IE63591B1 - Circuit for detecting a frequency reference signal - Google Patents

Abstract

The invention relates to a circuit for recognising a reference frequency signal which is transmitted with a data signal in time- division multiplex mode. A circuit of this type is provided for mobile radio systems in order to correct the frequency standard of the mobile stations through transmission of a reference frequency from the (fixed) base stations. In order to ascertain the temporal position of a reference frequency signal of this type, a solution is to be described in the implementation of which expensive circuits and high processing speeds are avoided. The invention provides a circuit with two transverse filters (1) which serve to distinguish between the data signal and the FC burst (frequency correction burst). They are designed so that the transmission factor of one filter shows a minimum at the FC frequency, while that of the other shows a maximum at that frequency. A device (2, 3) to determine the absolute value of the filter output signals, an arrangement (4 to 9) to form the sliding mean value over the period of a burst length and a device (10) to form the quotient of the output voltages of both paths are in each case connected downstream from the transverse filters (1). <IMAGE>

Description

The invention relates to a circuit for detecting a frequency reference, signal which is transmitted in time-division multiplex with aa information signal, A circuit of this type is known from 5 iyS~A~4 358 737. This describes an improved filter detector circuit for cheeking input signals ©f various frequencies and for providing a detection signal when the input frequency has a desired predetermined value. The circuit is suitable for use as a sound detector for con10 tiauously sound-controlled squelch systems.

In connection with a future mobile radio system, it is provided to correct the frequency standard ©>£ the mobile stations (2SS) by transmitting a reference frequency from the (stationary) base stations (BS) . The frequency standard of a mobile station must be corrected, ea the one hand in order to ensure the accuracy required in generating its ows transmitting frequency but cs the other hand also ia order to facilitate the demodulation of the received information. This reference frequency is transmitted from the base station ia time-division multiplex with the information signal in such a manner that alternately she reference frequency is transmitted over a particular period (about 0.5 ms) and the information signal is transmitted in the subsequent longer period (about SO aas>. In this case, th® information signal transmits digital data at the rate ft (for example ft κ 271 kHz) and with a particular bandwidth (about 200 kHz). The reference frequency ia the form of a sinusoidal oscillation is transmitted with a spacing of ft/4 above the centre of the respective transmission frequency band. Since there is as yet no synchronism between base station and mobile station af the time of the frequency correction, the mobile station requires a device which responds to the particular characterist is of this so-called frequency correction burst The invention is therefore based on the object of specifying, for th© determination of the position in time of a frequency reference signal transmitted in the WM frame together with an information signal, a solution, in the implementation of which elaborate circuits and high processing speeds ar® avoided., According te the invention, this object is t„ achieved hy means of a circuit consisting of two transversal filters which are used for distinguishing between information signal and FC burst (frequency correction burst) from the received signal converted to an IF position and periodically sampled, and, for this purpose,' are constructed in such a manner that the transfer factor of one filter exhibits a saiaimusa af the FC frequency whilst that of the other one has a saassisatm, at that point, IS and furthermore consisting &£ one device each for forming the square value of the filter output signals and as arrangement, following in each case, for forming the .moving average over the period ©f one burst length, and a device la which the output voltages of both paths are continuously correlated with one another and the time of the maximum is determined.

Advantageous embodiments and further developments of the subject-matter of the invention are specified is, the subclaims.

Xa th® text which follows, the invention will be explained in greater detail with reference to aa illustrative embodiment shown in the drawing.

Figure 1 shows th© structure of a matched filter by means of which the detection circuit is implemented.

