RU44019U1 - BROADBAND VIDEO CONVERTER OF NOISE SIGNALS - Google Patents

Abstract

The utility model relates to the field of receiving noise signals in the VHF and microwave ranges and can be used to convert radio frequencies to video frequencies in radio interferometric terminals for receiving and recording signals from space sources of radio emission. The video converter contains an input common-mode power divider (1), two mixers (2, 3), a quadrature power divider (4) and a sum-difference device (5). In order to reduce interference in a wide range of operating frequencies and expand the bandwidth, two multistage phase-shifting circuits (6, 7) are introduced on operational amplifiers, and the quadrature power divider of the heterodyne signal is made in the form of a frequency doubler (8) with two antiphase outputs, which are through frequency dividers two (9, 10) are connected to the heterodyne inputs of the mixers. Multistage phase-shifting circuits provide high accuracy of the phase shift of the video signals at the shoulders of the video converter and eliminate bandwidth limitations, and by doubling, phase inversion, and dividing the local oscillator frequency by two, high phase shift accuracy is achieved at the heterodyne inputs of the mixers in a wide operating frequency band. 1 ill.

Description

The utility model relates to radio engineering, more specifically to the field of receiving noise signals in the VHF and microwave ranges and can be used to convert frequencies in radio interferometric terminals for receiving and recording noise signals from space sources of radio emission. Such video converters without distortion transfer the spectra of noise signals from the working ranges of radio frequencies or intermediate frequencies of the receivers into the video frequency band. The side frequency bands (upper and lower), which are mirrored relative to the tuning frequency of the local oscillator, are separated in such converters by a phase method.

A video converter for a modern radio interferometric terminal should have a working frequency range with an overlap factor f _max / f _{min of} at least 10 and with sideband separation (attenuation of specular interference) of at least 25 dB. Even more should be weakened Raman noise arising from the conversion of frequencies in the video converter. The frequency band of the input signals of modern video converters reaches 900 ÷ 1000 MHz and a tendency to its expansion is visible. At the same time, the frequency bandwidth of the video converter outputs reaches tens of MHz. To increase the sensitivity of radio telescopes and the accuracy of the results of radio interferometric observations, a number of video converters are used that are offset from each other in frequency so as to cover the entire frequency band of the input signals. The wider the frequency band, the higher the sensitivity and accuracy.

To work in narrow frequency ranges, relatively simple video converters with one quadrature frequency conversion are used, which contain a pair of mixers connected to the signal input and a local oscillator through power dividers with hard-defined parameters (see, for example, review: A.N. Abramov, A.S. .Astakhov, V.P. Kamishkintsev, D.A. Usanov. “Microwave Mixers with Phase Suppression of the Mirror Channel in Hybrid Integrated Design.” Reviews on Electronic Engineering. Ser. 1. Microwave Electronics. Issue 5 (1447). M Central Research Institute "Electronics". 1989, or an article : S. Padin, B. Arend, G. Narayanan. "A Wideband SSB Mixer Using High Frequency Operation Amplifiers." Microwave Journal, March 1992, p. 131-133). Thus, video converters of radio interferometric terminals were constructed, which are used for relatively narrow input signal frequency bands - up to 500 MHz. For example, video converters of the widely used VLBA 3 terminal operate at frequencies from 500

up to 1000 MHz, that is, only with a 2-fold overlap in frequency. Video converters of another common Mark III DAS terminal, although they work with 5-fold frequency overlap (100 ÷ 500 MHz), can’t work at frequencies above 500 MHz.

These limitations are related to the fact that in order to ensure the required isolation of the sidebands (weakening the mirror noise), it is necessary to maintain relative phase shifts with very high accuracy in the entire frequency band of the input signals, as well as the equality of the amplitudes of the signals supplied to the inputs of the mixer pair and to the sum-difference device. To attenuate specular interference by 25 dB, it is necessary to withstand a phase shift with an accuracy of no worse than 6 ° with uneven power division of no more than 1 dB. The fulfillment of such stringent requirements (especially in terms of phase shifts) can be ensured only in a relatively narrow frequency range. Despite the various ways of performing power dividers, mixers, and phase-shifting devices, known video converters with one quadrature frequency conversion cannot provide a wide range of input operating frequencies with a high degree of suppression of specular noise at the outputs. The ratio of the maximum working frequency to the minimum for video converters operating at frequencies above 500 MHz does not exceed 2.

