DE1275631B - Demodulator for frequency-modulated electrical high-frequency oscillations - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

H03d

German KL: 21 a4- 29/01

Number: 1275 631

File number: P 12 75 631.9-35 (C 32218)

Registration date: February 21, 1964

Opening day: August 22, 1968

The invention relates to a demodulator for frequency-modulated electrical high-frequency oscillations with several impedances, which are arranged in a back circuit, one of which The high-frequency oscillation is applied to the diagonal, while the demodulation product is applied to the other diagonal forming rectifier are connected.

A frequency discriminator formed from electrical circuits usual design, which is based on the medium frequency of, for example, 11.5 MHz Intermediate frequency filter is set in the middle, has no particular manufacturing difficulties. A Such a discriminator is able to produce a linear amplitude response for a frequency deviation of ± 50 kHz around the center frequency with sufficient slope to excite a low frequency amplifier to own. However, its stability as a function of time and temperature remains below of ± 10 "· '. This is reflected in a possible center deviation from its zero point a frequency change of about ± 11.5 kHz around a mean frequency of 11.5MLIz is noticeable. Although this center deviation has practically no effect on the demodulation, it does arise in contrast, a deterioration in the signal-to-noise ratio due to the asymmetry of the introduced Demodulation with respect to the noise band of the intermediate frequency amplifier. The tube noise is amplified because the components of opposite signs on both sides zero are no longer cancel each other out.

Discriminators stabilized by a piezoelectric crystal are already known. Certain of those Discriminators are characterized by a very narrow pass bandwidth, which is determined by the Characteristics of the crystal are determined. Other such discriminators have an input impedance, the value of which changes sharply between the cutoff frequencies of the passband, which makes it difficult to match them, the level the output signal is also insufficient, especially if this signal is for the automatic Frequency control is used. A specially tailored crystal leaves the properties of a Improve discriminator of this type, but also disproportionately increases its manufacturing costs.

In the case of the aforementioned discriminators, according to a recent proposal, a single crystal has been replaced by a series or parallel connection of two crystals which give resonance at different frequencies, which are the limits of the demodulator for frequency-modulated
electrical high frequency vibrations

Applicant:

Company Francaise Thomson

LIouston-Hotchkiss Brandt, Paris

Representative:

Dipl.-Ing. Dipl. Oec. publ. D. Lewinsky,

8000 Munich, Gotthardstr. 81

Named as inventor:

Claude Rouxel,

Aubervilliers, Seine (France)

Claimed priority:

France of March 1, 1963 (926 502)

Define the pass frequency band. Such a circuit arrangement leads to good stabilization, and the bandwidth can be selected in advance. The amplitude response of such a discriminator however, it is difficult to adjust and its input impedance is not easy to adjust.

U.S. Patent No. 2,243,417 discloses a discriminator known that at its output a difference signal depending on the frequency gives away. In one embodiment, the RF signal is applied via an input reactive resistor two opposite nodes of a bridge are placed, while the output signal is across two diodes is removed from the other two bridge nodes. The bridge branches are connected in series in parallel constructed in such a way that their series resonance frequency is lower or higher than the external frequencies of the transmitted band and the counter-parallel resonance frequency of one branch are the same is the series resonance frequency of the other. To find the band to be demodulated and the associated Determining the frequency response is an exact setting of the sensitive to mechanical vibrations Resonance circles, which also require a large amount of space, are necessary.

Furthermore, from the German patent specification 662 456 a circuit acting as a demodulator is known known, in which the RF input signal via a

809 597/166

symmetrical transformer is fed into a bridge circuit with a symmetrical output. One quartz located in a bridge arm, matched to the carrier, and one in the opposite one Capacitors located in the bridge arm serve to convert the sidebands with respect to the carrier 90 ° each time so that the output signal at the terminals of the symmetrical output as voltages of the same size but in phase opposition appear. On the characteristic of this also very much Bulky elements having circuit only affects the selectivity of the crystal, whereby a exact setting of the frequency response is not possible.

