RU185006U1 - Frequency modulator / demodulator based on a phase-locked loop - Google Patents

Abstract

The proposed utility model relates to radio engineering and can be used to generate frequency-modulated signals, demodulate them, synthesize frequencies with a uniform grid spacing of the generated frequencies. Achievable technical result is to provide frequency modulation and demodulation in one device with the IFAPC system, while improving its spectral characteristics This is achieved due to the fact that the first (2), second (5) and third (8) switches and the second low-pass filter (7) are introduced into the frequency modulator / demodulator. Moreover, the second inputs of the first (2), second (5) and third (8) switches, as well as a frequency divider with a fractionally variable division coefficient (10) are connected to the output of the control device. The output of the third switch (8) is the output of the demodulator. The third input of the first switch (2) is the input of the demodulator. Due to the switching using the second (5) and third (8) switches, the first (6) and second (7) low-pass filters with different passband, the reduction of side spectral components at the output is achieved modulator when transmitting both speech and binary signals, and a compromise is achieved between the speed of the IFAH system and the purity of the spectrum of the output signal. The introduction of the first switch (2) and additional connections with the control device allows you to use the proposed device as a demodulator of FM signals, which leads to the expansion of its functionality. 2 ill.

Description

The proposed utility model relates to radio engineering and can be used to generate frequency-modulated signals, their demodulation, frequency synthesis with a uniform grid spacing of the generated frequencies.

In radio communications, frequency-modulated (FM) signals are often used. At the same time, FM signal generation devices must have a set of qualitative characteristics:

- high stability of the carrier frequency;

- a wide band of modulating frequencies;

- low level of nonlinear distortion of the law of FM modulation;

- low concomitant amplitude modulation;

- a low level of spectral power density of phase fluctuations.

There are simple methods for generating frequency modulation in HF and VHF radios by directly affecting the oscillator frequency of the oscillator, however, this method does not allow to obtain a constant value of deviation when the frequency of the generated oscillations changes. Attempts to eliminate this drawback lead to a complication of the modulator. Also, a parametric LC oscillator even in autonomous mode has a large amount of instability of the middle frequency (of the order of 10 ^-3 -10 ^-4 ) [1].

A sufficiently high frequency stability can be achieved with the indirect FM method using controlled digital frequency dividers with constant parameters of the reference generator (OG) [2]. With this method, the signal from the exhaust gas is fed to the frequency divider with variable division coefficient (DPCD) and the FM is achieved by changing the division coefficient of DPCD according to the law of the modulating signal. However, with integer values of the division coefficient of the DPKD, it is impossible to form a uniform pitch of the frequency grid and, therefore, the linearity of the modulation characteristic. Also, with this FM method, at the moment of changing the coefficient, a signal phase gap occurs, which leads to a deterioration in the purity of the spectrum of the FM signal. The aforementioned disadvantages are eliminated by introducing a pulse-phase-locked loop (IFAP) with a frequency divider with a fractionally variable division coefficient (DDPKD) [3].

The formation of FM signals using digital frequency synthesizers with the IFAPH system is quite widely known in the literature [2, 3, 4, 5.]. The use of relatively simple single-point FM generation methods for relatively narrow-band modulating signals (up to 100 kHz) allows one to obtain highly stable modulators with respect to the average frequency (about 10 ^-6 );

The passband of the loop LPF in this case should correspond to the spectrum width of the modulating signal. When using a modulator, both for speech signals and for binary signals with a certain symbolic transmission rate (the so-called telegraphic), the cutoff frequency of the low-pass filter is determined by the second type of signal, as having a large (2-3 times, depending on the desired symbolic speed) the width of the spectrum compared to the first. As a result, when transmitting a speech signal at the output of the modulator, we obtain an increased level of secondary spectral components.

In the absence of a modulating signal, it is possible to use the device as a local oscillator, and in this case, when choosing the cutoff frequency of the loop low-pass filter, a contradiction arises between the speed of the system and the purity of the spectrum of the output signal.

The specified disadvantage have similar devices described in patent No. 6011815 (USA) H04 L 25/49, copyright certificate No. 869068 (USSR) H03 7/18.

The closest in technical essence to the proposed one is the FM signal generation device that implements the indirect single-point method of generating FM signals on a frequency synthesizer, the functional diagram of which is shown in Fig. 1.9b on page 25 in the book [1], adopted as a prototype.

