RU221689U1 - Digital device for separating narrowband signals from a wideband signal - Google Patents

Abstract

The utility model relates to radio astronomy systems and can be used in signal conversion and recording systems for radio telescopes. The technical result is an expansion of the operating frequency range of input signals while reducing the used hardware resources. This result is ensured due to the fact that an M -channel quadrature converter is included in the digital device for separating narrowband signals from a wideband signal between the m -channel distributor of digital samples and the switch, each channel of which consists of a series-connected quadrature mixer and a digital filter, and the heterodyne inputs of the quadrature The mixer is connected to a quadrature signal synthesizer, the clocking input of which is connected to the output of the m -fold frequency divider. 2 ill.

Description

The utility model relates to radio astronomy systems and is intended for use in signal conversion and recording systems for radio telescopes. The model can also be used in radio equipment for various purposes (for example, in communications technology) and in measuring equipment.

When conducting radio astronomy observations on most operating radio telescopes, video converters extract narrow-band video frequency signals from a broadband noise signal received by an antenna, which are then formatted and recorded for subsequent processing on a computer. This is necessary both for very long baseline radio interferometry (VLBI) and for spectral observations. Currently, the most common are systems for converting and recording signals on radio telescopes, in which a broadband noise signal is first converted into a high-speed digital signal using an analog-to-digital converter (ADC), and then this signal is digitally processed before its registration (see, for example, the monograph Koltsov N.E., Grenkov S.A., Fedotov L.V. Digital radio-astronomical signal recording systems. St. Petersburg: Publishing house of St. Petersburg State Economic University "LETI". 2019. 155 p., ill.).

Domestic systems in which narrowband video frequency signals are separated from a broadband signal by digital methods are presented, for example, in patents for utility models: RU 188320 U1, IPC H04J 14/00, Digital device for isolating narrowband signals from a broadband signal (published 04/08/2019. Bulletin . No. 10); RU 175721 U1, IPC H03D 7/00, System for converting and formatting signals for a radio interferometer (published 12/15/2017. Bulletin No. 35); and also in the article Grenkov S.A., Koltsov N.E. Data stream converter for a radio telescope with a digital system for recording broadband signals. Digital methods for extracting narrowband video frequency signals from a wideband digital signal are also used in various foreign systems, for example R2DBE (see Akiyama Kazunori, Alberdi Antxon, Alef Walter et al. First M87 Event Horizon Telescope Results. II. Array and Instrumentation // The Astrophysical Journal Letters. 875:L2 (28pp), 2019 April 10. URL: https://iopscience.iop.org/article/10.3847/2041-8213/ab0c96), DBBC3 (see Tuccari G., Alef W., Haas R . et al New Observing Modes for the DBBC3 // IVS 2018 General Meeting Proceedings. P. 47-49. URL: https://ivscc.gsfc.nasa.gov/publications/gm2018/11_tuccari_etal.pdf), or CDAS (cm Zheng W., Zhang J., Wang G et al. Technical Progress of the Chinese VLBI Network // IVS 2018 General Meeting Proceedings. P. 7-11. URL: https://ivscc.gsfc.nasa.gov/publications /gm2018/02_zheng_etal.pdf).

The prototype closest to the claimed device is a device for separating narrowband signals from broadband, presented in the utility model patent RU 188320 U1, IPC H04J 14/00 (published 04/08/2019. Bulletin No. 10). It contains a serially connected analog-to-digital converter (ADC), an m-channel distributor of digital signal samples, an m-channel polyphase filter and a bandpass signal switch connected to several digital video converters (DVC), as well as a signal sampling frequency pulse generator F _D connected with clocking inputs of the ADC and m-channel digital sample distributor, as well as with the input of an m-fold frequency divider. This divider generates a clock signal F _T =F _D /m and its output is connected to the clock inputs of the polyphase filter and digital video converters. To ensure the ability to isolate narrowband signals at any frequency in the frequency band of the input broadband signal, an m-channel quadrature mixer is connected to the output of the digital sample distributor, connected by heterodyne inputs to a quadrature signal synthesizer, and its outputs to an additional m-channel polyphase filter, the outputs of which connected to additional inputs of the bandpass signal switch. The tuning frequency of the quadrature signal synthesizer is four times less than the clock frequency of the mentioned polyphase filters and digital video converters. Therefore, the bandpass signals generated by the additional polyphase filter are shifted in frequency by half the band relative to the bandpass signals generated by the main m-channel polyphase filter. This makes it possible to avoid distortions when isolating signals in the DCC in narrow frequency bands around zero frequency, _FT , _FT /2 and their harmonics, since in this case it is possible to use bandpass signals shifted in frequency using an additionally introduced complex polyphase filter.

