SU1707765A1 - Frequency-modulated digital frequency synthesizer - Google Patents

Abstract

The invention relates to radio engineering. The purpose of the invention is to reduce the frequency distortion of the output signal. A frequency modulated digital frequency synthesizer contains a reference oscillator 1. a frequency divider 2 with a fixed division factor, a first phase detector 3, a low-pass filter 4, a controlled oscillator 5. a frequency divider 6 with a variable division factor, a frequency setting unit 7, a phase module torus 8, modulating signal source 9, second phase detector 10, key 11, high-pass filter (HPF) 12, amplifier 13, timing indicator 14, element OR 15, one-shot 16. Phase modulator 8 is included in the auxiliary circuit cal adjustment of phase difference. which allows to stabilize the phase of the output signal when exposed to modulating disturbances in the frequency band of the HPF 12 This allows you to work out changes in the frequency of the controlled generator 5 from the lowest to the cut-off frequency of the HPF 12. 1 Il (L

Description

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The invention relates to radio engineering and can be used in receiving and transmitting equipment.

The aim of the invention is to reduce the frequency distortion of the output signal.

The drawing shows an electrical structural diagram of a digital frequency synthesizer with frequency modulation.

A frequency modulated digital synthesizer contains a reference oscillator 1, a frequency divider with a fixed division factor (DFCD) 2, a first phase detector (PD) 3, a low-pass filter (LPF) 4, a controlled oscillator 5, a variable frequency divider division factor (DCD) 6, frequency setting block 7, phase modulator 8, modulating signal source 9, second PD 10, key 11, high-pass filter (HPF) 12, amplifier 13, timing indicator 14, OR 15 element, one-shot 16 .

A digital frequency synthesizer with frequency modulation operates as follows.

In synchronism mode, when modulating voltage is applied from source 9 to the modulating input of controlled oscillator 5, its frequency modulation takes place (as in the usual direct single point method). From the output of the controlled oscillator 5, the frequency-modulated signal is fed to the input of the DPCD 6. where it is divided by frequency by the division factor N. From the output of the DPCD 6, the phase-modulated short pulses go to the input of the phase modulator 8 and to the input of the second PD 10, where they are compared with short reference pulses with a frequency of comparison fcp from the DFCD 2 output, resulting from dividing the frequency of the operative generator 1 by a constant dividing factor of DFCD 2. As a result of comparing these pulses in frequency and phase at the output of the second PD 10, The control voltage, which by the principle of negative feedback through the switch 11, the HPF 12 and the amplifier 13 goes to the control input of the phase modulator 8 and modulates its input pulses IR, so that the output of the phase modulator 8 the pulses are no longer modulated, as in the synchronism mode without the supply of a modulating signal (i.e., the modulation is completely removed).

In the proposed device, the phase modulator 8 is included in the auxiliary circuit of automatic adjustment of the phase difference of the output signal relative to the input signal. Such a circuit allows stabilizing the phase of the input signal of the phase modulator 8 under the influence of modulating disturbances in the frequency band defined in the passband frequency band of the HPF 12. Otherwise, all modulating disturbance pulses at the input of the phase modulator 8 and the distortion associated with them automatically compensated in the bandwidth determined by the passband of the HPF 12,

From the output of the phase modulator 8, pulses with a modulation removed in a frequency band equal to the passband of the HPF 12 are transmitted to the second input of the first PD 3. Where they are compared with the reference pulses from DFCD 2, From the output of the first PD 3, the control slowly varying The voltage across the LPF 4 is fed to the control input of the controlled generator 5 and adjusts it in frequency and phase to the reference generator 1,

Thus, the phase locked loop works out the changes in the frequency of the controlled oscillator 5, starting at the lowest and ending at the cutoff frequency of the HPF 12.

In a transient mode, when the phase difference at the inputs of the second PD 10 exceeds a predetermined maximum value, which is determined by the maximum necessary level of frequency deviation of the controlled oscillator 5 when it is modulated, the timing indicator 14 is triggered from the output signal of the second PD 10 the inhibit signal, which, through the OR circuit 15, arrives at the control input of the key 11, so that the key 11 is closed.

Thus, the auxiliary auto-regulation loop is opened and the frequency control of UG 5 in the phase-locked loop is done in the usual way, in which the phase-locked auto-tuning ring processes the frequency of the controlled oscillator 5, starting at the lowest frequency and ending with the cut-off frequency of the low-pass filter 4. Otherwise, the bandwidth of the auto-tuning ring expands and is determined only by the cutoff frequency of the low-pass filter 4, which can be significantly higher than the cut-off frequency of the high-pass filter 12.

Thus, the transient response rate is kept high with a wide modulation bandwidth, as with the two-point method, but there is no frequency distortion.

The key 11 is closed not only by the signal from the synchronization indicator 14, but also by the signal from the output of the one-shot 16, When switching division factors

DPKB from the second output of the installation unit 1 to the input of the one-shot 16 receives a pulse, which overturns it to the state when the inhibit signal is generated at the output of the one-shot 16. This signal is fed through the second input of the circuit OR 15 to the control input of the key 11, as a result of which the key 11 is closed. The duration of the inhibit signal from the one-shot 16 is not longer than the transition time of setting a new frequency in the digital frequency synthesizer. The amplifier 13 is necessary for the correct selection of the transmission coefficient of the control signal at the control input of the phase modulator 8 in accordance with its slope. The transmission coefficient of the amplifier 13 is selected so that at the output of the phase modulator 8 the modulation of the pulses in phase is completely removed in the frequency band determined by the HPF 12.

In the case of using a digital frequency synthesizer as a receiver's local oscillator, an auxiliary auto-control loop can be opened based on the second PD 10 by turning on the key 11 a prohibitory signal of the corresponding logic level.

In the digital frequency synthesizer, the compensation of the response of the phase locked loop to the modulating disturbance and the associated distortions in a given frequency band is automatically complete, since negative feedback acts here (by disturbing the forward frequency. This allows to obtain an extension of the range of modulating frequencies to the lower frequencies compared to single-point modulation at a given speed of 0

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while avoiding distortion of the output signal.

Claims (1)

The invention includes a frequency modulated digital frequency synthesizer comprising a ring controlled oscillator, a variable division frequency divider, a phase modulator, a first phase detector and a low pass filter, a reference division generator and a frequency division divider with a fixed division factor, output which is connected to the second input of the first phase detector, the source of the modulating signal, the output of which is connected to the modulating input of the controlled oscillator, and conductive input of the frequency divider with a variable division ratio connected to the first output of the frequency setting unit, characterized in that, to reduce the frequency of the output signal distortion between the second output of the frequency and the control input of the phase modulator introduced serially connected monostable OR element. a key, a high-pass filter and an amplifier, as well as a second phase detector and a synchronism indicator connected in series, the output of which is connected to the second position of the OR element, the first and second inputs of the second phase detector are respectively connected to the output of the q-q clock with a fixed division factor and to the output of a frequency divider with a variable division factor, and the signal input of the key is connected to the output of the second phase detector.