The functional diagram of the prototype device is shown in FIG. 1, where indicated:

1 - reference generator (OG);

3 - frequency divider with a fixed division ratio (DPCD);

4 - pulse phase detector (IFD);

6 - low-pass filter (low-pass filter);

9 - voltage controlled oscillator (VCO);

10 - frequency divider with fractional variable division coefficient (DDPKD);

11 - analog-to-digital Converter (ADC).

The prototype device contains a series-connected reference generator (OG) 1, a frequency divider with a fixed division ratio (DPCD) 3, a pulse-phase detector (IPD) 4, a low-pass filter (LPF) 6, a voltage-controlled oscillator (VCO) 9, the output of which is connected to the input of the frequency divider with a fractionally variable division coefficient (DDPKD) 10, the output of which is connected to the second input of the IDF 4. The control input of the DDPKD 10 is connected to the output of the analog-to-digital converter (ADC) 10, the input of which is the input of the modulator . The output of the VCO 9 is the output of the modulator. The second input DFKD 3 is connected to the output of the control device (UU) (not shown in Fig. 1).

The prototype device operates as follows.

The output signal of the exhaust gas unit 1, passing through the block DFKD 3, the division coefficient of which is set by supplying a control signal to the corresponding input from the output of the control unit, is fed to the first input of the IFD 4. The signal from the output of the VCO 9. An error signal from the output of the IFD 4 is fed to the input of the VCO 9 block through the low-pass filter 6. Modulation is performed by supplying signals from the output of the ADC block 11, which are formed according to the law of the modulating signal, to the control input of the DDPKD 10 block, which leads to a difference in the signal frequencies, by stepping on the inputs of IFD 4, and the appearance of an error signal at its output. An error signal proportional to the modulating signal is fed to the input of the VCO unit 9, at the output of which an FM signal is generated.

The problem to which the claimed utility model is directed is to improve the spectral characteristics of the frequency modulator, to find a compromise between the speed of the system and the purity of the output signal spectrum, as well as to expand the functionality - using the modulator path as a demodulator.

To solve the problem, a frequency modulator / demodulator containing a reference oscillator (OG), a serially connected divider with a fixed division ratio (DPCD) and a pulse-phase detector (IDF), a first low-pass filter (LPF), a voltage-controlled oscillator connected in series and a frequency divider with a fractional variable division coefficient (DDPKD), the output of which is connected to the second input of the IDF, as well as an analog-to-digital converter (ADC), the output of which is connected to the corresponding input (DDPKD), etc. the second DPCD input is connected to the output of the control device, the ADC input is the modulator input, the VCO output is the modulator output, according to the utility model, the first switch is inserted, connected between the exhaust gas and the DPCD, as well as the second switch, the second low-pass filter and the third switch, connected in series which is connected to the input of the VCO and is the output of the demodulator, in addition, the other output of the second switch is connected to the input of the first low-pass filter, the output of which is connected to the corresponding input of the third switch, while the second inputs the first, second and third switches and DDPKD are connected to the output of the control device; the first input of the second switch is connected to the output of the IFD; the third input of the first switch is the input of the demodulator.

A functional diagram of the proposed device is shown in FIG. 2, where indicated:

1 - reference generator (OG);

2, 5, 8 - the first, second and third switches;

3 - frequency divider with a fixed division ratio (DPCD);

4 - pulse phase detector (IFD);

6, 7 - the first and second low-pass filters (low-pass filters);

9 - voltage controlled oscillator (VCO);

10 - frequency divider with a fractionally variable division coefficient (DDPKD);

11 - analog-to-digital Converter (ADC).

The proposed device contains series-connected exhaust gas 1, the first switch 2, DFKD 3, IFD 4 and the second switch 5, the two outputs of which are connected respectively to the inputs of the first 6 and second 7 low-pass filters, the outputs of which are connected to the corresponding inputs of the third switch 8, the output of which is the output demodulator and through series-connected VCO 9 and DDPKD 10 connected to the second input of the IDF 4. The output of the VCO 9 is the output of the modulator. The third input of the first switch is the input of the demodulator.

The output of the ADC 11 is connected to the control input of the DDCPD 10. The input of the ADC 11 is the input of the modulator. The second inputs of the first 2, second 5 and third 8 switches, as well as DFKD 3 and DDPKD 10 are connected to the output of the control device (UU) (not shown in Fig. 2).

