SU488305A1 - Frequency modulated demodulator - Google Patents

Description

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The invention relates to telecommunications and can be used in receiving devices of systems for transmitting and receiving analog messages.

Known demodulator frequency-modulated signal, containing m processing channels, each of which consists of a series-connected bandpass filter and amplitude detector, the inputs of all channels are connected to the signal source, and the outputs through a series-connected decision circuit and a digital-to-analog converter - with one from the inputs of the adder. In addition, the demodulator contains a delay line, the input of which is connected to the signal source, and the output to one of the inputs of the multiplier with a band-pass filter; the other input of the multiplier is connected to the output of the controlled oscillator; frequency detector and output low pass filter.

The presence of a wideband frequency detector makes it difficult to implement a demodulator with large modulation indices and high demands on linearity of the operating characteristic.

In order to simplify the proposed demodulator, the input of the controlled oscillator is connected to the output of a digital-to-analog converter, and the output of the SINGLE CURRENT filter of the multiplier is connected to another input of the adder, the output of which is connected to the input of the output low-pass filter.

The drawing shows a structural block diagram of the proposed demodulator of the frequency-modulated signal.

The incoming signal with frequency modulation received by the receiver is fed to the input of the demodulator. With the modulation index used, the band of such a signal, when transmitting the full amplitude of the message from maximum to minimum, occupies the 2-Fs-m section, where 2Fs is the upper frequency of the transmitted message spectrum. The signal falling on the bandpass filters 1 is filtered out by that one, whose tuning frequency coincides with the value of the frequency of the received signal to the accuracy. The filtered signal is detected by detector 2 and fed to decisive circuit 3, which determines the number of the filter with the highest signal voltage.

Thus, filters 1, detectors 2 and decisive 3 carry out demodulation of the received signal with an accuracy of 2Pv. The number of the spectral region determined by the resolution scheme is converted by the digital-to-analog converter 4 into a beam, the value of which is proportional to the frequency of the signal received, determined with an accuracy of 2 /). This voltage is applied to one of the inputs of the adder 5.

With this, the incoming frequency-modulated signal arrives at the delay line 6, which delays it for harm, which is necessary for filtering and triggered by circuit 3. In an ideal circuit, the delay time should be equal.

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The delayed signal passes to the input of multiplier 7, to the second input of which voltage is supplied from the controlled oscillator 8. The generator frequency depends on the control voltage, which is used as the output voltage of the digital-to-analog converter 4. In this case, the difference between the frequencies of the received signal and the voltage of the controlled generator at any time lies in the limits

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where fcp is the average frequency of the filter bandwidth 9.

These conditions are feasible, since the voltage at the output of the digital-to-analog converter is proportional to the frequency of the received signal, taken to an accuracy of ± FB, and generator 8 inversely converts the amplitude to frequency (to the accuracy of consistency).

The difference frequency from the output of the multiplier 7 through a band-pass filter 9 is fed to a 3-band frequency detector 10, the passband of which, like filter 9, does not exceed 2F for any modulation index. The frequency detector performs demodulation of the difference frequency and, accordingly, the voltage at its output is equal to the difference between the actual value of the instantaneous value of the transmitted message (frequency) and the value obtained with its coarse determination by ftnltrami 1, detectors 2, decider circuit 3 by converter 4.

The voltage from the output of the frequency detector 10 is fed to another input of the adder 5, the total voltage from the output of which, corresponding to the frequency of the received signal, is additionally filtered by the low-pass filter 11 and is available at the output of the demodulator.

From the work of the proposed demodulator it can be seen that the controlled generator 8 has a finite number (equal to τ) of strictly defined frequencies. In connection with this, the requirements for it are significantly reduced. Likewise, lower requirements are placed on the linearity of the frequency detector 10. Since the maximum amplitude of the output voltage of the frequency detector is not critical,

the maximum amplitude of the entire message, its requirements decrease with increasing m. In conventional demodulators, on the contrary, they increase.

Claims (1)

Invention Formula Frequency modulated demodulator a signal containing m processing channels, each of which consists of successively connected band-pass filter and an amplitude detector, the inputs of all channels are connected to a signal source, and the outputs through a serially connected decider circuit and a digital-to-analog converter are connected to one of the inputs of the adder, as well as a delay line whose input is connected to a signal source, and an output to one of inputs of a multiplier with a bandpass filter, the other input of which is connected to the output of a controlled oscillator, a frequency detector and an output low-pass filter, characterized in that, in order to simplify the demodulator, the input of the controlled oscillator is connected to the output of a digital-analog converter, and the output of the bandpass filter and the multiplier through the frequency detector is connected to another input of the adder, the output which is connected to the input of the output low-pass filter. Output