After completed conversion into a suitable intermediatefrequency position and periodic sampling of the received signal, th® samples thus obtained are supplied to this filter. Having regard to the simplest possible circuit technology, the intermediate-frequency position and the sampling frequency are selected in such a manner that the > FC burst is converted fc© a frequency which has a rational relationship to this sampling frequency. In a preferred illustrative embodiment, the intermediate frequency is 2ft and the FC burst at this frequency position accordingly is 9.4ft, and 13/32ffc' after sampling at a rat® of ft' = 0/5£t. (evaluation of every 5th sample). The spectrum of the information signal is then within the range from 0 to l/2ft*. lh© matched filter contains at its Input end two transversal filters (FIB filters « finite Impulse response) with a common delay nT (T « I/fit'} and the coefficients *1, *1, and, respectively, el, -1, which are used for the actual differentiation between information signal and FC burst. These filters have the characteristic of ce^b filters with transfer factors cos(n»vf/ffc*) aad, respectively, siain-*£/£&'}, that Is to say with mutually offset pass-band aad rejection characteristic.

In this arrangement, the factor n Is selected In such a manner that the transfer factor of one filter exhibits a minimum at the FC frequency whilst that of the other ©ae has a maximum at that point. In the Illustrative embodiment referred to above, this is the case, for example, for a b 16. The sin filter then has the transfer factor X at the FC frequency whilst the transfer factor of the cos filter exhibits a zero crossing at that point. In the information signal having a noise-like characteristic, both filters emit approximately the same output power whilst greatly different output powers are issued on the occurrence of aa FC burst. This situation is registered ia the subsequent circuit section of the matched filter.

For this purpose, an arrangement of an FIS, aad HR (IIS = infinite impulse response) filter is provided in each path, consisting of a delay 4 and, respectively, 5 (delay time saT) , a summation 6 and, respectively, 7 and a further delay 8 and, respectively, 9, through which the two signals pass after a device 2 aad, respectively, 3 in which actually the square of the output values of the transversal filters would have to be formed but aa absolute value Is formed for simplifying the arrangement.

This arrangement is the computing-time-saving version of a transversal filter having a sqcarewave pulse response of length mT and supplies the moving average over this period. The output voltages of both paths are continuously correlated with one another, for which purpose the division device 10 is provided. With mT corresponding to the length of the FC buret (0.5 ms), this results in an extreme value at the time when the FC burst had become effective in its entire length, that is to say at the transition from the FC burst to the information signal. 'Thus, only the time of this extreme value (maximum) needs fc© be determined.

To circumvent possible problems in the circuit implementation of the division provided here in real time, the following solution is provideds instead of continuously dividing at high resolution, the value pairs (averages) at the inputs C and D of the division device 10 are continuously loaded into a BAM (shown dashed) having the capacity of approximately half a burst length and are simultaneously compared with one another. The BSM input is stopped as soon as a condition derived from the comparison is met. This condition consists in that the running ratio of the values drops below a threshold value after this threshold value has been exceeded a minimum period before. This threshold value is dimensioned in such a.manner that, oa the one hand, it is reliably below the extreme value to be expected during the division and, oa the other hand, if covers the least possible number of value pairs, having regard to the required BAM capacity. In this manner, only value pairs which are within the immediate vicinity of that extreme value are stored in, the BAM at this time. The evaluation of the stored value pairs can now begin and, in principle, must only be concluded when a sew FC burst is to be received. The position of the FC burst in time, that is to say its accurate time, can new be determined at leisure (sen-real time) frcm the KAM address at which the corresponding value pair was stored. The circuit new required is, real time instead ©£ the division can be implemented with much less esqpesdifure than the latter. Following the formation of the quotient, a further FIB filter ear. be used for additional noise suppression and thus further Improve the accuracy ia the determination of the burst phase.

Figure 2 shows in a graphic representation the signal variations for a circuit simulation after formation of the absolute values at points A and 3, after formation, ©£ the moving averages at the output of the arrangement of FIS. and 113 filters (point C and D of the circuit) and, finally, the quotient 3 » D/C, that Is to say the ratio of the moving averages. From the last curve, an extreme value can be very clearly seen at a time when the FC burst has become effective In Its entire length, that Is to say at the transition from the FC burst to the information signal.

