EP0662267A1

EP0662267A1 - If filter arrangement for fm reception signals

Info

Publication number: EP0662267A1
Application number: EP94921580A
Authority: EP
Inventors: Stefan Brinkhaus
Original assignee: Becker GmbH
Current assignee: Harman Becker Automotive Systems GmbH
Priority date: 1993-07-22
Filing date: 1994-07-21
Publication date: 1995-07-12
Also published as: DE4324603A1; US5584063A; WO1995003650A1

Abstract

The invention relates to an IF filter arrangement in an FM signal receiver. An IF filter arrangement is to be provided in which the adverse effects of a control loop in which controlled variables obtained from the MPX signal for tracking the IF filter do not arise. The solution of the invention is an IF filter arrangement with a filter unit having a defined and especially permanently set pass region and at least two outputs having opposed central frequencies or, with the same central frequency, offset IF frequencies. The signals from the outputs are taken to a weighting unit which is regulated by a controlled variable obtained from the demodulated FM signal.

Description

IF filter arrangement for FM received signals.

The invention relates to an IF filter arrangement for FM received signals.

From WO 88/08223 = EP 0358 649 an IF filter arrangement is known in which a narrow band IF filter with variable center frequency is arranged between the tuner and the demodulator, the center frequency of which can be tuned by means of capacitance diodes depending on the demodulated FM signal.

The desired advantage of this device compared to conventional fixed filters should, on the one hand, result in a significantly increased selection for adjacent channel interference and, on the other hand, a noticeable improvement in the signal-to-noise ratio due to the considerably reduced bandwidth and the stroke tracking used.

These improvements can only be seen in static operation with sufficient field strengths and a small stroke.

At high travel, the distortion factor in the demodulated FM signal rises very sharply to unacceptable values.

Another disadvantage of this arrangement is shown when pulse-like disturbances or rapid jumps in modulation occur. Then this IF filter arrangement can snap out so strongly that dramatic impulse disturbances become noticeable. In audio receivers with this circuit, these disturbances are known as aggressive scratching.

The invention has for its object to provide an IF filter arrangement which, with essentially comparable filter performance, shows a significantly reduced sensitivity to interference, in particular to impulse-like interference or rapid modulation jumps. This object is achieved by an IF filter arrangement with the features specified in claim 1 and the method for IF filtering with the features specified in method claim 17. Advantageous developments of the invention are presented in the subclaims.

In the case of low field strengths and strong adjacent channel interference, very narrow filters are required in accordance with the known IF filter arrangement, which filters are tuned by means of a control loop.

Contrary to the teaching of the prior art, the control of the tracked filters is dispensed with. According to the invention, the required regulation does not access the filters by tracking them, but rather links the output signals of a filter unit that has not been replicated with weighted defined transmission ranges. According to the invention, the IF filter arrangement with two or more filters with offset center frequencies shows the mode of operation explained below. At the output of these filters, the frequencies passed through the respective filters are present at the same time and with the appropriate phase compensation, which is not absolutely necessary. A subsequent weighting selects the optimum filter in accordance with the instantaneous modulation frequency or compiles a suitable resulting pass curve from the respective filters. This enables an optimal transmission curve to be formed without phase errors and practically without inertia. A control loop formed with this arrangement is orders of magnitude faster than when using a tracked filter and no longer has the disadvantages mentioned.

According to the invention, two or more filters of the same center frequency can also be used with offset IF frequencies. Signals are individually demodulated at the filter outputs and only then is the weighting of the demodulated FM signals to form a resultant signal. This allows the use of identical, inexpensive and easy-to-use filters while overcoming the disadvantages mentioned.

The signals emitted by the outputs of the FM demodulators are preferably routed to the weighting unit via a low-pass filter. There is a very fast control when the weighted FM signal from the output of the

Weighting unit is fed back to the weighting unit as a control variable via a control amplifier. This rapid embodiment of the invention ensures particularly low-interference filtering.

As an alternative to the permanently set filters, those are preferably used that can be adapted to the device-specific needs in the manufacturing process without the need for a separate filter type for each device type, whereby in this connection individual differences can be compensated for by component differences, and thus the best possible freedom from interference all devices is possible. The invention is based on the method in which a plurality of output signals of a filter unit which has defined passbands are linked to one another via a weighting unit. The weighting is controlled or regulated by demodulated FM signals. The output signals of the filter unit with the defined passbands either have mutually offset center frequencies or they show IF frequencies offset from one another at the same center frequencies. This method according to the invention with the regulation of the weighting of the filter unit output signals of filters that cannot be tracked overcomes the disadvantages mentioned.

