RU2477571C1 - Method of receiving and processing dme signals - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: when measuring the position of the front of a video pulse, a DME signal is received and digitised, frequency of the received signal is shifted by an intermediate frequency shift value in the corresponding Nyquist zone and divided by the quadrature by multiplying the digitised signal by the signal of the quadrature generator; the multiplied signal is filtered and decimated by 2, the resultant video signal is transferred to zero frequency; the video signal is filtered; the time position of the front of the video pulse is determined from the level of half the amplitude of the video signal at the intermediate frequency; signals from non-operating Nyquist zones are suppressed by filtration and decimation by 5 and the obtained signal is transferred to the sampling frequency which is equal to 10 MHz; the video signal transferred to the sampling frequency is filtered by a narrow-band low-pass filter; a detection signal is generated by comparing the modulus of the detection pulse from the output of said narrow-band low-pass filter with the video signal, wherein the detection signal allows transmission of the video pulse for subsequent decoding.

EFFECT: improved signal-to-noise ratio at the output of the detector, enabling to obtain stable group delay time when frequency of the input signal changes, preventing distortion of the shape of video pulses by the analogue detector.

Description

The invention relates to communication technology, and in particular to a method for receiving and processing DME signals, and can be used in short-range radio navigation and landing systems to determine the distance between an aircraft and a transponder in conjunction with standard on-board equipment of DME / P and DME / N radio-ranging systems.

Omnidirectional range-measuring beacon (eng. Distance Measuring Equipment, DME) is a type of radio navigation system that provides distance from a ground station to an aircraft based on measuring the duration of the radio signal. Both on the aircraft and on the ground station, a receiver and an UHF transmitter (DMV / UHF) are installed to operate in the DME system.

When determining the distance to the ground station, the transmitter on board the aircraft emits a pair of signals of a certain duration, with a certain delay. At a ground station, which in practice is usually combined with a VOR navigation system, a transponder is installed that transmits a response signal at a prescribed frequency. Sometimes, DME equipment can also be placed with a course-glide path (ILS) system to determine the distance to a station during an approach.

The combination of rangefinder equipment with an azimuth beacon (for example, VOR) allows you to uniquely determine the position of the aircraft in space.

Foreign and domestic DME / N transponders use superheterodyne receivers. As an intermediate frequency (IF), a frequency of 63 MHz is usually used, which makes it possible to simplify the equipment somewhat by simultaneously using one synthesizer as a master transmitter generator and as a receiver local oscillator.

A transponder is known from the prior art which comprises a serially connected transceiver antenna, an antenna switch, a two-channel receiver, a first delay unit, a priority unit, a first encoder and a first transmitter, a first decoder, the output of which is connected to the second input of the first delay unit, a second decoder connected in series and a second delay unit, a third decryptor and a third delay unit connected in series, and a second encoder and a second transmitter connected in series, whereby the inputs of the first, second and third decoders are connected to the second output of the two-channel receiver, the outputs of the second and third delay units are connected to the second and third inputs of the priority block, respectively, the outputs of the first and second transmitters are connected to the first and second inputs of the antenna switch, respectively, and the input of the second encoder connected to the second output of the priority block (see publication of application RU 94020849 A, IPC G01S 13/74, publ. 08/10/1996).

A receiver is also known in the art which includes a frequency converter, a band amplifier, a compensating amplifier, a delay line, a linear amplifier, a filter, a buffer amplifier, three comparators, three keys, a channel discriminator, a level meter, and a gain control device. The level meter contains an amplifier, an N-bit adjustable amplifier, a first comparator, a driver of control signals, a binary N-bit counter, N-1 delay circuits, a second comparator, a trigger, a differentiator, a logic element I. The input signal is received through a frequency converter and a strip amplifier parallel to the level meter and the delay line. The level meter is a linear device for fast automatic gain control with negative feedback and provides information about the amplitude of the input signal in binary code, which is transmitted through the control device to a linear amplifier identical to the amplifier of the level meter, and changes the gain of the linear amplifier before its input a radio pulse will arrive from the output of the delay line. The amplified intermediate frequency radio pulse from the output of the linear amplifier through the filter and the buffer amplifier goes to the first comparator, where the front of the radio pulse is fixed at half its amplitude without demodulation. The second and third comparators fix the front of the radio pulse at the levels of 0.05 and 0.30 of its amplitude. The outputs of the comparators are connected to the outputs of the receiver through keys that are opened by the channel discriminator if the received signal corresponds to a given frequency. The use of a linear amplifier and three comparators operating without demodulation of radio pulses reduces the error in fixing the front of the radio pulse, and the feedback in the meter and the identity of the linear and measuring amplifiers weaken the influence of destabilizing factors. Two feedback loops in the level meter increase the dynamic range of the receiver (see publication of patent RU 2097922 C1, IPC Н04 В 1/10, G01S 7/285, publ. 11/27/1997).