An extension of the circuit with the aim of limiting the computing range can be carried out by inserting an addition circuit 11 and, respectively, a subtraction circuit 12 in the two paths between the arrangement for forming the moving average and the division, device. Figure 3 shows this circuit section of Figure X between points C* and C and Ώ11 and 33. In this arrangement, the Inputs of each of the two circuits are supplied with the signals of both paths. The numbers 0, X and 2 entered ia the loops specify the signal values at the output off the arrangement for forming the moving average and after passing through the addition and summation circuit, the ones designated by a and a* applying to the normal signal variation and those designated by b and h* reproducing the signal values during the occurrence of an, FC burst. For the ratio of both signals at points D and C, this results in the value © or X, the maximum value 1 occurring at a time when the FC burst has become effective In its entire length (compare also Figure 4) . In contradistinction to the representation of Figure 2, this always results la a very accurate value of 1 or slightly less for the maximum, Independently of the transmission conditions (background noise, frequency offset)»

Claims (6)

1. Patent Claim®s

1. Circuit for detecting a frequency reference signal which is transmitted In time-division multiplex with aa Information signal, characterised by two transS vereal filters (IS which ar· used for distinguishing between information signal and FC burst (frequency correction burst) free the received signal converted to an IF position and periodically sampled, and, for this purpose, are constructed in such a manner that the 10 transfer factor ©£ one filter exhibits a minimum at the FC frequency whilst that ©£ the other ©ae has a ataxiaim at that point, and furthermore by ©ne device (2, 3) each for forming the square value ef the filter output signals .and an arrangement Ή to S’>, following In each case, for 15 forming the moving average over the period of one burst length, and a device (10} In *d&ich the output voltages of both paths are continuously correlated with one another and the time of the .maximum Is determined.

2. Circuit according fc© Claim 1, characterised in 20 that the two transversal filters (1) exhibit a ccaaoa delay si C? * 1/ffeQ aad the coefficients ·»·1,φ1 and, respectively, *1, -1 and have the characteristic of ©csaab filters with transfer factors of ©ee<»·£/££*) and, respectively, ei» 25

3. „ circuit according to Claim 2, characterised Is that the factor a of the transversal filters (1) Is selected in such a manner that the transfer factor ©f the filters exhibits the In each case opposite maxiausa/ niaiasnasa value at the FC frequency. 30

4. Circuit according fc© one ©f Claims 1 fc© 3, characterized in that the arrangement (4 to 9} for forming the moving average consists of an FIB and an IIB filter»

5. » Circuit according fc© Claim 4, characterized in 35 that the averages frcm the FIB and HR filter are continuously loaded into a RAM and are simultaneously compared with one another and the BAM input is stopped when a ©ssadifcl©» derived from the eemparison has been reached. '§» Circuit according t© Claim 5, characterised is that the condition coaaeisfcs in the dropping of the running ratio of ‘the values below a threshold value which 5 ie disseaeicsaed is such a manner that, oa 'the ©ae hand, it is reliably feels®? the extreme value te be expected during· the division and, cm the other hand, it covers the least possible number of value pairs, having' regard to the required EAM capacity. 10 7. Circuit according to Claim 6, characterised is that the SAMs exhibit a capacity ef approximately half a buret length.

6. 8. Circuit according fc one of the preceding claims, characterised in that, befewea the arrangement (4 to 9) 15 for fcraiag the moving average and the division device (10), an addition device (11) is inserted in one path and a subtraction device (12) is inserted in the other path, the few inputs of which are in each case supplied with the output signal of one path. 20 9. Circuit according to one of the preceding claims, characterized in that, between the transversal filter (!) and the device (4 to ,9) for forming the moving average, a device for forming the absolute value of the filter output signals is in each eatse inserted. 25 10. A circuit for detecting a frequency reference signal which is transmitted with a message signal in a time-division multiplex r substantially as hereinbefore described with reference to Figs. 1*3 of the accompanying drawings.