Receiver channel selection

(54) Designation: Control signal generator for a channel selector device in a radio receiver fframes within brackets indicates international identification code, INID code. Letters in clamps indicate international document code

7802477-5

The present invention relates to a control signal generator for a channel selector device in a radio receiver for frequency or phase modulation that is movable relative to fixed transmitter stations transmitting at mutually different frequencies.

Such a device can be used, for example, in a radio link between a train and fixed stations along the railway. For example, there may be three possible transmission frequencies and two consecutive stations transmitting at mutually different frequencies. If the train passes between two stations, then the receiver in the train must be tuned so that at a certain moment the frequency of the next station is received instead of the frequency of the station just passed.

In order to find a solution to this kind of problem, it is advisable to use a receiver in the train which can be tuned to the various possible transmission frequencies and which for this purpose is equipped with an automatic channel selector device, e.g. a circuit that automatically switches on local oscillators. The control signal generator has the task of delivering a signal for controlling the channel selector circuit when a certain deterioration in the reception occurs. However, it should be noted that in order to maintain a certain reception quality in the train, the automatic channel selection must be made before the reception becomes impossible, ie from the time when the smallest possible disturbances begin to occur. This is essential if the link is used for, for example, data transfer.

At the VHF / UHF transmission frequencies utilized in the phase or frequency modulator, it appears that the first deterioration in the link, which occurs when the moving body with the receiver moves away from a fixed station, is not due to a

7802477-5 decrease in the mean of the received field strength but instead of rapid and in principle cyclic changes in said magnitude caused by the receipt of components that propagate along different propagation paths.

Legs of the present invention utilize the deterioration in signal-to-noise ratio «

which occurs during these cyclic disturbances to generate in the receiver a control signal for controlling the automatic frequency selection, thereby simultaneously solving the problem arising from the significant changes in the speed of the moving body. The frequency of the disturbances in the received field is proportional to the speed of the moving body, while below a certain field strength threshold the duration of each disturbance is inversely proportional to said speed.

According to the invention, a control signal generator for the channel selector device in a phase or frequency modulation radio receiver is arranged to include: means for forming a logic signal indicating whether the level of the received signal is above or below a given threshold value; a gate circuit adapted to pass pulses from a clock pulse generator if said logic signal assumes a value indicating that the level of the received signal is below the threshold; means for calculating the clock pulses passed by the gate circuit for specified time intervals derived from the clock pulse emitting a control signal if either for a period of n time interval The number of counted pulses during each time interval Γ reaches a first given threshold value, or if the number of counted pulses during a time interval ύ reaches a second given threshold value that is higher than the first mentioned threshold value.

The invention will be described in more detail below with reference to the drawings, in which: Fig. 1 shows an FM radio receiver comprising a control signal generator according to the invention; and Figures 2 and 5 show diagrams of the received field strength as a function of the distance between the receiver and a transmitting station, each corresponding to two ways in which the control signal generator according to the invention may be arranged in the receiver.

Legs of the simplified FM receiver shown in Fig. 1 include a receiver antenna 1, a portion 2 comprising an amplifier for amplifying the received high frequency signal and may also include a step for converting the amplified high frequency signal into a lower frequency. Bone first intermediate frequency, which may be different in size depending on the received frequency (three values ​​in the present example) is converted to a second fixed intermediate frequency in a mixer stage 3, to which, depending on the received frequency, a frequency of three derived from the quartz oscillators 4, 5.6 is added. The output of the mixer stage 3 is connected to an amplitude limiter 7, which consists of two high power amplifier stages 8,9 · The output of the limiter 7 is connected to a frequency discriminator 10, which emits a low frequency signal which is filtered in the filter 11 and then amplified in the amplifier 12. The filter 11

7802477-5 pass bands range from, for example, 0 to 5 kHz in accordance with the band of the affected useful signal.

The receiver further comprises an automatic channel selector device 13 which cyclically activates the three quartz oscillators 4.5.6 if the channel selector device receives a control signal at the input 14 · The cycle of switch-on times is generated in a known manner starting from the pulses of the clock pulse generator 15.

