RU2546542C1 - Controlled preselector integrated with magnetic ferrite antenna - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: present invention relates to radio engineering and can be used in designing input circuits of software-defined radio receivers. A controlled preselector integrated with a magnetic ferrite antenna comprises a ferrite rod, two coils placed at the ends of the ferrite rod, first and second variable capacitors, a tuning control circuit, a matching transformer, a preamplifier and two additional inductance coils. The disclosed controlled preselector provides bandwidth consistency with variation of the tuning frequency thereof, identical input resistances at both the inverting and non-inverting inputs and is characterised by low input resistance.

EFFECT: providing the best matching with low-impedance series circuits of a ferrite antenna.

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Description

The present invention relates to radio engineering and can be used in the construction of input circuits of software-defined radio receivers.

The quality of the radio receiver can be greatly improved through the use of a tunable input selector. Such a preselector must have a fixed bandwidth in the entire tuning range, the minimum number of adjustable components. Using a preselector allows already in the input path to significantly reduce the influence of unwanted signals from interfering radio stations, interference on the mirror channel, industrial interference, which can cause interference interference. Especially strong influence is caused by interference interference on useful signals using quadrature (multiphase) amplitude modulation (see Arkhipkin V.Ya., Meshkovsky K.A. Comparative noise immunity of communication systems with broadband and narrowband signals. Information and space, No. 3, 2004 , p.23), which is widely used in modern methods of digital communication.

From the application for US patent No. 2001/0036811 (published 01.11.2001; H03J 5/24), a device is known in which a parallel circuit with a set of capacities is used to control the tuning frequency of a magnetic ferrite antenna, which are connected to the loop coil when tuning the receiver using electronic keys (see Figure 3 of this application). A similar solution was used in US patent No. 4403347 (published on 09/06/1983; H03F 3/193, H03G 3/18, H03G 3/20, H03G 3/30, H03J 3/06, H03J 3/20, Н04В 1/18), with the exception that a capacitor of variable capacitance was used instead of electronic switching of capacities.

A device is known according to US patent No. 4805232 (published 02/14/1989; H01Q 5/00, H01Q 7/00, H04B 1/18), in which varicaps connected to a magnetic antenna loop coil are used to control the tuning frequency.

The disadvantage of these devices is the need to use an amplifier stage with a high input impedance; This ensures minimal shunting of the parallel circuit by the input impedance of the amplifier stage. However, high-impedance cascades are particularly sensitive to interference and industrial interference received by a ferrite antenna.

A technical solution is known (Zadorozhniy S.M. Suppression of common mode interference to radio reception. Radioamator, No. 1, Kiev, 2008), in which common mode interference is suppressed by using a symmetrical input circuit formed by a parallel circuit and a balanced transformer. However, with this solution, only partial suppression of the common mode noise can be achieved despite the stabilization of the operating point of the input source repeaters.

Closest to the proposed solution is a dual coil ferrite antenna described in US patent No. 6529169 (published 04.03.2003; G08B 13/24, H01Q 7/08), containing a ferrite rod, two coils located at the ends of the ferrite rod, tunable capacitor parallel to the coils, matching transformer, pre-amplifier, the outer ends of the coils connected to a tunable capacitor, and the inner ends to the primary winding of the matching transformer, the output of which is connected to the input at the pre-amplifier.

The disadvantage of the prototype is the use of sufficiently high-resistance coils, which, together with a ferrite core, form a low-frequency autotransformer for mains interference, the voltage from which is supplied together with a useful signal to the input of a high-impedance pre-amplifier on field-effect transistors, thereby creating conditions for the occurrence of intermodulation interference. Such a phenomenon is used, for example, for the simplest remote control circuits using a magnetic loop (see Wojciechowski J. Remote control of models. Translated from Polish, edited by A. P. Pavlov and N. N. Putyatin. M.: “Communication ", 1977, p.20, fig. 2.1.a). The magnetic loop, made in the form of a single-core wire, laid along the perimeter of the zone of responsibility, receives control commands from the low-frequency amplifier at frequencies of several hundred hertz. The receiver is a ferrite antenna, the coil of which is connected to another low-frequency amplifier. Thanks to electromagnetic induction, a voltage with the frequency of the transmitted commands is induced in the coil of the ferrite antenna. Similar processes occur in urban areas, penetrated by thousands of wires of industrial power supply network, whose interference due to the phenomenon of electromagnetic induction is induced on the coils of ferrite antennas.