In the above solutions, the selectivity for the mirror channel is provided by a narrow-band preselector (a device for amplifying the signal of the DME receiver), so the preselector is tunable and must be tuned to a specific frequency channel.

The disadvantage of the above solutions is the use of such preselectors - they are difficult to configure, and, in addition, the transponder must be tuned to a specific frequency channel during manufacture, i.e. “Attached” to a specific place of operation.

In addition, in known solutions, the output signal of the mixer is fed to a narrow-band analog filter that emits a video signal having a special “Gauss” shape, which is then transmitted, most often, to a logarithmic detector.

The disadvantage of these solutions is that conflicting requirements are imposed on this filter. On the one hand, its band should be wide enough to allow the video signal to pass through without distortion and to ensure a minimum change in the group delay time when the frequency of the input signal changes. On the other hand, the bandwidth should be minimal to ensure selectivity on the adjacent channel (in accordance with ICAO, the frequency spacing between the channels is 1 MHz) and to suppress interference from other radio equipment, which may be at frequencies close to the operating frequency of the transponder, since after passing through the detector, it will be impossible to separate the useful signal from interference. Typically, the IF filter bandwidth is ± 500 kHz.

In receivers with such a narrow-band filter having an infinite impulse response, it is not possible to obtain the stability of the group delay time when the input signal frequency is changed, which is necessary for signal processing. Changing the group delay time increases the range error. This is especially important in DME / P transponders, where the accuracy requirements are very stringent.

Also, an analog detector assumes the presence of a low-pass filter, which introduces, to one degree or another, distortions in the shape of the video pulse, i.e. gives an additional error in measuring the position of the front.

An object of the present invention is to provide a method for receiving and processing DME signals that is free from the above disadvantages.

The problem is solved by the proposed method for receiving and processing DME signals, according to which the DME signal is received and digitized, the frequency of the received signal is shifted by the frequency offset of the IF in the corresponding Nyquist zone and divided by quadrature by multiplying the digitized signal by the signal of the quadrature generator; filter and decimate by 2 times the signal, and then the resulting video signal resulting from filtering and decimation is transferred to the intermediate frequency; filtering the intermediate frequency video signal; determine the temporary position of the front of the video pulse at the level of half the amplitude of the video signal of the intermediate frequency; perform suppression of signals of non-working zones of Nyquist by filtering and decimation by 5 and transfer the received signal to a processing frequency of 10 MHz; filtering the video signal transferred to the processing frequency with a low-pass filter of low frequency; generating a detection signal by comparing the module of the detection pulse from the output of the aforementioned narrow-band low-pass filter with an intermediate-frequency video signal, the detection signal allowing the passage of a video pulse for subsequent decoding.

The technical result of the invention is to improve the signal-to-noise ratio at the output of the detector.

Another technical result of the proposed method is the ability to obtain a stable group delay time when changing the frequency of the input signal.

Also, another technical result of the invention is to prevent distortion in the shape of the video pulses by the analog detector and to reduce the error in measuring the position of the front of the video pulse.

The indicated technical results are achieved by the proposed method for receiving and processing DME signals, in accordance with which the DME signal is received and digitized, the frequency of the received signal is shifted by the frequency offset of the IF in the corresponding Nyquist zone and divided by quadrature by multiplying the digitized signal by the signal of the quadrature generator; filter and decimate by 2 times the signal, and then the resulting video signal resulting from filtering and decimation is transferred to the intermediate frequency; filtering the intermediate frequency video signal; determine the temporary position of the front of the video pulse at the level of half the amplitude of the video signal of the intermediate frequency; perform suppression of signals of non-working zones of Nyquist by filtering and decimation by 5 and transfer the received signal to a processing frequency of 10 MHz; filtering the video signal transferred to the processing frequency with a low-pass filter of low frequency; generating a detection signal by comparing the module of the detection pulse from the output of the aforementioned narrow-band low-pass filter with an intermediate-frequency video signal, the detection signal allowing the passage of a video pulse for subsequent decoding.

According to the present invention, a higher frequency than the above is selected as the inverter. For example, a frequency of 180 MHz is selected. In this case, the specular frequency is related to the working frequency of the channel at 360 MHz. Therefore, selectivity over the mirror channel can provide an irreplaceable broadband preselector. It has a frequency band matched with the receiving frequency range of the DME (1025 ... 1150) MHz.

The proposed method for receiving and processing DME signals does not require tuning the equipment to any particular frequency channel; changing the frequency channel is achieved by software tuning the local oscillator synthesizer.

The IF amplifier is linear. Its output signals with the help of an analog-to-digital converter (ADC) working with IF signals are converted to digital form. The conversion frequency, for example, equal to 100 MHz, is lower than the intermediate frequency, i.e. discretization at harmonics occurs. In this case, the discretization of the signal limited to the fourth Nyquist zone.

