DE2901124C2 - Doppler direction finder - Google Patents

Description

The invention relates to a Doppler direction finder according to the preamble of claim 1.

Such a Doppler direction finder is known from DE-OS 26 31 008. In this known direction finder the two output signals of the two receivers to each other, possibly after one or more frequency conversions, added by amplitude fluctuations of the To compensate for the received signal.

Both with the conventional Doppler direction finders with only a single receiver and with the aforementioned known Doppler direction finder with two independent receivers, the sum signal generated at the output of one or at the outputs of the two receivers largely corresponds to that of a continuously connected individual antenna that is located on the circle C moves at a constant speed and the rotational frequency / «. The phase deviation Δφ _{α of} the sinusoidal phase modulation of the sum signal of the frequency to be tracked is then

(1)

D - diameter of the circle C of the antenna group, Ap - wavelength in free space at frequency /

of the wave to be tracked, Ca - speed of light in free space.

The phase deviation is thus the frequency / is proportional to fox hunter shaft and therefore increases with increasing f. / "The Umlaufrequenz the scan, so the above phase shift corresponds to a frequency deviation _Δφ υ A <p _u of the frequency / _u

50

55

(2)

which therefore grows with increasing frequency / the incident wave The high-frequency bandwidth B of a signal · with the modulation frequency /, and the is alio approximately according to equation (2) β - 2 Uf - /. + f _n ) - If _n (l + ITf-

(3)

ie is dependent on the operating frequency / and proportional to the rotational frequency of the sampling. Since DF systems are mostly used for large ranges of frequency /, the bandwidth of the intermediate frequency amplifier and the frequency demodulator of the receiver must either be dimensioned to the largest bandwidth B of the bearing signal, which occurs at the highest bearing frequency /, or when changing the bearing frequency of the respective bearing frequency be adjusted. Since the bandwidth of the intermediate frequency amplifier determines its noise, the aim is not to make the bandwidth unnecessarily large, so that an adaptation of the bandwidth to the respective / and 4 fCr the smallest possible value according to equation (3) is advantageous DF signals with a higher DF frequency / disadvantaged because "they result in a larger frequency deviation with the same f" and require a larger bandwidth B. The following numerical example results in the following strokes and bandwidths B for an antenna circle diameter D-6 m and bearing frequencies between 30 Hz:

When the operating frequency / and the same rotational frequency f _{u change} , there is a phase deviation between

Δφ _υ - 138 rad (for 30 MHz) to 183 rad (for 300 MHz)

According to equation (2), this corresponds to a frequency deviation of

Af _u - 244 Hz (for / -30 MHz) to 2.44 kHz (for / -300MHz)

Figure 2 shows the spectrum of this modulation for Δφ ^ " 13; 33; 9.4; 19. The distance between the spectral lines is equal to the orbital frequency /«. From FIG. 2 it follows that for the transmission of the bearing information depending on the carrier frequency / the incident wave a more or less large HF bandwidth B are expended

Frequency deviation Af _n is nihenings-wise B - 2 (f, + Af _n X 65 must. In the numerical example, approximation is B - 2 (Af ₁ , + Λ) - 748 Hz (! Or / - 30 MHx) to 5.148 Uta (door- 300 MHz)

(5)

10

25th

All of the above Doppler direction finder arrangements have the following disadvantages:

1. The bandwidth B of the bearing signal changes at bearing frequencies / between 30 MHz and 300 MHz like 1: 7. The IF amplifiers therefore either have to be adjusted in terms of their bandwidth, or when dimensioning for the largest bandwidth of 5 «5.2 kHz at / = 300 MHz for the lower frequencies, there is a noticeable deterioration in the signal-to-noise ratio.

2. The same applies to the demodulator, which has to be set either specifically to the expected frequency deviation Af _u or to its maximum value Afunux · " 2.44 kHz. With such a setting, for / <300 MHz there are bulges in sensitivity compared to the optimally possible sensitivity

3. The advantages of frequency- or phase-modulated systems only come into play if the distance carrier power / V to noise power Pr at the input of the demodulator is a certain minimum value z. B. exceeds 10 dB The noise power Pr is however proportional to the IF bandwidth, ie even with optimal IF bandwidth, there are unfavorable values for the sensitivity of the DF system due to the large bandwidth at carrier frequencies near the upper frequency limit of the operating frequency range.

