DE2342409A1 - Receiver for FM radio links - uses reduction of channel cross-modulation by adjacent channels - Google Patents

Abstract

Selection is increased only when input power is low. Two band-stop filters are tuned to frequencies on both sides of the passband of an IF bandpass filter, to which diodes are added which strongly damp the band-stop filters when the input signal is high, and remove the damping when it is low. The band-stop filter L-C ratio is such that the propagation time distortion is symmetrical. The diodes in series with a capacitor are in parallel with the band-stop filters tuned to the sum or difference of the useful channel and of the adjacent channels centre frequencies. For automatic gain control the current is derived from the gain control voltage.

Description

Method for reducing cross-modulation by adjacent channels in FM directional radio receivers

The invention relates to a method and a circuit for performing the method for reducing the Cross modulation through channels in FM directional radio receivers that are adjacent to the useful channel.

In the case of FM radio links, the RF and IF selection means are generally designed so that adjacent channels do not have any significant interference. Normal propagation conditions, as they prevail about 99% of the time, are assumed. In the remaining 1 % of the time, however, the adjacent channels can give rise to cross-modulation, namely when the useful carrier is very strongly attenuated, while the interfering adjacent channels are received practically undamped (selective fading or device interference on the transmitting side). The cross modulation manifests itself, for example, in understandable crosstalk and can therefore lead to quite unpleasant effects in monitoring circuits, among other things.

The invention is based on the object of specifying a solution for suppressing or reducing the cross modulation even in the event that the useful carrier is very strongly attenuated, but the interfering adjacent channels are practical can be received unattenuated.

This task is carried out with one of the methods mentioned at the beginning. Art according to the invention by increasing the selection only for the operating case of reduced input power solved.

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To carry out this method, a circuit is advantageously used with two bandstop filters, which are _{tuned to frequencies (f - Afι f Q} + 2Xf) on both sides of the pass band of an IF band limiting filter and dimensioned according to the RF channel spacing and which are connected to diodes controlled in this way are that the bandstop filters are heavily attenuated when the input level is high and undamped when the input power is low.

The L-C ratio of the bandstop filters is more advantageous Way chosen so that an occurring delay time distortion is symmetrical.

In an advantageous embodiment of the subject matter of the invention it is provided that the diodes in series with a capacitor, in each case parallel to the in particular on the sum or Difference frequency from the band center frequency of the useful channel and adjacent channel spacing matched bandstop filters are switched and a control current is fed to the junction of the diode and capacitance.

With a directional radio receiver with automatic gain control the control current is advantageously derived from the control voltage of the automatic gain control. derived.

The invention is described below with reference to the diagrams shown in the figures and an exemplary embodiment explained in more detail.

In Fig. Ϊ are the usual selection relationships in FM directional radio receivers shown. In this case, the attenuation a is plotted as a function of the frequency f in a diagram. In the middle of the pass band of the filter curve of the IF band-limiting filter is the Hutzkanal with the

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Center frequency fo and one corresponding to the hatched area Bandwidth. On both sides of the useful channel, i.e. on both sides of the pass band of the IF band-limiting filter, there is one adjacent channel each at frequency fjr.

In Fig. 2 are the thermal receiver noise (curve a), the intermodulation noise (curve b) and the cross modulation (Curve c) with variable input power and constant selection shown over the signal-to-noise ratio in baseband. The numerical example in FIG. 2 does not relate to a specific device; only the qualitative connections should be considered clarify. It can be seen from this diagram that the signal-to-noise ratio is reduced by cross-modulation when the Useful input power Ng decreases and the interference power of the Adjacent channels remains constant. The ones with the steeper slope. running curve of cross modulation and the curve of thermal Receiver noise intersects at a point corresponding to an input level of -40 dBm. From this one Input power, the cross modulation is greater than the thermal receiver noise.

Considerations within the scope of the invention have shown that a substantial reduction in the cross-modulation sensitivity can be achieved through an increased selection. However, a larger selection leads to larger delay time distortions and larger intermodulation noises and this always , ie not only in the short period of time, that is to say about 1 % of the time in which the higher selection is required, but during the entire time.

According to the invention, the selection is only increased when the input power is low. In one of FIG. 1 corresponding Diagram is shown in Fig. 3, the selection against the adjacent channels with a variable additional selection. The dotted curves in the area between the ifatz canal and the neighboring canals show how if the input power is reduced, the selection is increased.

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4 shows the thermal receiver noise (curve a), the intermodulation noise (curve b) and the cross modulation (curve c) with variable input power and variable additional selection, with the signal-to-noise ratio in the baseband as a function of the input power N _{E is} applied. Compared to the representation in FIG. 2 with constant selection, the intermodulation noise also increases as the input power decreases, but it still remains significantly below the thermal noise. Above all, however, it is essential that the cross modulation, the curve of which runs with approximately the same slope as that of the thermal noise, is considerably reduced by the additional selection.

5 shows, as a basic circuit diagram, an exemplary embodiment of a circuit for carrying out the method for reducing the cross modulation. This consists of two bandstop filters that operate on frequencies "on both sides of the pass band of the IF band-limiting filter with the center frequency fo, its inductive and capacitive element are drawn in dashed lines in the figure, are matched. The bandstop filters with the frequencies fo- ^ f or fo + Zif exist from the parallel connection of a capacitor C1 or C2 with a coil L1 or L2, each of which according to the invention a diode D1 or D2 is switched on, which has an attenuating or de-attenuating effect on the bandstop filters. The diodes D1 and D2 are in series with a capacitor C3 and C4, respectively. At the connection point of diode D1 and capacitor C1 or diode D2 and capacitor C2 is the control current ig via an ohmic resistor R1 or R2 to the diodes D1 and D2 fed to an FM directional radio receiver with automatic Gain control is derived from this control voltage. The regulation now takes place in such a way that the Band-stop filters with the diodes are strongly attenuated as long as

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the input level is high and must be undamped, d. H. can be made effective when the input power is small is. The L-C ratio of the two circles will be like this chosen so that the delay time distortion that occurs remains symmetrical.

5 claims
5 figures

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Claims (5)

Claims 1. Procedure for reducing cross-modulation Channels in FM directional radio receivers that are adjacent to the "useful channel", characterized by an enlargement of the selection only for the operating case of reduced input power. 2. Circuit for performing the method according to claim 1, characterized by two bandstop filters on either side of the passband of an IF band-limiting filter lying frequencies (fo- ^ f, fo + ^ f) matched and dimensioned according to the RF channel spacing and connected to such controlled diodes are that the bandstop filters are strongly attenuated with a high input level and with a low input power undamped s ind. 3. A circuit according to claim 2, characterized in that the L-C ratio of the bandstop filters is selected is that any delay time distortion that occurs is symmetrical is. 4. A circuit according to claim 2 or 3, characterized in that g e k e η η that the diodes in series with a capacitor, each parallel to the particular on the sum or difference frequency of the band center frequency of the useful channel and the adjacent channel spacing of matched bandstop filters are connected and a control current for the diodes is supplied to the connection point of the diode and capacitance is. 5. A circuit according to claim 4 for an FM directional radio receiver with automatic gain control, thereby g e - VPA 9/641/3012. - 7 - 509810/0474 indicates that the control current from the control voltage of the automatic gain control is derived. VPA 9/641/3012 509810/0474