The invention is based on the object of creating a demodulator of the type mentioned at the beginning, while avoiding the disadvantages inherent in the circuits described above, a linear one Frequency response that is temperature and time stable, a large, adjustable bandwidth and a has practically constant input impedance within the bandwidth and also a low one Circuit effort and accordingly low manufacturing costs. This works with the The demodulator proposed here in that, according to the invention, between the one input terminal and the two ends of those bridge diagonals to which the rectifiers are connected, two mutually equal impedances are switched on that between the other input terminal and the one of said ends a third forming a series resonance at a certain upper frequency Impedance is turned on that between said other input terminal and the other a fourth impedance between the ends of the said ends and between the ends of the bridge diagonals that the rectifiers are switched on, a fifth impedance is switched on and that the third and fifth impedance are dimensioned so that they - if necessary together with the same Impedances - form a series resonance at a certain lower frequency for which the an the voltage across the terminals of the fourth impedance is zero.

Such a demodulator is usually located between the output of the last stage of the intermediate frequency filter and the input of the low frequency amplifier stages. It is therefore desirable to adjust its input impedance and that it be within the passband im remains essentially constant and the level of its output signal is adjustable.

An advantageous embodiment of the demodulator according to the invention is that the third Impedance a quartz, as a fourth impedance an adjustable capacitor to compensate for the capacitive Reactance of the quartz at a medium frequency and as a fifth impedance one with the Quartz at the lower frequency series resonance forming inductance are used.

The use of quartz ensures a particularly good stability of the frequency response and by choosing an adjustable capacitor as the fourth impedance, a shift in the band center frequency, enables the setting of the bandwidth and the steepness of the frequency response.

It is of further advantage. when the first and second impedance capacitors with a small capacitance value are. The input impedance of the demodulator is determined by the appropriate choice of capacitors easy and precise to adjust.

In the case of a receiver provided for frequency-modulated signals, the distortion is due to the demodulation and the tube noise is minimal when the demodulator is stabilized. The changeable one The impedance of the quartz used to stabilize the bridge has a capacitive component, which changes depending on the frequency. This property is preferred to this used to influence the course of the frequency response of the bridge. It follows that the stability of the band center frequency, which corresponds to the zero point of the frequency response, depends only on the Stability of the capacitor used depends and also both extreme values of the frequency response are determined. The relative position of a third curve point of the frequency response is among other things through given the correct choice of inductance lying in the bridge diagonal. Their value is in Dependence of the desired pass bandwidth and the required linearity of the frequency response chosen.

In the drawing, a demodulator of the type proposed according to the invention is shown, for example Selected embodiments and the invention explanatory diagrams illustrated schematically. It shows

1 shows a circuit diagram of a circuit diagram according to the invention, which is simplified for a more detailed explanation of the mode of operation Demodulator,

Fi g. 2 shows an embodiment of a stabilized demodulator in a likewise simplified circuit diagram and

Fig. 3 a and 3 b an example of the course of the characteristics (frequency response) of such a demodulator.

F i g. 1 shows a demodulator in the manner of a bridge with the bridge nodes A, B, C and D. The high-frequency signal HF is applied to the bridge nodes A and D and transmitted via two impedances Z of high value, which form the bridge branches AB and AC. The RF signal is rectified via _{diodes D 1} and D ₂ connected to bridge nodes B and C, respectively. The size of the impedance Z ₁ is heavily dependent on the frequency of the applied RF signal. In addition, the bridge nodes B and C are connected to one another by an impedance Z ₀ forming the bridge diagonal BC.