If necessary, change the sub-band of the received waves in the known device, it is required to use additional coils or taps from the existing coils directly on the ferrite antenna, which is not technologically advanced in mass production.

In the case of using a varactor as a tunable capacitor, the symmetry of the input circuit and its balance with respect to common mode noise are violated.

The objective of the present invention is to eliminate the above disadvantages of devices known from the technical field, to reduce intermodulation interference caused by low-frequency interference of an industrial network.

This problem is solved by the fact that the known device containing a ferrite rod, two coils located at the ends of the ferrite rod, a tunable capacitor connected by a control input to the output of the tuning control circuit, a matching transformer, a preliminary amplifier, the output of which is the output of the selector, and whose balanced input is connected to the output (secondary) winding of the matching transformer, additionally contains a second tunable capacitor connected by its control input the house to the output of the control circuit, and two additional inductors, the first outputs of tunable capacitors connected to a common bus, and the second outputs of tunable capacitors connected respectively to the first outputs of additional inductors, the second conclusions of additional inductors connected respectively to the internal outputs of the coils located on ferrite rod, the external terminals of the coils located on the ferrite rod are connected respectively to the terminals of the primary coil of the matching transformer, the middle point of the primary winding of the matching transformer is connected to a common bus, each of the coils located on a ferrite rod, represents one or more series-connected flat turns.

As tunable capacitors, discrete tunable capacitors, varactors, etc. can be used.

Further, the invention is disclosed in more detail with reference to the figures, which represent embodiments of the invention.

Figure 1 presents the structural diagram of a controlled preselector, combined with a magnetic ferrite antenna, using a discrete tunable capacitor.

Figure 2 presents the structural diagram of a controlled preselector, combined with a magnetic ferrite antenna, when used as a tunable capacitor varactor.

Figure 3 shows the design of single-coil coils, their location on a ferrite rod, as well as the method of their connection and fastening to the printed circuit board.

A controlled preselector combined with a magnetic ferrite antenna contains:

1 - microcontroller. It controls the transfer of data over a data bus, such as I ² C or SPI. A general purpose microcontroller can be used, for example, with an ARM Cortex-M3 core of the LPC1700 series from NXP Semiconductors. The value of the generated data code determines the value of the capacitance of tunable capacitors.

2 - register. It receives and stores digital code sent from the microcontroller via the data bus. It has its own unique address. For example, a 16-bit PCA9535 register for the I ² C data bus from NXP Semiconductors can be used.

3 is a control configuration diagram. Generates at its output a digital code according to the embodiment of the invention shown in FIG. 1, or an analog voltage according to the embodiment shown in FIG. 2, which control the capacitance of tunable capacitors 4 and 5.

4, 5 - tunable capacitors. Capacitors whose capacitance value varies depending on the control action (digital code or voltage).

6, 7 - digital keys controlled by analog keys. For example, Analog Devices type ADG811 switches with ultra-low short-circuiting resistance of less than 0.8 ohms can be used.

8, 9 - a bank of capacitors, the value of the capacities of which is distributed according to the binary law: 2 ^N-1 · C, where N = 1, 2, 3 ... is an integer equal to the number of bits of the control code.

10 - single-coil coils L _A. Located on a ferrite rod. They are made of thin sheet of copper (foil).

11 - inductor L ′ _A. Additional high-quality inductor. Can be performed on an armored or toroidal core.

12 - inductor L ′ _B. Additional high-quality inductor. Can be performed on an armored or toroidal core.

13 - single-turn coils L _B. Located on a ferrite rod. They are made of thin sheet of copper (foil).

