DE1279782B - Method for demodulating a frequency-modulated signal and circuit for carrying out the method - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

H03d

German class: 21 a4 - 29/01

Number: 1279 782

File number: P 12 79 782.9-35 (S 103528)

Filing date: April 29, 1966

Opening day: October 10, 1968

The invention relates to a method for demodulating a frequency-modulated signal the instantaneous output signal is determined within each period of the carrier frequency, the demodulated output voltage occurs only on a single line and is proportional to the deviation the carrier frequency is from the center frequency and at which the received, frequency-modulated Signal is fed to a limiter amplifier. In the known demodulation method for frequency-modulated Signals are used to convert the frequency-modulated carrier wave back into telegraphic character phase discriminators. These phase discriminators contain LC resonant circuits and the dynamic behavior of the discriminator depends on the quality of the oscillating circuit.

The object of the invention is to provide a method which, when used, causes telegraphic distortion the demodulated telegraph characters is significantly smaller compared to the previously known methods. Furthermore, the efficiency of the arrangement should be more favorable compared to the previously known circuits lying, d. H. large output voltage change with small frequency change. One aim of the invention it is also to show a method that works independently of the quality of the oscillating circuit and thus allows lower quality of the oscillating circuits.

A frequency demodulator that does not use LC resonant circuits is already known. This demodulator measures the duration of the limited half-waves by adding several from the rising edge monostable multivibrators are switched on and when the falling edge occurs by a Galter arrangement is determined in which time range of the monostable trigger circuits the trailing edge falls. Depending on this, an output voltage appears on one or the other line. Of the The efficiency and transient response of this arrangement are very unfavorable, especially when when the frequency deviation is small. There is large distortion. if on a transferred step a few periods of the carrier oscillation fall.

Another known frequency demodulator circuit works on the backmixing principle. In this type of demodulator, the limited, frequency-modulated carrier wave is fed to a gate, at the other input of which the frequency of a multivibrator is applied. The multivibrator is controlled by the output signal of the gate, which also represents the demodulated signal. However, the gate output must first be fed to a filter. The arrangement is connected in a ring, so that method for demodulating a
frequency-modulated signal and circuit for
Implementation of the procedure

Applicant:

Siemens Aktiengesellschaft, Berlin and Munich, 8000 Munich 2, Wittelsbacherplatz 2

Named as inventor:

Wilhelm Grafinger, 8000 Munich

it is very susceptible to failure, since errors that occur circulate in the ring and are amplified. In addition, the circuit has because of the coincidence of the multivibrator frequency with the carrier frequency an instability. The circuit needs LC circuits for the filter at the output of the gate.

The object is achieved by a circuit arrangement designed according to the claim. With a Circuit arrangement according to the method according to the invention can be used in a telegraphy channel of the FM-WT 240 can achieve a telegraphic distortion that is less than 2% - Compared to the Hitherto known circuits, such a circuit has a significantly more favorable settling with the same efficiency. It also has a high output voltage with good settling achieved. Furthermore, the method according to the invention is particularly suitable for implementation without use suitable for coils, since fluctuations in the quality of the oscillating circuits have only an insignificant effect. RC circuits or astable multivibrators can thus be used as resonant circuits. As a result, the circuit arrangement is used to an ever increasing extent for the application coil-free circuit technology is particularly suitable.

Details of the invention are explained using the figures:

F i g. 1 and 2 show the previously known method of a phase discriminator for demodulation the frequency modulated signals;

F i g. 3 shows the principle of the method according to the invention;

F i g. 4 shows a basic circuit diagram for implementing the method according to the invention;

F i g. 5 shows an advantageous circuit arrangement of the invention.

809 620/111

F i g. 1 shows the basic circuit diagram of a conventional phase discriminator in an FM telegraphy receiver. The signal voltage U ₀ arriving at the input is amplified and limited to a constant amplitude in the amplitude limiter B. The limited signal is now given on the one hand directly, on the other hand reversed polarity (180 °) on each contact of an electronic switch S1. The changeover contact Sl is controlled by a switching voltage U _s , where U _{s is the} same frequency as U ₁ , but is phase-shifted by a frequency-dependent angle φ with respect to CZ _1. This phase shift ψ is achieved in the phase shifter P.

F i g. 2 indicates the phase shift between U ₁ and U _s achieved with the phase shifter P. The phase angle between U ₁ and U _s is 180 ° at the frequency f _L and 90 or 0 ° at the frequency f ₀ and f ₂ . If U ₁ and U _{s are} square-wave voltages, the curve in FIG. 1 applies. 2 also for the direct voltage component contained in the output voltage U ₃ , ie for the voltage U ₄ present after the low-pass filter TP .

