DE1248123B - Circuit arrangement for altimeter receiver - Google Patents

Description

GERMAN M PATENT OFFICE

EDITORIAL

German class: 21 a4 - 48/61

Number: 1248 123

File number: J 22622IX d / 21 a4

1 248 123 filing date: November 8, 1962

Opened on: August 24, 1967

The invention relates to a receiver for working with frequency-modulated waves Altimeter, FM altimeter for short, in which the evaluation is carried out by the fact that the zero crossings of the beat signal derived from a cyclically frequency-modulated signal by means of a counter are counted. The apparatus also contains an upstream of the counting device Amplitude limiter that limits the signal to a certain level.

In the case of impulsive disturbances, as detailed in the main patent 1 214 753, there is the possibility of that as a result of the additional zero crossings caused by these pulse disturbances, incorrect measurements occurrence.

As has also been stated in the main patent, a reduction in the disruptive effect is achieved by a the integration preceding the count and an amplitude limitation of the signal is achieved.

However, it has been shown in practice that the integration process itself errors in the information can bring in. Namely, if the frequency modulation of the signal is not exactly sinusoidal, are the deviations of the integrated signal voltage from the mean integrated signal voltage this is not symmetrical, and the integrated signal voltage then sometimes does not fall within the working range of the amplitude limiter. When such conditions occur, the signal will be zero crossings not counted.

The measures of the invention enable the mean level of the integrated Signal and thereby a reduction in the number of errors.

The invention thus relates to a circuit arrangement for altimeter receivers in connection with a device for eliminating the influence of interference pulses on the altitude display at low altitudesji of frequency-modulated, continuous, electromagnetic waves based on the reflection principle working altimeters, in which the waves reflected from the earth's surface again received and mixed with those supplied directly by the transmitter and the resulting Beat frequency is evaluated by the zero crossings of a square wave derived from it are counted, the square wave via an integration element with a low-pass effect is fed to a gain limiter and then to the counter, and wherein the The integration constant is chosen so that a Be circuit arrangement is only used at the lower of the frequencies occurring

for altimeter receivers

Addendum to the patent: 1214 753

Applicant:

International Standard Electric Corporation,
New York, NY (V. St. A.)

Representative:

DipL-Ing. H. Ciaessen, patent attorney,
Stuttgart W, Rotebühlstr. 70

Named as inventor:

William Littery Garfield, London

Claimed priority:

Great Britain 10 November 1961 (40 395)

limitation already occurs through the low-pass effect, and the square wave continues to dem The amplifier limiter is also supplied via a second signal path, which consists of a transistor amplifier with a frequency-controlled threshold, but each with a flat gain characteristic, whose control electrode has a bias voltage that is approximately inversely proportional to the frequency of the square wave and in addition a manually adjustable pre-tension counteracting the frequency-dependent constant bias is applied in such a way that the transistor amplifier operates at the lower of the occurring frequencies is blocked (main patent 1 214 753).

According to the invention, between the integration element and the amplifier limiter there is already one of keeping constant the signal amplitude serving clamping circuit switched on, so that the output voltage of the integration element - possibly after amplification - via a capacitor to the connection point two diodes connected in series, the free poles of which are oppositely biased, supplied will; the output voltage of the clamping circuit is taken from the connection point of the diodes

709 638/192

and entered into the input of the amplifier limiter.

The invention is illustrated using an example and voltage curves that illustrate the mode of operation, explained in more detail.

F i g. Figure 1 shows the form of a frequency modulated signal such as the receiver of an FM altimeter supplies;

F i g. 2 shows the curve shape after the integration of the signal according to FIG. 1;

F i g. 3 shows the signal after it has passed through the clamp circuit;

F i g. 4 shows the part of the circuit of the receiver of an FM altimeter which is essential for the invention drawn schematically.

The integration circuit according to FIG. 4 with input terminals 3 and 4 consists of a resistor 1 and a capacitor 2. In the circuit, one pole of the signal voltages is connected to ground. The voltage of the capacitor 2 is applied as a control voltage to the input of a linear amplifier 5 fed. Its output voltage is connected to the connection point of two in Series-connected semiconductor diodes 7 and 8 supplied. The anode of the diode 7 is on the negative Pole 9 of a DC voltage source (not shown separately), the positive Poi of which is connected to ground. The cathode of the diode 8 is connected to the positive pole 11 of a further DC voltage source (not shown), the negative pole of which is connected to ground.