Without being limited to these, further exemplary embodiments of the invention are explained with reference to FIGS. 2 to 9. Show it: 1 is a block diagram of the known IF filter arrangement,

2 shows an embodiment of the IF filter arrangement according to the invention as a block diagram,

3 shows a modification of the IF filter arrangement according to FIG. 2 with an additional IF train,

4 pass curves of an IF filter arrangement according to FIG. 2,

5 shows an exemplary embodiment for the weighting unit as Gilbert cell Gz in the IF filter arrangement,

6 shows an exemplary embodiment of the IF filter arrangement according to FIG. 2 with three outputs with three center frequencies offset from one another,

7 shows an exemplary embodiment of an FM receiver digitized in the IF plane with an IF filter arrangement similar to FIG. 3,

8 shows another exemplary embodiment of the IF filter arrangement according to the invention with demodulator Dem and low-pass filter Tp for each output signal, and FIG. 9 shows an IF filter arrangement according to FIG. 8 with several different ones

Output signals and with another implementation of the weighting unit by a processor, which receives the demodulated Fm signal from before the weighting unit.

1 shows the block diagram of an FM receiver which contains the known IF filter arrangement WO 88/08223. In this FM receiver, the IF is generated in the known modules from the signal received with the antenna. The demodulated FM multiplex signal MPX is available at the output of the demodulator Dem, from which an LF mono or stereo signal and RDS data are obtained.

In the known IF filter arrangement, the IF is fed into an IF filter ZF-Fi with a bandwidth which is significantly smaller than that of a normal IF filter

Center frequency can be tuned with capacitance diodes CAP. The ZF passes this narrow-band "filter with a considerable delay time Td.

The FM signal is demodulated in the demodulator Dem and is available for further processing as a multiplex signal MPX. The tuning voltage for the capacitance diode CAP is readjusted in a control loop via the low-pass filter TP and the control amplifier RV in order to track the resonance frequency of the narrowband filter BP with the current position of the IF frequency.

Because of the considerable delay time Td, the filter BP can be thrown completely out of the tracking range in the event of disturbances occurring in pulses, since the control loop disengages and takes considerable time until it engages again. The acoustic interference impression is a reception interrupted by very unpleasant loud bangs and slaps, which is itself quite low-noise due to the narrow-band filter. If the receiver is a satellite TV receiver, then in comparison to the conventional receiver, interference in the video signal only occurs at a significantly lower input level on the antenna, which initially appears on the screen as white and black lines (in technical terminology as Spikes or fish).

2 shows an embodiment of the IF filter arrangement according to the invention. An IF filter ZF-F1 F2 with two outputs, which have mutually offset center frequencies F1 and F2 and which are fed to the weighting unit Gew. The output signals F1 and F2 are summed with variable weighting and then fed to an FM demodulator Dem. To control this weighting, the demodulated FM signal from the FM demodulator is fed back to the weighting unit Gew in a control loop. Depending on the demodulated FM signal, the weighting of the individual signals supplied to the weight unit is changed. By changing the weighting, an effect corresponding to the tracking of an L-C filter is achieved, but according to the invention the running time of the IF filter, which is disadvantageous for a control loop, does not apply, since the control loop does not control the filters, but only intervenes after the filters. It is also possible to work with defined, non-dynamically changeable filters, which can be realized in terms of production technology as well as in terms of price, and also much more temperature-stable. A low-pass filter Tp and also a control amplifier can be provided in the control loop. Both can already be implemented by the demodulator Dem. The low-pass filter Tp only ensures that only the spectrally lowest signal is used as the reference variable for the control, which considerably simplifies implementation. The control amplifier Rv should, insofar as the tapped demodulated Fm signal is not of sufficient signal strength for the weighting unit, amplify it to the necessary extent or, if necessary, vaporize it, so that there is always a safe and clearly defined weighting.

In addition to the optional low-pass filter Tp and the control amplifier Rv, phase correction elements are preferably provided at the inputs of the weighting unit assigned to the filter unit, by means of which a phase-correct connection is ensured. In this way, nonlinear effects during the combination due to phase differences between the individual output signals of the filter unit can be avoided.

4 shows the transmission curves of an IF filter arrangement according to FIG. 2 for three different weightings, namely: 1. Fl 100% F2 0% (curve a)

2. Fl 50% F2 50% (curve b)

3. Fl 0% F2 1.00% (curve c)

By means of such a link, in addition to preventing the unpleasant interference effects of the invention, the signal-to-noise ratio is also improved.