The receiver is mobile relative to the fixed stations which transmit at mutually different frequencies (3 in the present example). The control signal generator according to the invention is designated in Figure 1 and outputs the control signal to the input 14 of the automatic channel selector device 13 if the quality of the radio link to a fixed transmitting station from which the mobile receiver moves away becomes too poor, in order to connect to another fixed transmitter station as the receiver approaches. As described above, it is essential that the quality deterioration at reception is detected as early as possible.

The control signal generator is based on the phenomenon that will be described below with reference to the diagram in Fig. 2. In this diagram, the waveform C repres represents the variation in the received field strength H at the receiver antenna 1 as a function of the distance between the receiver and the transmitting station located in origin 0. However, the field strength H is shown only in the vicinity of a point A beyond which the problem of the impaired reception arises. It can be seen that in the frequencies used for frequency modulation the VHF / UHP field strength, represented by the waveform Cj, decreases on average with increasing distance, but in addition the field is affected by short-term attenuation x the field strength due to interference as the signal propagates. paths between the transmitter antenna and the receiver antenna. The distance between two successive attenuations (decreases) in the field strength is theoretically equal to λ / 2 for interference between the signal passing the straight propagation path and the signal which. passing a propagation egg via a single reflecting object, for example, Å / 2 = 35 cm at a frequency equal to 450 MHz. If H + indicates the field strength above which the limiter 7 in the receiver is no longer saturated, it will be seen that beyond the point A the received field H, represented by the waveform C of the limiter 7 and, consequently, at the output of the frequency discriminator 10. Since the field strength H decreases on average with increasing distance, it can be seen that the duration of the noise pulses also increases with the distance as this increases at a given speed. But on the other hand, this speed can vary considerably (for example, in the case of the mobile body being a train) and from this it can be seen that the frequency of the pulses is proportional to said speed, while the duration is inversely proportional to this speed. The control signal generator utilizes these noise pulses to generate a control signal to control the automatic selection of the frequency by making itself substantially independent of the moving body speed.

In the embodiment of the receiver shown in Fig. 1, the control signal generator 16 is connected to the output of the frequency discriminator 10. This includes a handpass filter 17 which filters out the noise (interference) outside the band for the useful signal filtered out through the filter 11 in the receiver. The filter 17 pass hand extends from, for example, 3 to 6 KHz. Circuit 19 forms the noise pulses detected by rectifier circuit 18 · This circuit produces logic pulses that correspond to the noise pulses and whose values ​​are 1 or 0 depending on whether the received field strength H is less than or greater than a given field strength threshold.

AND circuit 20 passes the pulses from clock pulse generator 15 when the logic pulses supplied by circuit 19 carry the value 1, indicating the presence of noise.

The pulses from the clock pulse generator 15 are applied to the cascade coupling of two frequency dividers 21 and 22, which output synchronized pulses separated by the time interval T 2 and by the time inverter interval n Z 2 (n integer).

The clock pulses from the gate circuit 20 are supplied to the counter 23, which is reset to the initial position by each output pulse from the divider 21, that is, at times separated by the time interval L. In addition, counter 23 is reset to its initial state each time the number of counted pulses assumes a predetermined value N, thereby emitting an output pulse. The output pulses from counter 23 are applied to a D flip-flop 24, which is reset at times spaced time interval ooh which changes state only once during a time interval l if counter 23 has counted IT clock pulses. Each time the flip-flop 24 changes state, the contents of the counter 25 are increased by one. The counter 25 is reset to the initial position by each output pulse from the divider 22, that is, at times the time interval n L is separated. When the content of the counter 25 becomes equal to the number n, it emits a pulse which is used via the OR circuit 26 as a control signal to control the automatic channel selector circuit 13 · This control signal can alternatively be obtained by the counter 27 · The counter receives the output pulses from the counter 25 and is reset to the initial state at times separated by the time interval ~ C. When the content of the counter 27 assumes the value n, it emits a pulse which is supplied via the OR circuit 26 as a control signal to the automatic channel selector 13 ·

The control signal generator 13, described above, operates as follows. AND circuit 20 emits a series of clock pulses. The number of pulses in a series depends on the length of a disturbance pulse. The frequency of the pulse train is proportional to the frequency of the interference pulses.