4. In order to increase the distance between carrier power Pt and noise power Pr , a so-called comb filter can be used in the IF, which preferably transmits the spectral lines of the Peil-PM and weakens the noise components between these spectral lines (distance f “) Such a filter is very complicated by the number of spectral lines to be taken into account, which varies greatly with the carrier frequency f. According to FIG. 2a would have to transmit approximately seven spectral lines for Δφ _α = 1.9 (corresponding to a carrier frequency of / »30 MHz), with <d <p« = 19 according to FIG. 2d (/ «300 MHz) approximately 43.

The invention is based on the object of influencing the described relationships between DF frequency and bandwidth B in a generic Doppler direction finder so that a bandwidth that has been optimized once can be maintained even when the DF frequency is changed, with the bandwidth being optimized as desired depending on the DF task , e.g. B. in the sense of a favorable signal / noise ratio.

This object is achieved according to the invention with the characterizing features of claim 1 solved.

Antenna 1 U ₁ = i / cos Qt + A φ. cos (t> ja- Αφ / 2) Antenna2 U ₂ « Ucos Qt + Αφ _κ cos (v _u ta + Αφ / 2)

Ω = 2 π /: angular frequency of the carrier, bearing frequency, «, = 2 π / y. Circular frequency of the revolution, _{Αψ κ} = DnIX _n : phase deviation of the first and second sum sigml, a: Azimuth angle of the incident wave according to Fig. S,

A ψ: Apparent lag of antenna 1 compared to antenna 2 in the circuit according to FIG. 5.

The phase angles of the HF oscillations of the two sum signals (7) and (8) are:

35 An example of a bearing arrangement with two scanning processes, similar to that described in DE-OS 26 31 008, is shown in FIG. The individual antennas are A \ to A ₆ . Each antenna has two outputs or connections for taking the high-frequency signal voltage. A cable leads from each output (Zi _n or Zauber a switching device (S \ _B or Szjjzii to one of the receivers (Ei or E ₂ ). Ei receives the first sampling process and delivers the first sum signal (branch 1) at its output). Ei receives the second scanning operation and provides at its output the second sum signal (branch 2). the switching devices are used for each scanning operation by a respective control means (B ₁ and B j) for each of the two scanning operations connected the outputs of e \ and e ₂ are connected to the evaluation device W, at the output of which the direction of the incident wave is obtained from the third sum signal.

F i g. 4 shows an example of an evaluation circuit W with a mixing process for the two scans. The evaluation circuit has two inputs for the two branches 1 and 2. Each branch has at least one mixer (Mi or Mt), an upper position oscillator (O \ or O ₂ ) and an intermediate frequency amplifier ZFi or ZF ₂ . Both branches are brought together in a common mixer Afe and a common intermediate frequency amplifier ZFy. The local oscillator Oi of branch 1 oscillates at the frequency fo \ and, together with the input frequency, generates the intermediate frequency f _z \. Accordingly, Oz vibrates on the frequency / 02. The two intermediate frequencies must be different so that an intermediate frequency / _Z 3> 0 can arise when mixing in M _{3, & h.} f _z 3 = f _z ι - / _z2 .

In the formation of the intermediate frequency f _z3 as the difference between f _z] and f _z i disappears a possibly existing interfering external frequency modulation of the message due to the staggered scanning operations in the two receiving channels is obtained at the intermediate frequency f _z3t ie after mixing, a phase deviation of the Bearing modulation that depends on the time offset between the two scanning processes

If one replaces as in FIG. 5 the two scans of the individual antennas by two antennas 1 and 2 moving on circle C, then the time delay At of the second scan corresponds to a lag of antenna 1 compared to antenna 2 by the geometric angle Δψ This angle is determined by a corresponding digital circuit in Control unit adjustable and freely selectable. Without taking any FM transmitter side into account, the output voltages of the rotating antennas according to Fig. 5 (or the correspondingly scanned antenna group) are:

O).