The disadvantage of the prototype is the limited operating frequency range of the input signal Δƒ. In accordance with Kotelnikov’s theorem, the sampling clock frequency of the input signal F _D ≥2Δƒ, and the clock frequency F _T =F _D /m, at which the main functional units of the device operate on a programmable logic integrated circuit (FPGA), is limited. Therefore, with an increase in Δƒ, it is necessary to increase not only the multiplicity m of the frequency divider, but also the number of parallel operating channels of the quadrature mixer, as well as the number of channels of the main and additional polyphase filters, which is also equal to m. In this case, the number of inputs of the 2m switch doubles. All this leads to an unacceptable increase in hardware costs and FPGA resources, which are also limited. For example, with Δƒ=1000 MHz, F _D =2048 MHz and the maximum permissible FPGA clock frequency of 256 MHz, it is necessary to have m=8. Accordingly, you will need an 8-channel quadrature mixer, two 8-channel polyphase filters and a 16x16 switch when using sixteen video converters. If the input frequency range doubles to 2000 MHz, then at the same clock frequency of the FPGA, a 16-channel quadrature mixer, two 16-channel polyphase filters and a 32x16 switch will be required, which is problematic to implement with limited FPGA resources.

The purpose of the claimed utility model is to expand the operating frequency range of input signals while reducing the used hardware resources. This goal is achieved by the fact that in a digital device for separating narrowband signals, containing a series-connected ADC of an input wideband signal, an m-channel signal sample distributor and a bandpass signal switch connected to several digital video converters, as well as a signal sampling pulse generator connected to the clock inputs of the ADC and an m-channel distributor of digital samples, as well as with the input of an m-fold frequency divider, the output of which is connected to the clocking inputs of digital video converters; an M-channel quadrature converter is connected between the m-channel distributor of digital samples and the switch. Each channel of this converter consists of a quadrature mixer and a digital filter connected in series, and the heterodyne inputs of the quadrature mixer are connected to the corresponding quadrature signal synthesizer (heterodyne), the clocking input of which is connected to the output of the m-fold frequency divider.

In the device claimed as a utility model, the m-channel digital sample distributor and the m-fold frequency divider serve only to reduce the clock frequency of the digital signal to a value acceptable for the FPGA. In order to avoid signal distortion in the process of frequency conversion and separation of sidebands in the PCB, the entire frequency range of input signals Δƒ using an M-channel quadrature converter is divided into M equal in width (equal to 2F _T ) and partially overlapping subranges. The amount of this overlap must be greater than the maximum bandwidth of the allocated narrowband signals. The boundary frequencies of the indicated subbands are determined by the tuning frequencies of the local oscillators of the corresponding quadrature converters, which are calculated by the formula

F _Gi =3 ⋅ (F _D /2m) ⋅ i (1),

where i=0,1,…, (M-1) is the serial number of the subrange. Within each subband, any of the digital video converters connected to it by the switch can be reconfigured. In this case, the number of subbands M is much less than the number of channels m of polyphase filters in the prototype of the utility model. In the proposed device, all digital conversions are performed on signals in a complex form, which also reduces hardware costs and allows the formation of subbands with a width of 2F _T. For example, for the input frequency range 0...2000 MHz we have F _D =4096 MHz, F _T =256 MHz and M=5, the tuning frequencies of the local oscillators of the quadrature converters will be 0, 384, 768, 1152 and 1536 MHz, and the boundaries of the subranges will be -256, respectively …256; 128…640; 512…1024; 896…1408; 1280…1792 MHz. In the complex region, the frequency band from 1792 to 2000 MHz falls into the first of these sub-bands.

Figures 1 and 2 (Fig. 1 and Fig. 2) show a block diagram of the proposed utility model, where it is indicated:

1 - analog-to-digital converter (ADC),

2 - m-channel signal sample distributor (demultiplexer),

3 - M-channel quadrature converter,

4 - bandpass signal switch,

5 ₁ ... 5 _N - digital video converters (DVC),

6 - pulse generator of frequency F _D signal sampling,

7 - frequency divider m times,

8 - quadrature mixer,

9 - digital filter,

10 - quadrature signal synthesizer (channel local oscillator of the quadrature converter).

