JPH0365819A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To exclude the effect onto a demodulation characteristic due to a temperature change without need of an exclusive reference crystal oscillator by providing a phase comparator using an oscillated output from a voltage controlled crystal oscillator used at a clock recovery section of a demodulation circuit as a reference frequency. CONSTITUTION:An oscillated output of a voltage controlled crystal oscillator (VCXO) 12 used in a PLL of a clock recovery section 11 recovering a clock for demodulating an MSK (Minimum Shift Keying) signal from the standpoint of frequency stability is used for a reference frequency of a phase comparator 8. Then the PLL of the clock recovery section 11 follows the clock at the sender side with highly stable state when the clock is recovered. Thus, no exclusive reference oscillator is required and the frequency stability of the recovered clock is equal to that at the sender side and the effect due to temperature change is not affected.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an automatic frequency control circuit (hereinafter referred to as 'AFC) provided before a demodulation circuit.
This is related to the requency control (referred to as J).

Chie Aishutachi Traditionally, one of the wireless digital signal transmission methods was MSK (Minis+uw 5hift Ke
ying) method has been proposed. That is, this MSK
The method is FSK (F
Since the clock is frequency-modulated with a digital signal of 0.1, it has a constant envelope and has the advantage of being resistant to nonlinear distortion. Conventionally, in order to demodulate this MSK signal, a demodulation circuit using a synchronous detection method is provided, which performs clock recovery on the receiving side that is phase-synchronized with the clock on the transmitting side and demodulates it. Because of the need to narrow the frequency pull-in range of the PLL circuit to suppress loop noise, an AFC circuit was provided before the demodulation circuit to reduce frequency errors and improve demodulation characteristics against noise.

Specifically, such an AFC circuit is configured as shown in Fig. 2, where (1) is a signal input terminal into which a high frequency MSK signal of f, 4 is input, and (2) is a voltage controlled oscillator which will be described later. The intermediate frequency f is mixed with the output of the oscillation frequency fLo of
, (3) is the intermediate frequency re
-l fi, a bandpass filter for extracting the MSK signal of fLol; (4) is an MSK demodulator that demodulates the MSK signal using a synchronous detection method;

(5) is the first frequency dividing the MSK signal of frequency f by 1/m.
Frequency divider, (6) is a reference crystal oscillator that outputs the reference frequency f IEF, (7) is the reference frequency f l1tF that is 1
/n, the second frequency divider (8) is the first frequency divider. A phase comparator that compares the divided outputs from the second frequency divider (5) and (7) to detect a frequency shift, that is, a phase difference, and (9) is a phase comparator that is supplied through the loop filter (10). vessel(
This is a voltage controlled oscillator (hereinafter referred to as rVcOJ) whose oscillation frequency is controlled based on the output of 8).

Therefore, the MSK signal of frequency f input to the MSK demodulator (4) is once divided by 1/m by the first frequency divider (5), and then divided by 1/n by the phase comparator (8). The MS
The K signal is always kept at a constant frequency fo.

For example, the frequency fo of the MSK signal input to the signal input terminal (1) fluctuates by Δf, resulting in f'IN""flN+Δ
f, the output from the phase comparator (8) proportional to the variation Δf causes the oscillation frequency of the V CO (9) to immediately change from rto to r'to. The extracted frequency fl is to”
=lf'+w f'Lol=l (f+N+Δf)
(f,.+Δf)l −l f+N−fLol
Therefore, the frequency of the MSK signal input to the MSK demodulator (4) is always maintained at f, and stability equivalent to that of the reference crystal oscillator (6) is obtained.

In the past, AFC circuits with such conventional configurations required a dedicated reference crystal oscillator, and the frequency stability of this reference crystal oscillator against temperature changes also became a problem, leading to increased costs and This resulted in deterioration of demodulation characteristics.

The present invention has been developed in view of these points, and it is an object of the present invention to provide an AFC circuit that does not require a dedicated reference oscillator and can obtain demodulation characteristics that are not affected by temperature changes.

In order to achieve the above object, the present invention provides an automatic frequency control circuit that is provided before a demodulation circuit and controls the frequency of a modulated signal input to the demodulation circuit. It is equipped with a phase comparator that uses the oscillation output from the voltage-controlled crystal oscillator used in the section as a reference frequency.

According to the AFC circuit configured as described above, the stability of the reference signal used in the phase comparator becomes equal to the frequency stability of the clock on the transmitting side, and a dedicated reference crystal oscillator is not required.

Hereinafter, the AFC circuit of the present invention will be explained with reference to the drawings.The same parts as those of the conventional one will be given the same reference numerals and the explanation thereof will be omitted.

In this embodiment, as shown in FIG.
The oscillation output of a voltage controlled crystal oscillator (hereinafter referred to as rvcxo) (12) used in L from the viewpoint of frequency stability is used as the reference frequency of the phase comparator (8).

In other words, once the clock regeneration is established, the PLL of the clock regeneration unit (11) will follow the highly stable clock on the transmitting side, so the frequency stability of the regenerated clock will be the same as that on the transmitting side, and the temperature will change. This is because they are no longer affected by change. Therefore, if the output signal of this vCX○ (12) is used as the reference frequency of the phase comparator (8), there is no risk of frequency fluctuation due to temperature changes, and deterioration of demodulation characteristics due to temperature changes is prevented.

Note that until the clock regeneration of the clock regeneration unit (11) is established (that is, until the synchronization state is reached), the VCX O(1
The frequency f CLll of the oscillation output in 2) will fluctuate, but the oscillation frequency of V CX O (12) changes with respect to the control voltage, that is, the conversion gain is small and hardly changes, so it was divided by 1/n again. Since the reference frequency is used as the reference frequency, its fluctuation is also reduced by L/n, and has little effect on the AFC circuit.

Furthermore, as mentioned above, according to the AFC circuit of the present invention, there is no need for a dedicated reference crystal oscillator, and the influence of temperature changes on the demodulation characteristics can be eliminated.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing a conventional example. (8)...-phase comparator, (9)...-voltage controlled oscillator. (11)--Black reproducing section, (12)--V C
XO.

Claims (1)

[Claims] (1) In an automatic frequency control circuit that is provided before a demodulation circuit and controls the frequency of a modulated signal input to the demodulation circuit, an oscillation output from a voltage-controlled crystal oscillator used in the clock regeneration section of the demodulation circuit. An automatic frequency control circuit characterized in that it is equipped with a phase comparator that uses as a reference frequency.