3 shows an exemplary embodiment of the IF filter arrangement according to the invention which, in addition to the advantages already mentioned, permits a further increase in the selection at least up to the values of a tracked filter and thereby achieves a significantly higher cut-off frequency. In the exemplary embodiment according to FIG. 3, the IF filter arrangement according to FIG. 2 is connected in parallel with another IF train including demodulator with the output MPX2, which has an IF filter Fis with a trackable one Center frequency included. The tracking does not take place as a control loop which uses its own demodulated FM signal MPX2, but is controlled by the demodulated FM signal MPX1 obtained earlier in time. As a special feature, a delay element Del is provided with a delay time Tds, which is selected so that it is the sum of all delay times in this

Tracking arrangement corresponds; the runtime in the filter ZF-F1 / F2, in the demodulator Dem and the phase shift in the low-pass filter of the control element STG are relevant here. In contrast to the known IF filter arrangement, the tracking takes place in phase with the position of the current IF frequency. The advantage of this development over the known IF filter arrangement, depending on the optimization, lies on the one hand in a higher cut-off frequency of the multiplex signal (higher channel separation of the stereo signal and less disturbed RDS reception) and / or on the other hand in a larger selection for mono reception and a better noise-signal ratio (Remote reception). Compared to the embodiment of the invention according to FIG. 2, the advantage with the same cut-off frequency lies in a higher selection and a further improvement in the noise-signal ratio.

FIG. 5 shows an IF filter arrangement corresponding to FIG. 2 for a TV satellite receiver. The IF frequency of these receivers in front of the FM demodulator is 480 MHz. Surface filters OFW have proven themselves here. A bipolar mixer serves as the weighting unit. Such is known in the literature as a four-quadrant multiplier or Gilbert cell GZ and is therefore shown only schematically without DC paths. However, the coupling of the two filter outputs is a special feature. In contrast to the Gilbert cell, where the input alternating currents il and i2 are coupled in as identical signals, but with the phase position rotated by 180 degrees, the currents here are a function of the outputs Fl and F2, independently of one another and are summed (weighted) at the resistors R3 and R4 depending on the output voltage of the control amplifier RV.

6 shows a further development of the IF filter arrangement according to the invention as shown in FIGS. 2 or 5. This IF filter unit has three outputs with three center frequencies F1, F2 and F3 offset from one another. By increasing the number of outputs of the IF filter arrangement with appropriate cascading of the weighting units, which are designed here as Gilbert cells Gz, practically every filter curve can be achieved in accordance with the present reception conditions or reception priorities. The extreme values of the resulting filter curve are limited by the filter outputs that are the most distant from the center frequency, which leads to a corresponding number and corresponding cascading. Another possibility with a comparable effect as the cascading arrangement is given by the parallel arrangement of the weighting units with the subsequent summation ring.

The IF filter arrangements according to FIGS. 2 to 6 can be used for analog FM receivers. Conventional L-C filters as well as resonator filters or surface acoustic wave filters, for example on ceramic or quartz substrates, are conceivable as IF filters. However, the choice of a digital circuit from the IF level turns out to be much more flexible. For fast control loops, the arrangements according to FIG. 2 to FIG. 6 again make sense, since here, in the same way as with the analog filters, digital filters with constant or filter functions that can only be changed slowly, which in themselves are not very suitable for extremely fast tracking.

FIG. 7 shows the basic circuit diagram of an FM receiver digitized from the IF level in accordance with the principle shown in FIG. 3. Compared to the analog solution with, for example, fixed filter properties that can be selected in the production process, this system is far more flexible. About an assessment of the reception situation in

Receiving condition analyzer module RCA, all filter parameters can be preset according to the reception situation via a corresponding control processor, the delay time of the delay device Del can be selected according to the current filter settings and the control loop of the weighting of filter 1 or the control chain of the weighting of filter 2 can be optimized.

A technically simple solution is shown in FIG. 8. The outputs of the filter units ZF-F1 / F2 are routed to two parallel FM demodulators Dem and low passes Tp. Alternatively, two identical filters with the same center frequencies but offset IF input frequencies can be routed to the two FM demodulators Dem become. The alternatives are shown in dashed lines. The two low-pass filters Tp are constructed identically and are therefore in phase at their outputs. Their cut-off frequency can therefore be tailored to the useful signal and filters out large parts of the interference amplitudes. In addition, low passes Tp are much cheaper to implement than delay elements. The subsequent weighting by the weighting unit Gew takes place practically without delay in a control loop from the MPX output. The weighting is therefore essentially determined only by the useful signal. The control amplifier Rv shown, insofar as the signal strength requires it, ensures a sufficient input signal to the weight unit Gew by amplification or damping.