14 - ferrite core. It is selected depending on the operating frequency range. For the frequencies of the long-wave and medium-wave range, ferrite of the 400NN or 600NN brands can be used. For higher frequency ranges, 150VCh grade ferrite or combinations of ferrite rods of different magnetic permeabilities can be used (see Khomich V.I. Receiving ferrite antennas. M.-L.: Gosenergoizdat, 1963; RF patent No. 2256264 (published July 10, 2005; H01Q 7/08)).

15 - matching transformer. Carries out the coordination of the impedances of the loop coils of the ferrite antenna and the preliminary differential amplifier. It can be made in the form of an RF transformer, for example, type TT25-1-SMD from Mini-Circuits or another type, depending on the required operating frequency range.

16, 17, 18, 19 - resistors. The gain and input impedance of a balanced differential amplifier are determined.

20 - pre-amplifier. It amplifies the useful signal while suppressing common mode interference.

21 is a symmetrical operational amplifier. An operational amplifier such as THS 4520 from Texas Instruments can be used, for example.

22 - digital-to-analog converter. It receives a digital code from the data bus and converts it into a voltage proportional to the value of the digital code. For example, a digital-to-analog converter from the NXP Semiconductors PCF 8591 chip can be used.

23, 25 - varactors. They are a pn junction whose capacitance is determined by the magnitude of the reverse bias voltage applied to the junction. Varactors of type 1SV149 from Toshiba or BB202 from NXP Semiconductors can be used, for example.

24 - terminating resistor. Serves to prevent shunting of low output resistance of the digital-to-analog converter of the capacitive element (varactor) of the oscillatory circuit. It has a value of 100 ÷ 200 kOhm.

26 - tuning resistor. Serves for balancing varactors in order to compensate for the technological spread of their nominal capacities. It has a value of 100 ÷ 200 kOhm.

27 - flat ferrite core. A round ferrite core may also be used.

28 - single-coil coils L _A and L _B.

29 - contact pad on the printed circuit board.

30 - printed circuit board.

31 - transition metallized holes of the printed circuit board. Together with printed conductors, they provide a serial connection of adjacent single-turn coils.

The operation of a controlled preselector combined with a magnetic ferrite antenna occurs as follows. The magnetic component of the electromagnetic wave of the useful signal induces a high-frequency magnetic field in the ferrite core 14, which, due to the high magnetic permeability of the ferrite core, is concentrated in magnitude and, due to the phenomenon of electromagnetic induction, induces a high-frequency voltage in single-turn coils 10 and 13, which is applied, on the one hand, to the primary winding matching transformer 15, and on the other hand to high-quality inductors 11 and 12. Tunable capacitor 5, sub connected to the inductor 11, which, in turn, is connected in series with the coil 10 connected to the matching transformer 15, form the first sequential oscillatory circuit. Similarly, tunable capacitor 4 together with coils 12 and 13, as well as matching transformer 15 form a second series oscillatory circuit. In the case when the resonant frequency of these circuits coincides with the frequency of the received signal, the resistance of the successive oscillatory circuits reaches its lowest value, equal to the ohmic resistance of the series-connected coils and half of the primary winding of the matching transformer, and the high-frequency current of the received signal in the primary winding of the matching transformer reaches its maximum value. For other frequencies at which the resonance conditions are not observed, the resistance of successive oscillatory circuits increases sharply, and the current in the primary winding of the matching transformer decreases. Thus, when the capacitance of tunable capacitors changes, the preselector is tuned symmetrically (in two channels) to the desired frequency.

The capacity of tunable capacitors can be controlled both discretely, in accordance with the embodiment shown in FIG. 1, and using varactors, as shown in FIG. 2.