Let the frequencies J ₀ and f ₂ be those frequencies which limit the pass bandwidth of the demodulator. At the frequency J ₂ , the impedance Z ₁ forms a short circuit. The rectification then takes place via the diode D ₂ - ß ^ß i of the frequency / o, the bridge nodes C and D are at the same potential and can be regarded as short-circuited. The impedance seen by the diode D ₂ is then zero. The rectification takes place via the diode D ₁ . Since the diodes D ₁ and D ₂ are connected in opposite directions, the output voltage with the frequency J _{2 has the} opposite sign to the voltage obtained with the frequency / O- In this way, the frequency-modulated RF signal is demodulated.

In order to achieve this mode of operation, it is therefore necessary to connect the terminals of the bridge

branches BD and CD with the frequency J ₂ or / _{0 of} the applied RF signal to let the voltages present become zero. The HF voltage V applied to the demodulator generates the voltages V _BD and V _CD at the connections of the bridge branches BD and BC . With Z _e as the input impedance of the demodulator, the following equations are obtained:

At frequency J ₂ :
Z ₁ -O;

IO

V _BD = O;

y - V

Z ₀ Z ₂

ZZ ₀ + ZZ ₂ + Z ₀ Z ₂

7 ₌ ^ + 1 ^ZZ « ^Z *

2 ⁺ 2 2 Z (Z ₀ + Z ₂ ) + Z ₀ Z ₂ '

At the frequency / i:

Z ₁ - Z ₂ ,

15th

At frequency / ₀ :

Currently ₀

Z ₁ - -

so

Z ₀ = -

Z ₀ + 2Z
2ZZ ₁

Z- ^ cZ ₁

25th

30th

35

= 0;

Ken =

^Zc - 2

2 '

In the case of the embodiment of FIG. 2, all impedances are pure reactances. The impedances Z are capacitances C \ ; the impedance Z ₀ is an inductance L ₀ , the impedance Z ₂ is an adjustable capacitance C ₂ , and the impedance Z ₁ is a quartz or other circuit element that is highly dependent on the frequency with regard to its impedance value. This quartz has, as in Fi g. 3b shows a frequency-dependent (positive or negative) reactance. The HF voltage occurring at the terminals of the impedances Z ₁ and Z ₂ is applied to the rectifier diodes D ₁ and D ₂ . At the output of the circuit, the voltages emitted by the diodes are used to form the difference between them, which is then available as an output signal from the demodulator circuit. At the frequency J ₁ , the bridge symmetry means that the voltages across the diodes D ₁ and D _{2 are the same.} So it applies

Vbd = V _cd for Z ₂ = X.

The resulting output voltage is then zero. This point corresponds to the zero point in the frequency response of the demodulator.

Some noteworthy properties of the circuit proposed by the invention should now be mentioned. Since the impedance Z _{1 is} a crystal, the frequency stability of the zero point of the frequency response depends practically only on the capacitance of the capacitor C _{2 selected as impedance Z 2} and can therefore be of the order of magnitude of 10 ~ ⁴ , i.e. about 1 Hz at 11.5 MHz . A quartz of the usual construction and therefore of normal price is suitable for this use. If the capacitance values and inductance values are inserted into the above equations, the voltages V _CD at frequency / ₂ and V _BD at frequency / _{0 are} approximately equal to one another, which ensures an acceptable symmetry of the useful range of the frequency response of the demodulator after rectification.

The input impedance of the demodulator depends, among other things, on the impedances Z of the bridge branches AB and AC. By choosing a large value for Z, it is possible to give the input impedance the desired value depending on the required adaptation and the required level of the output voltage. In addition, the correct choice of the impedance Z makes it possible to make the input impedance of the bridge discriminator essentially constant within the frequency band used.

In Fig. 3a shows the frequency-voltage curve of the demodulator and FIG. 3b shows the course of the reactance X of the quartz as a function of the frequency.

