JPS5818018B2 - Automatic frequency control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic frequency control system used in digital satellite communications and the like.

1 In satellite communications, etc., a variable oscillator installed in the receiving device for signal regeneration is used to reproduce synchronization pilot signals received through space far away or clock signals from received digital signals. Automatic frequency control (AFC) devices have become indispensable.

Two typical conventional systems of this type of automatic frequency control device will be described with reference to the block diagrams of FIGS. 1 and 3, respectively.

First, in the conventional example shown in FIG.
, ■ indicates an output signal with frequency f, 1 indicates a voltage controlled oscillator (hereinafter referred to as vCO), and 2 indicates a local oscillator with frequency fL.

Further, 3 is a mixer that mixes the frequencies of the VCO1 and the local oscillator 2 and outputs a signal of the difference frequency (fo-fL) between the two.

4 mixes the output of mixer 3 with the input signal V, and the difference frequency between the two (fi-fo+fL-Δf+fL, where Δf
-fi-fo] signal.

5 is a frequency discriminator having an output versus frequency characteristic as shown in FIG. 2 with the frequency fL as the center.

Since the frequency of the input signal of the discriminator 5 is fL+Δf,
At the output, a voltage V proportional to Δf=f--fO1, that is, the frequency error of this circuit, is obtained, thereby making it possible to control the frequency of the VCO1.

However, according to such a system, the frequency discriminator 5 operates around the frequency fL, so the local oscillator 2 and the mixer 3 are essential to match the frequency relationship, and the discriminator 5' A frequency error occurred due to variation in the O center frequency.

Furthermore, although it is possible to widen the frequency control range of the discriminator 5, it is difficult to increase the control sensitivity.

Along with this, the SN ratio of the output signal was inevitably poor.

Next, to explain the conventional example shown in FIG. 3, 11 is a vCO 112 which is an output of vColl.

It is a phase shifter that shifts the phase by π/2, and 1
3 and 14 are phase comparators that detect the phase of the input signal V· with respect to the output phases of the VCOll and the phase shifter 12, respectively.

The output of the phase comparator 13 is the input signal V and the output signal V with respect to the output of the phase comparator 14.

Does the phase advance by π/2 depending on the magnitude relationship of the frequencies?
Or be late.

13a and 14a are low-pass F wave generators inserted after the phase comparators 13 and 14, respectively; 15 is a tear wave generator 13a and 14;
This is a sign detector that detects the sign (positive or negative) of the frequency error based on the phase relationship of the output of a.

16 is a D/A conversion circuit that generates an analog voltage based on the output of the code detector 15 to control the VCOll.

In such devices, the frequency discrimination characteristic is obtained by a D/A conversion circuit, and the conventional D/A conversion circuit is a combination of an up/down counter and a D/A converter. This has the disadvantage that the circuit becomes complicated.

Furthermore, in terms of performance, frequency discrimination malleability is not affected by changes in component constants, so accurate frequency control without errors is possible, but the frequency control range cannot be widened, and the effects of input noise cannot be removed. There wasn't.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks, provide an automatic frequency control system that allows the frequency control range and control sensitivity to be arbitrarily selected, and that does not generate frequency errors due to constant fluctuations.

According to the present invention, there is provided an oscillator whose frequency can be changed by voltage control, a phase shifter that branches the output of the oscillator into two and shifts the phase of one of the signals by π/2, and an output signal of the phase shifter. and first and second phase comparators that respectively compare and detect the respective phases of the other two-branched signal and the phase of the separately applied input signal, and these first and second phase comparators. a first low-pass F-wave device connected to the output side of one of the first and second low-pass F-wave devices; and the phase of the output signal of the first low-pass F-wave device and the other of the first and second phase comparators. a third phase comparator that compares and detects the phase of the output signal of the
This is the output of the third phase comparator.

and a second low-pass filter that removes the alternating current component;
An automatic frequency control system (% formula) in which the frequency of the oscillator is controlled by the output of a low-frequency P-wave generator of the invention. This will be explained with reference to the configuration diagram and the frequency discrimination characteristics shown in FIG.

In FIG. 4, 11, 12, 13 and 14 are the same symbols as in the conventional example shown in FIG.

31 is a low frequency wave generator connected after the phase comparator 13, and 32 is a phase comparator for comparing the phases of the output of the low frequency p wave generator 31 and the output of the phase comparator 14. It is a vessel.

33 is a low-pass p-wave filter for removing AC components from the output of the phase comparator 32.

The operation of the device configured in this way will be explained below.

Input signal V, is applied to phase comparators 13 and 14. The phase of the output of phase comparator 13 is relative to the phase of the output of phase comparator 14 of input signal 7. and output signal V.

Depending on the magnitude relationship of the frequencies, it advances or lags by π/2.

Now, when f·>fo, assuming that the phase of the output of the phase comparator 13 advances by π/2 with respect to the phase of the output of the phase comparator 14, the output of the phase comparator 13 v1. (t) and the output of the phase comparator v14(t) are as follows.

However, ωi) ω0 are the input signal v, and the output signal V, respectively.
is the angular frequency of , and ω is the magnitude of the difference between the two, that is,
ωb=+ωi−ω.

It is 1. The transfer function of the low-pass F wave generator 31, F3□(S)=
1+ST where □, and the output v > 1 (t) of the low-pass p-wave generator 31 becomes.

Here, S indicates a differential operator, and T31 indicates a time constant of the low-frequency wave transducer 310.

Therefore, if the DC component of the output of the phase comparator 32 is vf, then the following equation is obtained.