The method according to FIG. 1 has the disadvantage that the telegraph signal by lengthening the step edge is distorted. This extension is the same as that

Running time T =

Λ ψ Λω

of the phase rotator P. This disadvantage is avoided in the proposed method.

F i g. 3 shows the basic circuit diagram of the method according to the invention. In the F i g. 3 a, 3 b and 3 c the associated timing diagrams are shown. The essential elements according to the invention are startable and stopable generators. The output voltage U _{1 of} the limiter amplifier B starts the generator Gl with every rising edge, which then outputs the voltage U _s with the frequency f _G. Each falling edge starts the generator Gl, which then outputs the voltage U ₂ with the frequency f _G. The frequency f _G output by the generators is an odd multiple of the channel center _{frequency / 0} . Immediately before the start, the generator Gl is _{stopped by the rising edge of U 1} , while the generator Gl is stopped by the falling edge of U ₁ .

In Fig. 3 a the ratios at the higher of the two characteristic frequencies (f ₂ ) are given. The points in time at which the generators Gl and Gl are started or stopped can be seen from the thick flanks of the voltages U ₅ and U _2. These times coincide with the times of the edges of the limited received signal U ₁ . From Fig. 3 it can be seen that the voltages U _s and U _{2 of} the start-stop generators operating at the same frequency are in phase. Thus occurs at the low-pass output in accordance with FIG. 2 the maximum negative voltage U ₄ . The timing diagram in FIG. 3 b results when the input voltage U ₁ has the channel center frequency f _Q. The generator frequency / g remains unchanged. However, since the phase shift between U _s and U _{2 is} now 90 °, the DC voltage U ₄ occurring after the low-pass filter TP will be zero. Finally, FIG. 3c shows the pulse diagram for operation at the lower characteristic frequency (J ₁ ). The phase shift between U ₅ and U ₂ is 180 ° there, so that the positive output voltage U _{4 of} the low-pass filter TP according to FIG. 2 reaches a maximum.

The control of the Umschaltekontaktes Sl is carried out with the voltage U delivered by the generator G1 _5, wherein the changeover contact alternately the voltage U by means of switches ₂ ode / phases 180 ° shifted voltage U ₂ to the low-pass filter TP.

To determine the frequency ratio - + §-

the following points must be observed:

At the center frequency f ₀ , the phase shift between U ₅ and U _{2 must be} 90 ° (270 °). This is achieved when on a period duration

T ₀ = -r an odd number of half-waves of the span

U _{5 is} not applicable.
The mathematical relationship results from this

J_
./0 ~
So is

(2b +

Ever. / 0

«= 1,2,3,

f ₀ = center frequency of the channel,
f _G = frequency of the start-stop generators.

The value η must now be determined in such a way that when the frequency changes from f ₀ to f ₂ according to FIGS. 3 b and 3 a, there is approximately a time difference of half a period of f _G :

J 1_

fi

/ o 2 (Z ₂ -Z ₀ )

F i g. FIG. 4 shows a basic circuit diagram for implementing the method according to the invention with the aid of FIG periodically triggered oscillating circuits. So far, the start-stop generator was in terms of its internal Mode of action not considered. Any circuit that is shown in FIG. 3 works, can be used for it will. It would be B. an astable multivibrator that can be started and stopped sufficiently quickly, useful.

F i g. 4 shows another possible implementation the start-stop generators, namely with periodically triggered oscillating circuits.

The output voltage U _{1 of} the limiter amplifier B is differentiated in a differentiating stage D. The rising edges of U ₁ after the rectifier result in positive pulses U ₃ , while negative pulses U ₆ arise from the falling edges. With the pulses U ₃ and U ₆ , an oscillating circuit located in the generators Gl and Gl is triggered, whereupon the voltages U ₄ and U _{7 are} emitted. The resonant circuits are followed by limiter amplifiers S1 and B1, at whose outputs the square-wave voltages U ₈ and U ₈ are available (corresponding to U ₅ and U ₂ in FIG. 3). The resonant circuits are relatively weakly damped, so that the time between two voltage zero crossings corresponds to the resonance frequency of the circuits.

The changeover contact Sl is controlled by the voltage U ₅ , while the contact alternates

the voltage U ₈ switches through to the low-pass filter directly or shifted by 180 ° in phase. In the momentum diagram for F i g. 4 shows the generation of the voltage U _s . Line 1 shows the limited input voltage U ₁ and line 2 the differentiated voltage U _{3 of} the positive edges. Lines 3 and 4 show the voltage emitted by the resonant circuit before and after the limiter stage Bl [U ₄ , U _s ).