(S)

A ψ _κ cos (ft _u t-tt- A φ / 2)

Λ f _u cos (β.ί -α-A φ / 2)

(10)

As an example, ψ _λ and f ₂ and fi-ψχ for an azimuth of a - 30 * and Αφ «30 ° are plotted in FIG. By mixing and converting into the intermediate frequency /] (see FIG . 4), the difference arises as the phase angle of the third sum signal

Λ - Ψι-9ι - ^. [cos (β, ί-β + Af / 2) -cos (*>, ta-Awnyi - 2J # „sin The third sum signal is therefore with the stroke

cot (· "* -" + π / 2).

(12)

phase modulated, determining the time sequence of these 15 antenna groups).

Modulation of the azimuth angle «contains For a certain desired intermediate frequency

«By comparing the frequency phase deviation Δψ3 must 4ψ according to equation (12)

Phase modulation ψ3 with the timing of the antenna rotation (or the sequence of the scanning of the

^ lr-2arcein -r- ^ M -2arcsin -

to get voted. The evaluation of equation (13) 25 d) can either take place in a central computer that z. B. is also responsible for the frequency setting of the receiver or with an independent e)

Microprocessor or in the simplest case also by hand. ao

The possibility of being able to set Δψ3 to practically any desired value offers the following advantageous applications:

1. Δψ3 can be independent of the carrier frequency
be kept constant. This can

35

a) the IF amplifier can be operated with a constant bandwidth without loss of sensitivity at low carrier frequencies:

b) the frequency discriminator optimally to a certain constant frequency deviation

Afu3 = ΐυΔψ3 (14)

be set,

c) instead of the frequency discriminator, a phase discriminator such. B. a synchronous detector can be used if Aq> 3 <st is selected. As a comparison frequency, for. B. the difference frequency of the quartz-stabilized oscillators fo ι and fo2 according to F i g. 4 serve;

(13)

because of the carrier frequency / independent Spectrum an optimal 'comb filter can be selected;

the phase deviation Δψ ₃ can be set so small that practically only two spectral lines appear next to the carrier. Then you can work with a very small IF bandwidth B and the ratio of carrier power to noise power at the discriminator is improved accordingly. At / «» 300 MHz the bandwidth to be transmitted is / u-130 Hz and D-6 m without time delay Bf »52 kHz, with optimal phase compression B _K «300 Hz. With constant carrier power, the improvement in the distance between carrier power and noise power is V <-101g (Β / Βκ) ^ \ 2ΑάΚ For higher carrier frequencies or a larger base of the direction finder, even more favorable values result.

45 The compensation of fluctuations in the group delay time of the receiver by alternating clockwise and counterclockwise rotation according to German OS 26 13 055 can be done in the same way with the inventive concept as with the simple Doppler direction finder.

In addition 6 sheets of drawings

Claims (3)

Patent claims: 1. Doppler direction finder with simulated antenna rotation, in which the direction finder antenna is subjected to two equally fast, but temporally offset from one another by a certain temporal shift At , scanning processes in the same direction of rotation and in which short-term signals obtained for the two two scanning processes have two independent receivers for the separate formation of two different frequencies Sum signals are provided, characterized in that a mixer (M ₃ ) is provided which mixes a third sum signal from the two sum signals, the phase curve of which is a measure of the bearing angle, and that the Temporal shift At can be changed in accordance with the desired bandwidth of the third sum signal (FIG. 4). 2. Doppler direction finder according to Claim 1, characterized in that the time shift At is selected so that the bandwidth (B) of the third to sum signal when changing the bearing frequency / remains approximately constant 3. Doppler direction finder according to claim 1, characterized in that the time shift At is chosen so that the bandwidth (B) of the third is the sum signal is smaller than the bandwidth of the two sum signals at the inputs of the two receivers.