In the first case, i.e. at discrete management capacity tunable capacitors, digital code corresponding to the receiver tuning frequency is supplied to the MCU 1 to the register 2. The input data bus as the data bus is often used bus I ² C (see. The I ² C-Bus Specification. Version 2.1. 2000. Philips Semiconductors). The binary code from the output of register 2 then goes to the digital inputs of the controlled analog keys 6 and 7. When the code value is 0000 ... 00, all keys are open and the value of tunable capacitors is determined by the capacitance value C ₀ = C - some discrete capacitance value, the choice of which depends on the bit used binary code and the required range of tuning capacitor bank. When the binary code is increased by one, which corresponds to the code 0000 ... 01, the analog keys 6 and 7 of the least significant bits are activated, the capacitance of each of the tunable capacitors becomes equal to C ₀ + C, etc. In general terms, the value of the capacitance of tunable capacitors can be represented as:

where N = 1, 2, 3 ... is the number of bits of the binary code;

a _i ∈ (1, 0) are the bits of the binary code, a ₁ is the least significant bit.

For example, with N = 8, the capacitance of tunable capacitors C _Σ can vary from C ₀ with the binary code 0000 ... 00 to (C ₀ + 255 · C) with the binary code 1111 ... 11.

In the second case, when controlling the capacity of tunable capacitors using varactors according to Figure 2, a digital code corresponding to the receiver tuning frequency is supplied from the microcontroller 1 via the data bus to the input of the digital-to-analog converter 22, at the output of which a positive voltage is generated proportional to the value of the binary code. This voltage is supplied through the resistor 24 and the tuning resistor 26 to the cathodes of the varactors, the anodes of which are connected to a common bus. When voltage changes from 1 to 8 V, varactors, for example, type 1SV149, change their capacitance from 540 to 30 pF.

To reduce the influence of the technological variation of the parameters of the varactors, a tuning resistor 26 is used, with the help of which, within certain limits, it is possible to achieve greater identity of the characteristics of the change in the varactor capacitance depending on the applied voltage.

An important feature of the invention, especially attractive for software-defined receivers, is the constancy of the preselector bandwidth when changing its tuning frequency.

As is known from Shebes M.R., Kablukova M.V. A problem book on the theory of linear electrical circuits. M .: Higher. Shk., 1990, p.128, the bandwidth of any circuit is determined by the following expression:

where Δf is the passband; Q - quality factor; f ₀ - tuning frequency.

, а величина добротности Q для последовательного контура равна:As

, and the quality factor Q for a serial circuit is:

then the expression for Δf takes the following form:

Here R is the active resistance of the circuit and its load (the active resistance of the circuit coils 11, 10, 12, 13 and half of the primary winding of the matching transformer 15), L _K and C _K are the inductance and capacitance of the components of the circuit.

So, for the first circuit:

R _K1 = R ₁₁ + R ₁₀ + R ₁₅ ,

for the second circuit:

R _K2 = R ₁₂ + R ₁₃ + R ₁₅ .

Here:

R _K1 and R _K2 are the resistances, respectively, of the first and second circuit;

R ₁₁ and R ₁₂ are the active resistances of the inductors 11 and 12, respectively;

R ₁₀ and R ₁₃ are the active resistances of the coils 10 and 13, respectively;

R ₁₅ is the active resistance of half of the primary winding of the matching transformer 15.

L _K1 = L ′ _A + L _A + L ₁₅ ,

L _K2 = L ′ _B + L _B + L ₁₅ .

Here:

L _K1 and L _K2 are the inductance of the components of the first and second circuit, respectively.

L ′ _A and L ′ _B are the inductance of the inductors 11 and 12, respectively;

L _A and L _B are the inductance of coils 10 and 13, respectively.

C _K = C _Σ is the value of the capacitance of tunable capacitors 4 and 5.

As follows from (1), the bandwidth of the preselector does not depend on the value of capacitance C _Σ , which changes during the tuning process and is determined only by the value of the active resistance R and the value of the inductance L _K. This makes it possible, unlike the prototype, to set the necessary bandwidth by selecting the inductance value of the inductors 11, 12. The constancy of the bandwidth allows the software-defined receivers to set solid boundaries for further digital bandpass filtering in the signal processing processor.

The high-frequency voltage applied to the primary winding of the matching transformer 15, with a transformation coefficient k _0, is transmitted to the secondary winding and then fed to the differential input of the symmetrical operational amplifier 20.