FIG. 9 shows an IF filter arrangement with properties similar to the IF filter arrangement in FIG. 3, but here the construction with parallel branches is chosen analogously to FIG. 8. This embodiment will mainly be applicable to digital circuits. The main advantage over the IF filter arrangement in FIG. 3 is that all the weighting processes take place in the baseband (useful modulation frequency) and thus a significantly lower data rate and resolution compared to systems in the IF range are necessary. The weighting is carried out via the control processor Prz. With the demodulator outputs MPX 1 to MPX n, outputs are available which still enable analysis of the disturbances in the frequency neighborhood and can therefore be taken into account in the control processor, for example due to lower weighting of severely disturbed demodulator outputs. In the case of adjacent-channel interferers, it is foreseeable that these are the respective outer frequency groups; in the case of inband interferers, these can also be internal groups. As an alternative to the version shown, the MPX 1 to MPX n signals can be tapped directly after the demodulators Dem also after the following low-pass filters Tp 1 to Tp n, which is what Reduced data rate further ensures possibly even further reduced. Due to the different signal transit times of the signals in the different branches, a delay element Del 1 to Del n is provided in each branch, which compensates for the difference in transit time and also provides the control processor Prz with the time to evaluate the MPX signals and to control the weight unit Gew accordingly .

Claims

Claims.

1. IF filter arrangement for FM received signals, with a filter unit, an FM demodulator (Dem), a low-pass filter (TP) and a weighting unit (Gew), which links the output signals of the filter unit to one another depending on the demodulated FM signal, the Filter unit has defined passbands, which show a plurality of mutually offset center frequencies (F1, F2) or, if the center frequency is the same, a number of offset IF frequencies.

2. IF filter arrangement according to claim 1, characterized in that the weighting unit is arranged so that the output signals of the filter unit are supplied to it without the interposition of the FM demodulator (Dem) or the low-pass filter (TP).

3. IF filter arrangement according to claim 1, characterized in that the weighting unit is arranged so that the output signals of the filter unit with the interposition of an FM demodulator (Dem) or a low-pass filter (TP) are supplied for each output signal.

4. IF filter arrangement according to one of the preceding claims, characterized in that a phase correction unit is provided which ensures a phase-correct linkage.

5. IF filter arrangement according to one of the preceding claims, characterized in that the defined pass bands are adjustable.

6. IF filter arrangement according to one of the preceding claims 1 to 3, characterized in that the defined passband are fixed.

7. IF filter arrangement according to one of the preceding claims, characterized in that the weighting unit (Gew) is designed as a bipolar mixer (GZ).

8. IF filter arrangement according to one of the preceding claims, characterized in that a control amplifier (RV) is provided which amplifies or dampens the demodulated FM signal for the weight unit to a defined value.

9. IF filter arrangement according to one of the preceding claims, characterized in that the tap of the demodulated FM signal is behind the weighting unit.

10. IF filter arrangement according to one of the preceding claims 1 to 8, characterized in that the tapping point of the demodulated FM signal lies in front of the weighting unit.

11. IF filter arrangement according to one of the preceding claims 1 to 8, characterized in that the tapping point of the demodulated and low-pass filtered FM signal lies in front of the weighting unit.

12 IF filter arrangement according to one of the preceding claims, characterized in that the weighting unit (Gew) is designed as a cascading arrangement of several individual weight units (Gew).

13. IF filter arrangement according to one of the preceding claims, characterized in that a delay element (Del), a center frequency tracking IF filter (Fis), a control element (Stg) and a demodulator (Dem) are provided in a second IF train are, the IF filter (Fis) in addition to the delayed by the delay element (Del) input signal of the IF filter arrangement at the same time via the

Control element (Stg) the end signal MPX1 of the first IF branch is fed and the output signal of the IF filter is then demodulated to the further end signal MPX2.

14 IF filter arrangement according to one of the preceding claims, characterized in that the weighting unit (Gew) is designed as a parallel arrangement of several individual weight units (Gew) with subsequent summation.

15. IF filter arrangement according to claim 5, characterized in that a quality assessment is provided, through which the adjustable pass bands are adapted to the respective requirements.

16 IF filter arrangement according to one of the preceding claims, characterized in that it is designed as a digital circuit. 17. A method for IF filtering, in which a plurality of output signals of a filter unit which has a plurality of defined passbands, which show offset center frequencies (F1, F2) or offset IF frequencies at the same center frequency, are linked to one another via a weighting unit, the Weighting of the output signals is selected depending on one or more demodulated FM signals.