Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Patent
1989-10-16
1992-10-27
Eisenzopf, Reinhard J.
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
455307, 455340, H04B 116
Patent
active
051597098
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an arrangement of the type defined in the preamble of claim 1 and to an FM receiver equipped with such an arrangement.
Arrangements of this type are known from DE-OS 3,147,493 and DE-OS 3,438,286, EP-A1-75,071 or FR-OS 8,121,986, all of which originate from the inventor of the present invention.
In presently employed FM radio receivers, a broadband IF filter is provided. The center frequency f.sub.IF of such a broadband IF filter is 10.7 MHz in most standards while the channel bandwidth is about 200 kHz (.+-.100 kHz symmetrically with respect to the center frequency f.sub.IF). The passband characteristic is selected in such a manner that its bandwidth corresponds approximately to the transmission bandwidth. Due to the wide bandwidth, a relatively large amount of interference and noise components of the antenna input signal are passed by the prior art IF filter, with the result that relatively high input field strengths are required to ensure a useful signal (modulation) worthy of being received.
To increase reception sensitivity, it is known from the above-mentioned references to use as an IF filter a filter which has a narrow bandwidth relative to the channel bandwidth. The center frequency of this narrow-band IF filter is retuned in dependence on the modulation of the received signal. At any arbitrary point in time, selection thus occurs exactly where the momentary IF happens to be located. This results in much improved suppression of interference and noise components. The bandwidth of this controlled IF filter, which is also referred to in the literature as an "in-channel-select" filter or, abbreviated, "ICS" filter, is about 20 kHz as a result of which, however, only the monophonic useful signal component (L+R component of the useful MPX signal) is passed.
For this reason, it is only possible to operate the ICS filter in parallel with a broadband IF filter in an FM stereo receiver and to effectively connect the ICS filter with the LF stage of the receiver if, in any case, only monophonic reproduction is possible due to a received signal which is too weak or has too much interference (for example, below 25 .mu.V antenna input voltage).
SUMMARY OF THE INVENTION
In contrast thereto, the invention has the object of achieving in a filter arrangement of the type mentioned above, filtering of the entire signal, particularly the MPX signal, with adequate sensitivity without greatly increased engineering expenditures.
According to the invention, this is accomplished by the characterizing features of claim 1.
Advantageous features of the filter arrangement according to the invention are defined in the dependent claims. They also define an advantageous improvement of an FM stereo receiver equipped with the filter arrangement.
The invention is initially based on the idea that, in order to provide sensitivity, a sharp drop in filtering is required primarily in the immediate region close to the momentary IF so as to achieve a high signal
oise ratio with respect to noise components at that point. In contrast, it is immaterial whether a flat or steeply dropping curve branch exists in the further frequency curve of the filter passband characteristic.
Instead of, for example, a comb filter structure having discrete filter regions, a coherent passband is provided for the two L-R bands and for the ARI/RDS auxiliary carrier of the MPX signal which has a relatively level frequency curve. A narrow, distinctly higher filter passband with a sharp drop in filtering is superposed on this coherent passband around the center frequency f.sub.IF. The latter range is the cause of high sensitivity. The bandwidth of this narrow steep-edged filter passband is about 20 kHz, while the bandwidth of the remaining filter passband which has a relatively level frequency curve is about 100 kHz, taking into account the actually contradictory requirements for high sensitivity and a broad passband for the complete MPX signal.
Surprisingly, the advantages of the invention are not only an imp
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Eisenzopf Reinhard J.
H.u.C. Elektronik GmbH
Urban Edward
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