The diodes 7 and 8, which are appropriately biased by the two DC voltage sources, and the capacitor 6 form the clamping circuit. Your output voltage is at the connection point of the both diodes removed and connected to a terminal 12. The other output terminal 13 is present Dimensions.

As is well known, an FM altimeter works as follows: One that is frequency-modulated by means of a modulator A signal from a transmitter is emitted to the ground. That reflected off the floor Signal is received and passed into the receiver, where it is in a mixer with an over a cable of known length is mixed with the signal taken directly from the transmitter. The output voltage the mixer stage is a frequency-modulated beat signal, the mean frequency of which is the Flight altitude. The flight altitude is determined by measuring the number of zero crossings of the beats occurring during a modulation cycle. frequency determined by a certain level value.

The zero throughputs are with a known counting device in connection with a Integration circuit counted. For this, however, it is necessary that this is applied to the counting circuit Signal has a constant amplitude. For a widely used FM altimeter, the mean will vary Beat frequency between 1 kHz and 100 kHz, and the modulation frequency is 300 Hz.

The beat signal is amplified and limited, so that finally a square-wave voltage as shown in FIG. 1 drawn shape. The time between the edges A and B corresponds to half an oscillation of the modulation signal. The number of square wave trains occurring during half an oscillation of the modulation signal is in practice always very much greater than the number shown in FIG. Each of the flan-

The square wave train represents a zero crossing of the beat signal that is counted got to. Interfering impulses that the in F i g. 1 drawn square wave would be counted in the same way like the passages useful for height measurement.

In order to reduce the amplitude of any interference pulses present in the signal, the amplitude-limited beat signal is applied to the input terminals 3 and 4 of the integration circuit according to FIG. 4 laid. The wave train after integration, ie the voltage across capacitor 2, has the value shown in FIG. 2 plotted time course. The dashed vertical lines 10 A and 102? have the same time interval as before the integration, the edges A and B in Fig.l.

The amplitude of the integrated signal is inversely proportional to the signal frequency and changes during a half-wave of the modulation signal, as in FIG. 2 is drawn.

The dashed horizontal noise represents the mean level of the integrated signal. The wave train corresponds to a half of the modulation signal. Because the modulation signal, however is not exactly sinusoidal, the integrated wave trains are not symmetrical to the mean level of the integrated signal. The disturbances inherent in the mochilation signal are due to mechanical tolerances of the frequency modulator.

The in F i g. 2 drawn wave train must be reinforced, limited before entering it into the counter and converted to a rectangular shape. If that's unbalanced to its mean Level pulse-shaped signal while maintaining its mean level as Nullirrie, which occurs with the input signal - i.e. before reinforcement - is thought, but actually exists after reinforcement, would be amplified and then limited in such a way that the signal relative to the maximum Signal amplitude cut out only a small part above or below the mean level would, there would be some of the impulses, including the passages through the level line, to which it is at the count arrives, no longer present in the output signal of the limiter.

In the embodiment according to the invention, the integrated signal is sent to the linear amplifier 5 and then via the capacitor 6 of the diodes 7 and 8 with their bias sources Clamping circuit supplied. The level of the integrated Signal between two level values determined by the characteristics of the diodes 7 and 8 and are determined by the values of their polarity reversed biases.

In Fig. 3 shows the voltage curve at the output of the clamping circuit (terminals 12 and 13). The signal lies between the upper clamping voltage shown by the dashed horizontal line JV and the lower clamping voltage shown by the dashed line Y. Points 220, 221, 222 and 224 of the curve according to FIG. 3 correspond in time to points 20, 21, 22 and 24 of the curve according to FIG. 2. At a point in time that is shown in FIG. 2 corresponds to the vertical line 10 A , the amplitude of the integrated signal at the output of the amplifier 5 is as great as indicated by the point 20 in FIG. 2 entered