In connection with the foregoing, the IF filter can be made broadband. Its task is to filter noise and noise in non-working areas of Nyquist. Its bandwidth in this case can be increased to ± (3 ... 5) MHz. Consequently, the IF filter solves the problem of stabilizing the group delay time when the input signal frequency changes (in accordance with ICAO, the permissible frequency change is ± 100 kHz).

Adjacent channel selectivity is ensured by narrow-band digital filters with finite impulse response (CIC, FIR). This type of filter is characterized by a constant group delay time.

Detection of a Gaussian-type video signal is also performed digitally. It is performed by multiplying the signal by the quadrature of the intermediate frequency, followed by filtering the higher harmonics and calculating the modulus. This method provides linear detection that does not distort the waveform, i.e. does not introduce errors in determining the position of the pulse front and does not impair the accuracy of the transponder.

Consider the implementation of this method on the example of a two-channel receiver.

Due to the fact that according to the requirements of the International Civil Aviation Organization (ICAO), the dynamic range of the receiver must be at least 81 dB, and modern high-speed ADCs do not provide it, the receiver is made of two-channel.

After filtering and amplification, the intermediate frequency signal is divided into two channels, while in the first channel the signal is further amplified, and in the second it is attenuated, dividing the dynamic range of the input signal into two subbands: minus (5 ... 45) dBm and minus (45 ... 95) dBm

In each channel, a 16-bit ADC with a sampling frequency of 100 MHz is included. Digital information is processed by the Altera Cyclone III family programmable logic integrated circuit (FPGA). This FPGA has a built-in processor that performs the following main functions: programming the synthesizer to the channel frequency and performing monitoring functions.

The main characteristics of the receiver:

- dynamic range of at least 90 dB;

- sensitivity no worse than minus 95 dBm;

- suppression of the mirror channel at least 85 dB;

- suppression of the adjacent channel is not less than 95 dB;

- delay deviation (mat. expectation) no more than 5 ns;

- delay deviation up to minus 70 dBm (3σ) not more than 60 ns.

The claimed method of receiving and processing DME signals is as follows.

The frequency values are given for a specific implementation of the receiver with an IF of 180 MHz and a sampling frequency of the IF of 100 MHz.

1. Receive and carry out a frequency shift of the received input signal by 20 MHz (to zero frequency) and separation by quadrature. It is performed by multiplying the output of the ADC by the output signals of the quadrature generator sin (ωt) cos (ωt) with a frequency of 20 MHz.

2. Using filter CIC 2, filtering and decimation by 2 is performed. The mirror frequency of 40 MHz and the signals of the second Nyquist zone are suppressed for the quantization frequency f = 50 MHz and the transition to the processing frequency of 50 MHz.

3. Filtering with FIR 104 - basic video filtering. As a filter, a 104th order Chebyshev low-pass filter with a finite impulse response is implemented. The filter operates at a clock frequency of 50 MHz.

4. The determination of the temporal position of the front of the video pulse is provided using the “snap” scheme at the level of half the amplitude. This circuit is known as a DAC trigger.

5. Using the CIC 5 filter, filtering and decimation by 5 is performed. The Nyquist zones are suppressed for the quantization frequency f = 10 MHz and the transition to the processing frequency of 10 MHz.

6. Filtering with FIR 52 - signal isolation for the detection circuit. As a filter, a 52nd-order Chebyshev low-pass filter with a finite impulse response is implemented. The filter operates at a clock frequency of 10 MHz.

7. The detection signal is generated by comparing the detection pulse module from the output of narrow-band filters FIR 52 with the level of the input signal module (from CIC 2).

8. The detection signal permits the passage of the video pulse further to the decoding circuit.

After decoding a pair of request pulses, a signal is selected from one of 2 channels depending on the amplitude of the input signal.

Claims (1)

The method of receiving and processing DME signals, which consists in receiving and digitizing the DME signal, shifting the frequency of the received signal by the frequency offset of the IF in the corresponding Nyquist zone and dividing by quadrature by multiplying the digitized signal by the signal of the quadrature generator;
filter and decimate by 2 times the signal, and then the resulting video signal resulting from filtering and decimation is transferred to the zero frequency;
filter the video signal;
determine the temporary position of the front of the video pulse at the level of half the amplitude of the video signal of the intermediate frequency;
perform suppression of signals of non-working zones of Nyquist by filtering and decimation by 5 and transfer the received signal to a sampling frequency of 10 MHz;
filtering the video signal transferred to the sampling frequency with a low-pass filter of low frequency;
generate a detection signal by comparing the module of the detection pulse from the output of the aforementioned narrow-band low-pass filter with a frequency video signal, while in the presence of a detection signal, transmit a video signal transferred to a sampling frequency of 10 MHz for subsequent decoding.