To ensure broadband, all known video converters use double frequency conversion (see, for example, WTPetrachenko, M. Bujold, WHCannon, BRCarison, PEDewdney, GHFeil, P.Newby, A.Novikov, J.Popelar, RDWietfeldt. " The S2 VLBI System: DAS, RT / PT and Correlator. "International VLBI Service for Geodesy and Astrometry. 200 General Meeting Proceedings. February 21-24, 2000. Germany, p.378-382). Thus, video converters of Mark IV DAS and S2 DAS broadband radio interferometric terminals of foreign production were built. The use of double frequency conversion significantly complicates the circuit and design, since in this case it is necessary to use two local oscillators and to suppress interference from local oscillator signals penetrating from one frequency converter to another. Filters between frequency converters have very stringent selectivity requirements, which limits the possibility of expanding the bandwidth of the video converter. In addition, in filters with high selectivity it is difficult to ensure high phase-frequency linearity and uniformity of the amplitude-frequency characteristics in the passband, which is necessary for undistorted reception of noise signals. This drawback is especially growing in multichannel equipment, in which a large

number of video converters. The analogue closest to the utility model is the video converter of the radio interferometric terminal developed at Chalmers University of technology (see Emrich Anders. "Spectrometers and Receiver Systems for Ground, Ballon and Satellite Based (Sub) millimeter Radio Astronomy." School of electrical engineering. Technical report No. 225. Chalmers University of technology. 1992, p. 98-100). The specified video converter contains an in-phase power divider of the input signal, the outputs of which are connected to two identical mixers, and the outputs of the mixers are connected to the inputs of the sum-difference device through filters and second frequency converters, the outputs of which are the outputs of the video converter for the upper and lower side bands, respectively. Both local oscillators are connected to the corresponding inputs of the mixers through quadrature power dividers, which provide a phase shift of the local oscillator signals by 90 °.

The disadvantage of this video converter is the presence of Raman interference from the second frequency conversion and, especially, mutual Raman interference of the first and second frequency conversion. This places very stringent filter requirements between the mixers and limits the bandwidth of the video converter (less than 16 MHz), since a larger number of combinational noise gets into it when the bandwidth is expanded. The specified video converter uses narrow-band quadrature power dividers, made in the usual way on ferrite transformers. This limits the input frequency range of the video converter (frequency overlap no more than 2).

The purpose of the proposed utility model is to reduce interference in a wide band of operating frequencies and expand the bandwidth of the video converter. For this, two video mixers (2), a quadrature power divider (4) connected to the local oscillator, and an output sum-difference device (5) are introduced into the video converter (Fig. 1), which contains an input common-mode power divider (1), two multi-stage phase-shifting circuits (6, 7) of n phase links each, and the quadrature power divider of the heterodyne signal is made in the form of a frequency doubler (8) with two antiphase outputs, which are connected through two frequency dividers (9, 10) to the heterodyne inputs of the above-mentioned mixers (2, 3). The refusal of the second frequency conversion by introducing multistage phase-shifting circuits and performing a quadrature power divider in this way allows not only to get rid of the Raman noise associated with double frequency conversion, but also removes the bandwidth limitations of the video converter.

Multistage phase-shifting circuits on operational amplifiers, the theory and design methods of which are known (see, for example, S.A.Bukashkin, V.P. Vlasov, B.R. Zmiy and others. Reference for the calculation and design of ARC-schemes. M. Radio and communication. 1984.), allow to provide a phase shift of 90 ° between the video signals at the outputs of the mixers with high accuracy, which depends on the number of cascades (links) n and the required bandwidth of the video converter. For example, with n = 5 in the band up to 32 MHz, this accuracy is no worse than 3 °, which allows for absolute phase shift accuracy in the quadrature power divider to obtain attenuation of specular interference by more than 35 dB. To expand the bandwidth even further (for example, to 64 MHz), it is enough to only increase the number of stages n to 7.