In the inventive device, the ADC and pulse generator of frequency F _D are implemented on separate microcircuits, and other circuit elements are formed in the FPGA. The input of the device is connected to the ADC 1, the output of which is connected to the m-channel demultiplexer 2. The output of the demultiplexer 2 is connected to the input of the M-channel quadrature converter 3 so that m samples of the input signal are supplied in parallel to all M channels of the quadrature converter. The M outputs of the quadrature converter 3 are connected to the corresponding inputs of the bandpass signal switch 4. Each of the N outputs of the switch is connected to one of the digital control centers 5 ₁ ... 5 _N . The outputs of the DVC are the outputs of the claimed utility model. The clock inputs of the ADC 1, the m-channel demultiplexer 2 and the m-fold frequency divider 7 are connected to the output of the pulse generator 6. The output of the m-fold frequency divider 7 is connected to the clock inputs of the local oscillators of the M-channel quadrature converter 3 and the DCC 5 ₁ ... 5 _N . In each of the M channels of the quadrature converter 3, the input of the quadrature mixer 8 is connected to the output of the m-channel distributor 2. Two heterodyne inputs of the mixer 8 are connected, respectively, to the outputs of the sine (Sin) and cosine (Cos) signals of the corresponding local oscillator (quadrature signal synthesizer 10). The quadrature outputs (I and Q) of the mixer 8 are connected to the corresponding inputs of the complex digital filter 9, the output of which is the output of the corresponding channel of the M-channel quadrature converter 3.

The utility model works as follows. The input wideband digital signal with a sampling clock frequency F _D by the digital sample distributor 2 is divided into m streams with a clock frequency F _T =F _D /m, which are supplied in parallel to M channels of the quadrature converter 3. In each channel of the quadrature converter, a quadrature signal synthesizer 10 (local oscillator) ) is tuned to the frequency F _G determined by formula (1), and forms a pair of sine (Sin) and cosine (Cos) local oscillator signals, which are supplied to the corresponding heterodyne inputs of the quadrature mixer 8. The signal input of this mixer receives a digital signal from the output of the distributor 2. As a result of frequency conversion, a complex digital signal with a carrier frequency F _G and a band 2F _T is formed at the output of the mixer 8, the quadrature components of which (I and Q) pass through the complex digital filter 9, which suppresses side spectral components, to the output of the quadrature converter. Thus, using an M-channel quadrature converter 3, the input signal with a band Δƒ=F _D /2 is divided into M complex digital signals with a band ΔF=2F _T , and the spectrum of each received band signal is transferred to zero frequency (in the frequency range 0... 2F _T ). In this frequency band, DCCs 5 ₁ ... 5 _N operate, separating N randomly spaced frequency channels with a given relatively narrow bandwidth and separation of the upper and lower sidebands. The bandpass signals intended for operation of the TsVK 5 ₁ ... 5 _N are selected using switch 4. Since the frequency bands of the indicated signals partially overlap, and in total cover the entire frequency range of the input signals Δƒ, it is possible to isolate narrowband signals from the input broadband at any frequencies within Δƒ without losses and distortions associated with the division of Δƒ into subbands.

An experimental sample of the claimed utility model was manufactured on ADC chips EV10AQ190ACTPY from e2v, FPGA XC7Z045-2FFG900E from Xilinx and LTC695IIUHF, on which generator 6 is implemented. The implementation in the FPGA firmware of m-channel distributor 2, m-fold frequency divider 7 and digital digital control system 5 is no different from the prototype. Tests of the sample, which, as a channel of a signal conversion system on a radio telescope, provided a 2-fold expansion of the operating frequency range while reducing FPGA hardware costs, confirmed the effectiveness of the model and the fulfillment of the stated goal.

Claims (1)

A digital device for separating narrowband signals from a wideband signal, containing a series-connected analog-to-digital converter of the input wideband signal, an m -channel signal sample distributor and a bandpass signal switch connected to several digital video converters, as well as a signal sampling pulse generator connected to the analogue clock inputs. digital converter and m -channel distributor of digital samples, as well as with the input of an m -fold frequency divider, the output of which is connected to the clocking inputs of digital video converters, characterized in that an M -channel quadrature converter is connected between the m -channel distributor of digital samples and the switch, each the channel of which consists of a series-connected quadrature mixer and a digital filter, and the heterodyne inputs of the quadrature mixer are connected to a quadrature signal synthesizer, the clocking input of which is connected to the output of the m -fold frequency divider.