Claims (1)

is tet. The corresponding signal amplitude at output terminals 12 and 13 is at 220 in FIG. 3 shown. The diode 7 is conductive and the capacitor 6 is charged to a potential which corresponds to the potential difference between the lower level Y and the signal voltage 20. When the size of the integrated signal changes from point 20 to point 21 , the potential at terminals 12 and 13 increases from the value indicated at 220 to the value indicated at 221 . The point 221 corresponds to the upper clamp voltage level indicated by line X. When the signal amplitude at the output of amplifier 5 has reached the value at point 21 , the positive bias voltage of diode 8 is canceled and the diode becomes conductive, so that the potential at terminals 12 and 13 is held at the upper clamping level X. The voltage at the output of the amplifier 5 can assume a peak value, as indicated by the point 22 in FIG. 2 is indicated. The capacitor 6 is then charged to a voltage which corresponds to the potential difference between the potential indicated by the point 22 and the upper clamping level indicated by the line X. If the signal amplitude of the integrated signal falls in accordance with the part of the curve indicated by points 22 and 24 , the voltage at terminals 12 and 13 also falls, namely from the point 222 to point 223. If the amplified signal voltage drops further, the diode 7 becomes conductive again at point 223 , and the voltage at terminals 12 and 13 is held at the lower clamping level Y until the amplitude reaches point 24. This sequence of operations is repeated for each of the successive integrated signals, and a voltage of constant amplitude with the zero crossings of the beat frequency is generated at terminals 12 and 13. The capacitance value of the capacitor 6 is chosen so that when either the diode 7 or the diode 8 is conductive, the voltage applied to it can follow the fall or rise of the integrated signal voltage. If, on the other hand, both diodes are not conductive, the capacitor 6 is discharged or charged, and indeed slowly in comparison to the decrease or increase in the signal voltage. Instead of semiconductor diodes, diodes with a heated cathode can of course also be used. The output signal of the clamping circuit is then amplified in a further amplifier, in Converted to a rectangular shape and then entered into the frequency counter. If necessary, the DC voltage component of the signal appearing at terminal 12 can also be restored without the zero crossings for the count being lost. The further amplifier following the clamping circuit and the amplitude limiter, which are necessary because of the nature of the counting devices used in altimeters of this type, do not impair the operation of the circuit arrangement according to the invention. With many receiving arrangements for FM altimeters, one can do without the linear amplifier 5 (FIG. 4) if the signal amplitude is sufficient to actuate the clamping circuit. The circuit arrangement according to the main patent 214 753 is used to suppress impulse interference in altimeter receivers and is also the basis of this invention. The effect of the in Integrated circuits used in these receiving arrangements to the beat signal becomes higher at decreasing frequencies by adding a frequency-dependent network in parallel to another, the generation of a bias voltage serving integration circuit is placed. The impedance of the frequency dependent The network becomes smaller and smaller as the frequency of the beat signal increases, until finally a frequency is reached at which the integration circuit by the frequency-dependent Network is short-circuited. In a certain frequency range above and below this Resonance frequency, the beat signal is attenuated considerably in the integration circuit, and in the part of the frequency modulation cycle where the frequency is highest. The one described here Clamping has been used to avoid failures when counting zero crossings in particular Proven effective when the flight altitude is so high that the generated beat frequency in the here described frequency range falls. Claim: Circuit arrangement for altimeter receiver in connection with a device for eliminating the influence of interference pulses on the altitude display at low altitudes of altimeters working with frequency-modulated, continuous, electromagnetic waves according to the reflection principle, in which the waves reflected from the earth's surface are received again and with the from Transmitter directly supplied are mixed and the resulting beat frequency is evaluated by counting the zero crossings of a square wave derived from it, the square wave being fed via an integration element with low-pass effect to an amplifier limiter and then to the counting device, and the integration constant is selected so that a limitation already occurs through the low-pass effect only at the lower of the frequencies occurring, and the square wave continues to the amplifier limiter via a second signal path is performed, which consists of a transistor amplifier with frequency-controlled threshold value, but each flat gain characteristic, the control electrode of a bias voltage that is approximately inversely proportional to the frequency of the square wave and additionally a manually adjustable, constant bias voltage that counteracts the frequency-dependent bias voltage, so that the transistor amplifier at the lower of the occurring frequencies is blocked, according to main patent 1214753, characterized in that between the integration element (1, 2) and the amplifier limiter (terminal 12) a clamping circuit which is already used to keep the signal amplitude constant is switched on, in such a way that the output voltage of the integration element (1. 2) - if necessary after reinforcement (5) - via a con-