In order to provide the necessary attenuation of specular interference, in addition to the accuracy of the indicated phase shift at the video frequencies, the accuracy of the phase shift of 90 ° between the signals at the heterodyne inputs of the mixers is of great importance. To ensure it in a wide range of operating frequencies, a doubling of the local oscillator frequency, inversion of the signal phase with a doubled frequency, and then dividing the frequency into two (FIG. 2) were used. The resulting phase shift of 90 ° between the signals at the heterodyne inputs of the mixers is maintained with high accuracy (no worse than 1.5 °) in the entire frequency range in which the operability of the frequency doubler, phase inverter and frequency dividers by two is preserved. Moreover, the total error of phase shifts in the quadrature power divider and in multistage phase-shifting circuits does not exceed 4.5 °, which is significantly less than the permissible value (6 °). The current level of technology of integrated circuits, including VHF and microwave ranges, allows you to implement a frequency doubler, a phase inverter and frequency dividers into two in the operating frequency range with an overlap factor of 10 or more, which extends the operating frequency range of the video converter to the specified limits.

Figure 1 shows the structural diagram of the proposed broadband video converter noise signals. The diagram indicates:

1 - common-mode power divider,

2, 3 - mixers,

4 - quadrature power divider,

5 - sum-difference device,

6, 7 - multistage phase-shifting circuit,

8 - frequency doubler with two antiphase outputs,

9, 10 - frequency dividers into two.

Figure 2 shows the timing diagrams of the signals at the main points of the quadrature power divider circuit, explaining how the phase of the heterodyne signal is rotated 90 ° before it enters the mixers. Here u is the signal voltage, t is time.

Figure 3 shows the results of measuring the attenuation of noise at the outputs of the upper sideband (PFS) and lower sideband (NBP) of the video converter for two experimental samples of the device.

The input signal power of the video converter is divided equally and in phase between the signal inputs of the mixers (2, 3). As a result of a phase shift of the heterodyne signal by 90 ° at the radio frequency f _g (using a quadrature power divider) and phase shift of the signals of the differential frequency f _in the phase-shifting circuits after the mixers by 90 ° in one arm relative to the other, the signals of the upper sideband are converted according to the rule f _in = f _with -f _g turn out to be in phase and add up to the output of the adder, and the signals of the lower sideband, converted according to the rule f _in = f _g -f _c , are out of phase and mutually compensated. Here f _c is the frequency of the input signal of the video converter. Similarly, the signal is allocated the lower sideband at the output of the subtractor. To obtain sufficient attenuation of specular interference, it is necessary to withstand phase shifts and power division in half with high accuracy.

The circuit of the proposed video converter is implemented on conventional radio elements and integrated circuits. A frequency doubler with antiphase outputs can be implemented, for example, in the form of an analog signal multiplier for itself on differential stages in a microcircuit design. Another embodiment is also possible: in the form of a semiconductor frequency doubler, a common-mode resistive power divider, and an inverter at one of its outputs. An in-phase power divider is usually performed on resistors. Balanced mixers can be built on semiconductor diodes or on integrated circuits. Digital frequency dividers into two are implemented by conventional methods on digital integrated circuits. Moreover, all these elements can be made in the form of a single integrated circuit. Multistage phase-shifting circuits are implemented as a series connection of the same type of phase links on broadband operational amplifiers. Modern integrated operational amplifiers have a working frequency band of 100 MHz or more and, thus, practically do not limit the bandwidth of the video converter.

The authors developed the circuit diagram and design of the video converter, made two experimental samples. They successfully passed tests as part of noise signal conversion systems at RTF-32 radio telescopes at the Svetloye (Leningrad Region) and Zelenchukskaya (Karachay-Cherkessia) observatories. In the operating frequency range 100–1000 MHz and in the passband up to 32 MHz, the attenuation of specular interference was not less than 26 dB, and Raman noise was absent in the dynamic range of 40 dB. Studies of samples of the video converter showed that it can provide a wider bandwidth (at least up to 64 MHz). This significantly exceeds the best foreign analogues, in which in the range of operating frequencies 100 ÷ 1000 MHz the bandwidth does not exceed 16 MHz.

Claims (1)

A wideband noise signal video converter containing two mixers connected to the signal input of the video converter through an in-phase power divider, and with a local oscillator through a quadrature power divider and an output sum-difference device, characterized in that two multi-stage phase-shifting circuits are introduced to reduce interference and expand the bandwidth on operational amplifiers connecting the outputs of the mixers with the inputs of the sum-difference device, and the quadrature power divider is made in the form of a doubler frequencies with two antiphase outputs, which are connected through two frequency dividers to the heterodyne inputs of the above-mentioned mixers.