The counter 25 emits a control signal if, after n consecutive time intervals, the duration of the disturbance in each time interval i is equal to or greater than a specified time (otsnew N clock pulses, This ultimately means that the short interruptions

7802477-5 of the link defined in FIG. 2 has the effect of generating the control signal only when at least n consecutive disturbances occur with a duration equal to at least (F ',

The counter 27 emits a control signal when, after a time interval in the duration of the disturbance during this interval, exceeds a specified time period corresponding to mN clock pulses. For example, it is possible to select the count threshold value m for the counter 27 in such a way that the control signal is output only if the disturbance continues for the entire time interval τ '. The control signal supplied by the counter 27 appears before the signal of the counter 25 and indicates a very low quality connection.

If during the frequency selection the switching on of the quartz oscillators 4,5.6 results in no interference at the output of the discriminator 10, then the selection is canceled and the receiver remains tuned to the transmitting station in question.

By appropriate selection of the various parameters defined above, ie the frequency of the pulses from the clock pulse generator 15, the time interval the numerical values ​​n, M, m, it can be achieved that the generation of the control signal becomes virtually independent of the speed of the moving body. This choice must be made for each particular application and, in particular, taking into account the ratio of the maximum and minimum speed of the moving body.

In Figure 1, the control signal generator 16 is connected to the output of the frequency discriminator 10. It will be appreciated that the control signal generator 16 may alternatively be coupled to the output of the amplitude limiter 17 to detect substantially the same interference pulses when the received field frequency H does not assume a value below the field strength threshold 7. .

Alternatively, it is possible to use as the input to the control signal generator 16 the signal appearing at a point between the amplifier stages 8 and 9 which are included in the amplitude limiter 7. This modification is shown in Fig. 1 by dashed lines. The device 16 ', which corresponds to 16, is connected to the output of a voltage comparison circuit 28. An input of this comparison circuit is connected to the point 29 between the amplifier stages 8 and 9 »which is included in the limiter 7, and the second input of the comparison circuit is supplied with a reference voltage Vg. The comparison circuit 28 is arranged to output a signal only when the voltage at the input connected to the point 29 is less than the reference voltage Vg.

This modified embodiment is based on the fact that with signal at the inputs of the limiter 7, of such size that limitation is taking place, the amplifier step 8 will operate on its non-linear part of its amplification characteristic, whereby a distorted signal of very wide frequency spectrum occurs at point 29 »About the reference voltage V₂ is located within the output range of step 8 within which the signal is generated, so that, in case the voltage at the input of the limiter, and thus the received field strength, is below a given threshold corresponding to Vg, a signal of wide frequency spectrum is obtained at the output of comparison circuit 28. The control signal generator 16 ', which corresponds to 16, is provided at its input with a special bandpass filter which filters out a portion of the spectrum of the output signal from the comparison circuit. 28th

As can be shown by means of Fig. 3, this embodiment allows one to obtain the control signal at an earlier stage, that is, before the interference pulses appear in the output of the frequency discriminator, and thus just before the reception will be disturbed. In Figure 3, which, for the sake of clarity, is drawn opposite to Figure 2, the waveform indicates the received field strength adjacent to a point B, the distance OB to the transmitting station located in O being shorter than the distance OA in Figure 2. The waveform V,, which has a process similar to the waveform in Figure 2, is affected by the same disturbances, but on average the field strength H represented by the curve is greater than the average field strength represented by the waveform. The cold strength Hg corresponds to the threshold voltage Vg supplied to the voltage comparison circuit 28. The beam strength Hg is greater than the field strength H ^ in Fig. 2. If the received field strength H decreases and becomes lower than the field strength Hg, pulses are obtained at the output of the comparison circuit 28, which pulses correspond to the pulses used in the control signal generator 16 for generating the control signal for controlling the automatic channel selector device 13 ·