DE2928976A1 - Method for demodulating FSK data trains - using count output comparators for precisely detecting logic level changes - Google Patents

Abstract

The demodulator employs at least two high frequency counters which count the periods of positive half wave cycles contained in the message train. The method requires that at least two consecutive half waves are thus measured and retained in memory (2) in order to derive therefrom whether there takes place a transition from a lower frequency to a higher, or vice versa, or no change at all. It is the trend detector (2) which determines the clock rate inputted to an up/down counter (4). The counter counts up when there is a transition from a higher to a lower frequency (longer half wave period) and vice versa. The unit also comprises another counter whose digital count output is static and corresponds to the arithmetic mean half wave period corresp. to the mean of the two frequencies employed. By comparing the instant count of the trend counter with the digital status of the mean half period counter, it is possible to define very accurately the transition from one logic level to the other.

Description

Demodulator for frequency reversed loads

Prior art signals The invention is based on a demodulator for frequency-shift keyed signals as specified in the preamble of claim 1. A such a modulator is in the book by P.Bocker, "Data Transfer; Communication Engineering in data processing systems ", Volume II," Facilities and Systems ", Berlin 1977, on pages 32 to 34.

Frequency shifted signals can be used to transmit binary data be used. Here one frequency (e.g. 700 Hz) is one binary value (e.g. 1) and the other frequency (e.g. 500 Hz) assigned to the other binary value (e.g. 0). The timing of the change between the two frequencies depends on the cycle with which the data is transmitted, and on the transmitted information itself. Of the Clock can be regular or irregular. For recovery at the receiving end of the transmitted data, the received signals must be frequency demodulated and the rate of the data transmission must be known.

From the frequency-demodulated signal you can see which binary value is present. The clock is necessary to recognize how many identical binary values follow one another.

The object of the invention is to specify a new demodulator.

Solution This problem is solved with those specified in claim 1 Funds; advantageous developments can be found in the subclaims.

Advantages The new demodulator makes it possible to change keying times very much to be determined exactly. This is especially important if there is a small gap between the clock frequency, with which the data is transmitted (baseband) and the carrier frequencies between to which is keyed an advantage.

Description The invention is illustrated by way of example with reference to the drawings explained in more detail. 1 shows diagrams to explain the mode of operation of the Invention, Figure 2 is a block diagram of the essential parts of the demodulator.

The new demodulator is only used to demodulate the frequency shift keyed Signal, i.e. the demodulated signal only indicates which binary value is currently available is and not the transmitted data itself. To get out of the demodulated signal To receive the data, as already mentioned, must also be done on the sending side Modulation used be known.

As is known from the cited reference, the received Signal (Fig.la), the amplitude of which can fluctuate slightly, band-limited, amplified and converted into a unipolar square wave signal (Fig.lb).

This signal is used to control a counter that counts the number the periods of a high-frequency counting cycle that lie between the zero crossings, counts (Fig.1c). The maximum counter readings Z (Fig.lc) n are according to the device Fig.2 supplied, in which the frequency demodulated signal is determined.

As already mentioned, the signal to be demodulated is between switched two frequencies. With demodulation, it is difficult to precisely determine the switching point recognizable because - baseband and carrier frequencies can be close to each other, and - after switching, a certain settling time to the new frequency is necessary is.

If one stores the determined counter readings Z in each case n until If a new count is available, the one shown in Fig.ld is obtained Stair curve. This step curve is a simplified representation of the frequency curve as a function of the time of the signal to be demodulated. Must be determined for demodulation at what point in time the meter reading, of the length of the Half period of the center frequency between the frequencies switched between wircrl, the step curve falls below or falls below it. Here the switchover point however, can only be determined imprecisely.

However, a good measurement result is advantageously achieved, if you replace the stepped curve with a curve showing the individual meter readings Z1 to Z10 connects with each other. Such a curve is shown in Fig.le. The points of intersection U of this curve with the threshold value S give very precisely the times to which the frequencies are switched on.

In order to be able to represent such a curve, it is necessary to have the two to know consecutive counter readings, because this is the only way to connect straight lines be determined.

All meter readings must therefore be stored for at least this long (Tv) until the next counter reading is determined. This is the longest duration occurring half-period Tv, i.e. the reciprocal of the lower of the two frequencies, between which is switched.