F i g. FIG. 5 shows an advantageous exemplary embodiment in accordance with the basic circuit diagram according to FIG. 4. This circuit arrangement is particularly favorable because fluctuations in the quality of the coilless oscillating circuits have only an insignificant effect on the discriminator characteristic. The amplitude-limited input signal passes from input E via an Im-I ₅ pedanzwandler, which consists of the transistor Ts 8, to the downstream differentiating stage D, where the positive pulses at the diode Gr 3 and the negative pulses at the diode Gr4. The two series of pulses now reach two similarly structured circuit parts, namely an oscillating circuit and the downstream limiter amplifier with two phase reversal stages. The two KC resonant circuits in the generators Gl and G 2 were tuned to the same frequency. In the case of the limiter amplifiers B 1 and Bl , the amplitude is limited at the output voltage of the transistor Ts3 (or TsIl) by feeding back an amplitude-dependent negative feedback voltage via Cl, GrI, Gr 1, C6 (or ClX, GrS, Gr6, ClO) to the base of the Transistor Ts3 (or TsIl). The rectifiers GrI, Gr2 (Gr5, Gr6) are connected to a voltage divider /? 22 ... R15 (J? 62 ... R65) , which increases the effective forward voltage of GrI and Gr2 (Gr5 and Gr6). The limiter amplifier also contains two phase inverters with the transistors Ts4 and TsS (Ts 12 and Ts 13). These are followed by impedance converters with the amplification elements Ts6, Ts7 (Ts 14, Ts 15), which are also used as power amplifiers. The electrical switchover contact 51, which operates with strong throughput attenuation, can therefore be designed with a relatively low resistance. The electrical changeover contact Sl consists of the two electronic normally open contacts Ts 16 and Tsl7, of which alternately one collector-emitter path is always a short circuit, while the other collector-emitter path is blocked. R 82, RS3 and Gr 7 have the effect that the emitter of Ts 6 is loaded as heavily in the positive half-wave as in the negative, whereby the rectifying effect cannot charge C12. The resistors i? 80, R 81, R 86 are required for mutual decoupling of the electronic working contacts. The circuit of the electrical changeover contact has the properties of a ring modulator circuit, so that the telegraphic characters are produced at output A.

Claims (6)

Patent claims: 1. Method for demodulating a frequency-modulated signal in which within each 60 period of the carrier frequency the instantaneous output signal is determined, the demodulated output voltage occurs only on a single line and is proportional to the deviation of the carrier frequency from the center frequency and in which the received, frequency-modulated signal is fed to a limiter amplifier, characterized in that each rising edge of the Output voltage of the limiter amplifier triggers a first frequency generator, each falling edge triggers a second frequency generator, both frequency generators being tuned to the same natural frequency, which is significantly higher than the channel center frequency, and that the two output voltages are given to a circuit having ring modulator properties at whose output the demodulated telegraphic characters are issued. 2. Circuit arrangement for performing the method according to claim 1, characterized in that that means are provided which the edges of the frequency-modulated limited signal tax and separate the rising and falling edges and the rising edges to a frequency generator (Gl) and the falling pulse edges the other frequency generator (G2) and that means are provided which consist of the two The voltages emitted by the generators recover the telegraphic characters. 3. Circuit arrangement according to claim 2, characterized in that the steepening of the limited signal edges by a differentiating stage and the separation of the rising and falling Pulse edges occur through appropriately polarized diodes. 4. Circuit arrangement according to claim 2, characterized in that as frequency generators start and stop electronic trigger circuits, in particular astable trigger circuits, are used will. 5. Circuit arrangement according to claim 2, characterized in that when using downstream of oscillating circuits in the generators limiters (Bl, B2) and the limited Voltages of the switching device (Sl) are fed to the formation of the telegraphic characters. 6. Circuit arrangement according to claim 2, characterized in that the ring modulator properties The own circuit consists of two transistors and the voltage of one frequency generator (Gl) alternates one The transistor blocks while the other transistor is controlled to be conductive, and that alternately the output voltage of the other frequency generator (G 2) or the phase shifted by 180 ° Output voltage of this generator is short-circuited. Considered publications:
U.S. Patent Nos. 3,044,020, 3,233,181. For this purpose 2 sheets of drawings 809 620/111 9.68 @ Bundesdruckerei Berlin