The optimal value of the transformation coefficient k ₀ is determined by the following ratio (see Application Note on Transformers / AN-20-002. Www.minicircuits.com/app/AN20-002.pdf):

where Ry is the input resistance of the pre-amplifier, equal to the sum of the values of the resistances of the resistors 16 and 18; Ra is the output impedance of the series circuits.

After amplification of the useful signal and suppression of common mode noise, the output signal is supplied from the output of the preamplifier 20 to the output of the preselector for subsequent conversion in the radio receiver. The gain at the non-inverting and inverting inputs is determined by the ratio of the resistors 19, 18 and 17, 16.

In contrast to a conventional operational amplifier in differential switching, symmetrical operational amplifiers make it possible to obtain the same input impedances both inverting and non-inverting inputs. As a result, the condition of complete symmetry and the optimal coordination of the entire path are observed. The consequence of this is a more complete, in contrast to the prototype, common-mode interference suppression, the level of which is comparable with the common-mode interference suppression coefficient guaranteed by the operational amplifier, and can reach 60 ÷ 80 dB.

There is an important limitation that does not allow the use of broadband (including symmetrical) operational amplifiers in conventional high-impedance circuits. This is a requirement to provide low input impedance, typically of the order of hundreds of ohms. Only at such values is the stable operation of amplifiers in a wide frequency range guaranteed. Such recommendations can be found in the manuals for the use of both conventional broadband operational amplifiers such as AD8000, OPA847, and symmetric types AD8351, THS4509, THS4520.

However, it is precisely these requirements that are best fulfilled in the inventive preselector, where it is the low input impedance of the preamplifier that provides the best match with the low-impedance series circuits of the ferrite antenna.

Figure 3 of the inventive preselector shows an embodiment of single-turn coils when using a flat ferrite rod. Similarly, the design can be performed using a round ferrite rod. On each half of the ferrite rod 27 there is one or more single-coil coils 28, which are soldered to the printed circuit board 30 using the contact pads 29. The contact pads 29 through the metallized transition holes 31 and the printed conductors provide a series connection of the single-coil coils 28. The single-coil coils soldered to the contact pads 28, made of copper or silver-plated brass, provide mechanical fastening of the ferrite antenna to the printed circuit board. This design of the coils of the ferrite antenna can significantly reduce their ohmic resistance, reduce the value of parasitic inter-turn capacitances and obtain high quality factor, especially in the upper ranges.

The inventive controlled preselector, combined with a magnetic ferrite antenna, ensures constant bandwidth when changing the frequency of its tuning, the same input impedance for both inverting and non-inverting inputs and is characterized by a low input impedance, which contributes to best coordination with low-impedance serial circuits of a ferrite antenna. Due to this, the proposed controlled preselector more effectively, in comparison with devices known from the prior art, suppresses intermodulation interference caused, in particular, by low-frequency interference of an industrial network.

Claims (3)

1. A controlled preselector combined with a magnetic ferrite antenna, comprising a ferrite rod, two coils located at the ends of the ferrite rod, a first tunable capacitor connected by a control input to the output of the tuning control circuit, a matching transformer, pre-amplifier, the output of the pre-amplifier being the output of the pre-selector and the balanced input of the pre-amplifier is connected to the output (secondary) winding of the matching transformer, characterized in that it contains a second tunable capacitor connected by a control input to the output of the control circuit, and two additional inductors, the first leads of the first and second tunable capacitors connected to a common bus, and the second leads of the first and second tunable capacitors connected respectively to the first terminals of the additional inductors, the second conclusions of the additional inductors are connected to the corresponding internal conclusions of the coils located on the ferrite with the rod, the external terminals of the coils located on the ferrite rod are connected to the corresponding terminals of the primary winding of the matching transformer, the midpoint of the primary winding of the matching transformer is connected to a common bus, each of the coils located on the ferrite rod is one or more flat connected in series turns. 2. The controlled preselector according to claim 1, characterized in that discrete tunable capacitors are used as the first and second tunable capacitors. 3. The controlled preselector according to claim 1, characterized in that varactors are used as the first and second tunable capacitors.

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