However, it is advantageous to choose the storage time TV a little longer than Tv. To avoid distortion of the curve, all meter readings must be of the same length get saved.

So it can happen that at certain times more than two Meter readings are stored.

The determined counter readings Z are proportional to the lengths of n Half periods Tn + 1-Tn. In Fig.ld, Z5 is proportional to T5-T4. With knowledge of Z4, Z5 and T5-T4 can be the straight line connecting Z4 (the older count value) and Z5 (the more recent count) can be determined.

The curve that connects the counter readings Z1 to Z10 can be seen through Approximate the counter readings of another counter very well. It will be another staircase curve - but with a very fine graduation - formed. This further counter receives clock pulses with different pulse repetition frequencies in successive time intervals. The pulse repetition frequencies are chosen so that it occurs in an interval (e.g. T5-T4) counts exactly from the older (T4) to the younger (T5) meter reading.

By comparing the counter readings of the other counter with the (digital) Threshold S one obtains the polarity change of the baseband signal. With every or falling below the threshold value changes the amplitude of the demodulator output signal (Fig.lf) between two possible values.

That this signal is shifted in time compared to the received signal is, does not matter. This shift just adds to that anyway We didn’t know about the communication system between Selldet and El1pfänyer. It is important, however, that the time sequence of the frequency shift keying is accurate it is determined what is very precisely possible with the new demodulator.

The accuracy can be increased even further if one uses the linear Straight line connecting successive meter readings through the envelope the meter readings are replaced. For this, however, several consecutive counter readings must be made are evaluated and the pulse repetition frequency of the clock pulses for the further Counters can be changed almost continuously.

The block diagram of the new demodulator is shown in greater detail on the basis of FIG explained.

A counting discriminator sends the maximum counter readings to a memory (2) Z continue. The memory, e.g.

n a FIFO, can store several values at the same time.

A computer calculates from two consecutive meter readings, e.g. Z4 and Z5, the difference Z5-Z4.

According to the amount of this difference and the length of the interval T5-T4, the computer 3 selects a clock in a clock generator 6, which is an up-down counter 4 is fed.

The up / down counter that is set to Z4 at this point in time counts from Z4 to Z5 during the time T5-T4. The information about the counting direction he receives from the computer 3.

On the basis of the description of FIG. 1 it was explained that the interpolated Curve (Fig.1e) must be shifted by TV compared to the stepped curve (Fig.ld). That's why the computer 3 does not select until the time T4 + TV in the clock generator 6 select the clock belonging to the interval T5-T4. At time T5, the up-down counter 4 is supplied with the clock that belongs to the interval T4-T3.

The counts of the up-down counter 4 are digital Threshold value circuit 5 supplied, which above the threshold value S with a signal the value 1 and below the threshold value S emits a signal with the value 0. That The output signal of the digital threshold value circuit 5 is the baseband signal.

The calculator and the memory are not explained in detail, as this is facilities known per se can be used.

Claims (3)

Claims j. Demodulator for frequency-shift keyed signals with a counting device to determine the lengths of the half-periods of the received Signals, characterized in that the counter readings proportional to the half-periods periodically stored in a memory (2) and deleted again, namely so that at the same time at least the two successive half-periods are proportional Counter readings (Zn) are stored that in each case after a time (Tv) after the storage the older of two consecutive values that are at least equal to the longest expected half cycle (Tv) is fed to a further counter (4) clock signals that are chosen so that this counter is in a time equal to the half period, which is assigned to the more recent of two consecutive counter readings from the older (Zn) counts to the narrower (in + 1) counter reading that in a comparison device (5) it is determined when this further counter (4) reaches a counter reading (S), which is at least approximately half-period of the center frequency of the frequencies between to which is keyed, is proportional, and that to these points of reference by one first to a second value of the demodulator output signal is switched. 2. Demodlllator according to claim 1, characterized in that the interpolation between the younger and the older counter reading, which is carried out by the further counter, is linear. 3. Demodulator according to claim 1, characterized in that the interpolation between the younger and the older counter reading, which is carried out by the further counter, is carried out depending on further counter readings in such a way that the interpolation curve Is part of an envelope for a certain number of meter readings.