The curve C _d of FIG. 3a runs through a minimum located at the frequency J ₀ , a zero point located at the frequency J ₁ and over a maximum located at the frequency f _2. These three voltage points correspond to the previously defined values of the impedance Z ₁ . The maximum point corresponding to the frequency J _{2 of} the resonance of the quartz forms a fixed point around which the curve of the discriminator can fluctuate within certain limits. The steepness of this curve between the frequencies J ₁ and J ₂ is given by the position of the zero point on the abscissa axis and is mainly determined by the value of the impedance Z ₂ , ie the capacitor C2. This value is selected as a function of the desired useful bandwidth. The position of the minimum point of the curve of the discriminator depends above all on the value of the impedance Z ₀ , ie the inductance L ₀ . It is therefore possible to achieve the desired linearity of the curve of the bridge discriminator within the useful passband by influencing the values of L ₀ and C ₂ .

The curve C _d of FIG. Figure 3a shows an example of the results obtained with a bridge discriminator according to the invention; curve 3b allows the particular values of the capacitances chosen to be precisely determined. The resonance frequency / _{2 of} the quartz used is around 11.5 MHz. The zero point at the frequency J ₁ corresponds to Z = C ₂ = IOpF (cf. the curve in FIG. 3b). At the frequency J _{0 there} is an inductance of about 45 μΗ resonance with a capacitance of about 5 pF according to the above equation

1 «ι Q

4.75 pF
5 pF, where X - 8 pF is the capacity of the crystal

and C ₁ = 1.5 pF are the capacitance value of a bridge leg AB or AC . With a capacitance C ₁ = 1.5 pF, the input impedance changes between 2000 and 6000 ohms. The by the dashed part of the curve of Fi g. 3a limited useful band extends by ± 20 kHz. This part of the curve is linear and is therefore suitable for demodulation and for suppressing the noise due to its symmetry. It should also be noted that the stability of the zero point as a function of the temperature is very good. The deviation of the zero point for a temperature change of 150 ^° C. is below 500 Hz.

The stabilized demodulator in accordance with FIG Invention, it is possible to use crystals of conventional manufacture and low cost. aside from that Since it has several control parameters, it can easily be adapted to the circuits of a receiver and as a reference signal source for the automatic frequency control of an oscillator, e.g. B. in one Transceivers. It can also be used in a carrying device without any disadvantage.

Claims (3)

Patent claims: 1. Demodulator for frequency-modulated electrical high-frequency oscillations with several impedances, which are arranged in a bridge circuit, on one diagonal of which the high-frequency oscillation is applied, while the rectifier forming the demodulation product is connected to the other diagonal, characterized in that between the one input terminal (A) and the two ends (C, B) of those bridge diagonals to which the rectifiers (D ₁ , D ₂ ) are connected, two equal impedances (Z) are connected that between the other input terminal (D) and one of the ends mentioned (B) a third impedance (Z ₁ ) forming a series resonance at a certain upper frequency (J _{2 ) is switched on, so that a fourth impedance (Z 2} ) is connected between said other input terminal (D) and the other of said ends (C) and between the ends (C, B) of the bridge diagonal to which the rectifiers (D ₁ , D ₂ ) are connected, a fifth impedance nz (Z ₀ ) is switched on and that the third (Zi) and fifth impedance (Z ₀ ) are dimensioned so that they - possibly together with the same impedances (Z) - have a series resonance at a certain lower frequency (J ₀ ) form for which the voltage at the terminals of the fourth impedance (Z ₂ ) is zero. 2. Demodulator according to claim 1, characterized in that the third impedance (Z ₁ ) is a quartz (C Q), as the fourth impedance (Z ₂ ) an adjustable capacitor (C ₂ ) to compensate for the capacitive reactance of the quartz at a medium frequency (Z _{1 ) and an inductance (L 0} ) which forms series resonance with the quartz at the lower frequency is used as the fifth impedance (Z _0). 3. Demodulator according to claim 1 or 2, characterized in that the first and second impedance (Z) are capacitors (C ₁ ) with a small capacitance value. Considered publications:
German Patent No. 662456;
U.S. Patent No. 2,243,417. 1 sheet of drawings 809 597/166 p. 68 © Bundesdruckcrei Berlin