviation tracking therefore applied.
These improvements are only seen in the statistical operation at sufficient field strengths and low deviation.
At high deviation, the Klirr factor in the demodulated FM signal rises very intensely up to unacceptable values.
A further disadvantage of this arrangement is seen when impulse-like disturbances or quick modulation shifts occur. This I.F. filter arrangement can then get out of control so much so that dramatic impulse disturbances become noticeable. In audio receivers having such a circuit, these disturbances are recognized as aggressive scratchings.
SUMMARY OF THE INVENTION
the task of the invention is therefore to achieve an I.F. filter arrangement having a substantially comparable filter performance but which shows a significantly reduced sensitivity to interference especially with respect to impulse-like interferences or quick modulation shifts.
This task is achieved by an I.F. filter arrangement having the features set out in claim 1 and by a method for carrying out I.F. filtering having the features set out in method claim 17. Advantageous further embodiments of the invention are represented in the dependent claims.
According to the known I.F. filter arrangement at low field strengths and strong neighbouring channel interferences very narrow band filters are necessary which are tracked by means of a control loop.
In accordance with the invention, regulation of the tracked filters is not carried out and therefore goes against the teaching of the state of the art. According to the invention, the necessary regulation does not take place by way of the filters which are to be tracked, but rather the output signals of a non-tracked filters unit having defined passbands are connected or mixed in a weighted relationship with one another.
According to the invention, the I.F. filter arrangement is shown having two or more filters with staggered centre frequencies, whose manner of operation is explained in the following.
The frequencies having passed through their respective filter appear simultaneously at the output of these filters and with the correct phase whereby it was unnecessary to force any appropriate phase compensation. A weighting operation, switched on at a later stage, selects the respective optimal filter corresponding to the instantaneous modulation frequency and puts together a resulting passband curve which fits the respective filter.
As a result, an optimized passband curve may be formed with no phase error and practically no time lag. A control loop formed with this arrangement is around a few magnitudes faster than when using a tracked filter and no longer comprises the disadvantages mentioned above.
According to the invention, two or more filters having the same center frequency but staggered I.F. frequencies can also be used. In this way, signals at the filter outputs are individually demodulated and only then are the demodulated FM signals combined to a resultant signal by way of a weighting operation. This construction, while overcoming the mentioned disadvantages, allows the use of the same type of filters which are more favourable from a cost point of view and are easy to employ.
Preferably, the output signals of the FM demodulators are fed to the weighting device via a respective
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Faile Andrew
Nguyen Lee
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