The proposed device operates as follows.

An intermediate frequency signal is fed to the input of the demodulator after preliminary selection and amplification to the required level. In demodulation mode, the first switch 2 connects the input of the demodulator to the input of the DPCD 3, from the output of which the demodulated signal is fed to the IFD 4, to the second input of which the feedback signals from the output of the VCO 9 through the DPCD 10 are fed. In this case, the error signal received at the output of the IFD 4, will be the result of demodulation and after filtering, the first low-pass filter 6 or the second low-pass filter 7, depending on the control signal from the control unit, is supplied to both the input of the VCO 9 and the output of the demodulator. In this operating mode, there is no signal at the input of the modulator, the value of the dividing coefficient DDPKD 10 is set by the control signal from the output of the control unit, and remains fixed.

In the modulation mode, after receiving the corresponding control signal from the output of the control unit, the first switch 2 connects the output of the exhaust gas 1 with the input DFKD 3 and then, according to the above algorithm, the modulation takes place.

When modulating (demodulating) speech signals by supplying the corresponding control signals to the second 5 and third 8 switches, the first low-pass filter 6, which has a narrower passband, is connected to the IFAP system as a loop filter, unlike the second low-pass filter 7, intended for modulation (demodulation) binary signals.

As an implementation of the first switch, you can use, for example, ADG701 / 702BRT microchips from Analog Devices; DFKD, IFD, DPKD blocks can be implemented, for example, on a SKY72301-22 chip from Skyworks. The second and third switches can be implemented, for example, on an Analog Devices ADG419BR chip, the first and second low-pass filters can be implemented as second-order passive filters. As a VCO, you can use GK252-UN-S, manufactured by ZAO NPF BMG-plus. The control device and the ADC can be performed on an 1886BE6U microcircuit manufactured by Milander.

Thus, due to switching using the second and third switches, low-pass filters with different passband, the side spectral components are reduced at the output of the modulator when transmitting both speech and binary signals, and a compromise is achieved between the speed of the PLL and the purity of the output spectrum signal.

Introduction to the proposed device of the first switch and additional connections with the control device allows you to use the proposed device as a demodulator of FM signals, which extends the functionality of the device.

BIBLIOGRAPHY

1. Shitikov G.T. Stable oscillators meter and decimeter waves. - M .: Radio and communication. - 1983 .-- 256 s.

2. Tikhomirov N.M., Romanov S.K., Lenshin A.V. Formation of FM signals in systems in auto-tuning.-M .: Radio and communications, 2004.

3. Romanov S.K., Markov I.A. Determination of fragmentation interference in frequency synthesizers with PLL systems using delta-sigma modulators in fractional frequency dividers. // Theory and technique of radio communications. Scientific and technical collection, OJSC Concern Constellation OJSC, Voronezh, 2006, No. 1, pp. 97-102.

4. Yampurin NP, Bolznev VV, Safonova EV, Zhalnin EB The formation of precision frequencies and signals.-Nizhny Novgorod. state tech. un-t Nizhny Novgorod, 2003.187 s.

5. Popov P. A., Lenshin A. V., Shatalov E. V. Angular modulation of digital frequency synthesizers. - Voronezh: VI Ministry of Internal Affairs of Russia, 2001. - 262 p.

Claims (1)

A frequency modulator / demodulator based on a pulse-phase-locked loop system containing a reference oscillator (OG), a series-connected divider with a fixed division coefficient (DFC) and a pulse-phase detector (IPD), a first low-pass filter (LPF), a series-connected generator controlled by voltage and a frequency divider with a fractional variable division coefficient (DDPKD), the output of which is connected to the second input of the IFD, as well as an analog-to-digital converter (ADC), the output of which is connected to by a working input (DPCD), while the second DPCD input is connected to the output of the control device, the ADC input is the modulator input, the VCO output is the modulator output, characterized in that the first switch is inserted between the exhaust gas and the DPCD, as well as the second switch connected in series, the second low-pass filter and the third switch, the output of which is connected to the input of the VCO and is the output of the demodulator, in addition, the other output of the second switch is connected to the input of the first low-pass filter, the output of which is connected to the corresponding input of the third switch, while the second inputs of the first, second and third switches and DDPKD are connected to the output of the control device; the first input of the second switch is connected to the output of the IFD; the third input of the first switch is the input of the demodulator.

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