Or frequency error ω. If we use =ωi−ω0, is obtained.

Thus, the discrimination characteristic of the output V with respect to the frequency error ω is as shown in FIG.

From the above explanation, it has been found that a frequency discriminator can be obtained by combining the phase comparators 13, 14.degree. 32 and the low-pass P-wave device 31.

Therefore, the basic form of the frequency control system according to the present invention is shown in a block diagram as shown in FIG.

In the figure, numeral 35 denotes a discriminator obtained according to the present invention, which is composed of phase comparators 13, 14, 32 and a low-frequency F-wave unit 31.

Here, KF is frequency discrimination sensitivity (V/H2), Kv is VC
If the closed-loop transfer function H(s) is determined from FIG. 6 using the FM modulation sensitivity (H/vl) of Oll, the following is obtained.

Here, F□(s), F, (s) is f. (t). f,
The Laplace transform value of '(t), Ko represents the loop gain of the AFC by KO=KVKF, and F33(s) represents the transfer function of the low-pass filter 33.

If F33'(8)=□, then 1 +T3. S T33 becomes ・'-0 to 7 is '1+' to 'oK' AFC/
'− time constant.

Thus, the output frequency f for a step change of Δf in the input frequency f.

The response is: ゛・Final frequency error °“t・(f,”)”−t 1
(t)t□6(t-00).

That is, by frequency control, the frequency error is compressed to □ by 1+ compared to the case without control.

Therefore, it can be seen that the accuracy of frequency control increases as the loop gain increases.

Also, as is clear from the transfer function, it has the characteristics of a low-frequency P-wave device with an AFC lunip side time constant τ.

Therefore, by increasing τ, the circuit band can be narrowed and the influence of input noise can be removed.The control method shown in Figure 4 can produce an output with high accuracy and less frequency error. Can be played.

The frequency discrimination sensitivity can be increased by making the time constant of the low-frequency P wave generator 31 thicker.

But 5. If this time constant is made thicker, the control frequency range of the frequency discriminator becomes narrower, and when the frequency difference between the input and output is large, the pull-in action is not performed.However, according to the present invention, the following embodiment can easily solve the problem.

An embodiment suitable for this case will be described below with reference to the block diagram of FIG.

As seen in the figure, there are two low-frequency ν wave generators 31, 31-1 and 31-2, and two phase comparators 3'2-1 and 3'2-1 and 31-2, respectively.
2-2 are provided, and further phase comparators 32-1 and 32 are provided.
An adder 36 is provided to add the Ryokawa power of -2.

In other parts of the structure, the same functions 3 are indicated by the same symbols as in the block diagram of FIG. 4.

Here, the time constant T! of the low-pass filter 31-1! 1-1 is thick (and the frequency number T'5t-2 of the low-pass F-wave unit 31-2 is selected to be small), the frequency discrimination characteristics of the outputs of the phase comparators 32-1 and 32-2 are as follows. This can be seen in curves a and b in the frequency discrimination characteristics of FIG.

Therefore, the output of the adder 36 becomes a curve C which is a combination of the above curves a and b, and as a result, the frequency control range is wide and the control sensitivity at adjacent points of frequencies can be increased.

Next, an embodiment in which the automatic frequency control method according to the present invention is combined with a phase locked circuit will be described with reference to the block diagram of FIG.

In the figure, 34 is a phase comparator (from 3 to low frequency r-wave unit 3).
1 for passing the AC component and blocking the DC component.
C coupler, 41' is the output of the phase comparator 13 and the low frequency F
This is a voltage adder that adds the output of the wave generator 33 □ and outputs the sum of both.

42 is a low-frequency p-wave generator for smoothing the output of the adder 41 and inputting it to the VCO 11'.

The adder 41 forms a phase synchronized circuit by transmitting the output of the phase comparator 13 to the VCOll via the low-frequency p-wave generator 42, and at the same time transmits the output of the low-frequency lachrymal fluid generator 33 to the low-frequency p-wave generator 42.
An automatic frequency control circuit is formed by adding it to the VCOll via the VCOll.

The former path will be referred to as the AFC route, and the latter bus will be referred to as the AFC route.

The disadvantage of phase-locked circuits is that the locking frequency range becomes narrow in order to improve their anti-noise characteristics.
By combining it with an AFC circuit as in this embodiment, the pull-in range can be expanded.

When the initial frequency difference is outside the pull-in range of the phase locked circuit, the AFC path operates to reduce the frequency difference.

When the frequency difference falls within the pull-in frequency range of the phase-locked loop, the phase-locked loop pulls in and enters a normal operating state.

In a normal state, the signal (DC) is blocked by the AC coupler 34, so the synchronization phase is determined by the phase synchronization circuit.

If the AC coupler 34 is not inserted, the loop consisting of the phase comparators 14, 32, low frequency waveform generator 33, adder 41, low frequency waveform waveform 42 and VCOll will operate as a phase locked circuit. There is a risk of performing phase locking to a phase that differs by π/2 from the desired one.

Therefore, by setting the DC loop gain of the AFC path to zero using the AC coupler 34, this risk can be eliminated and stable phase synchronization can be achieved.

It is clear that the same operation can be obtained even if the AC coupler 34 and the low-frequency F-wave device 31 are provided between the phase comparators 14 and 32 instead of between the phase comparators 13 and A2.

In addition, the input noise is A when the phase synchronization is established.
In order to prevent the APd path from being affected through the FC path, a switch is provided at the input or output of the low-frequency p-wave converter 33 to cut off the AFC path at the same time as synchronization is established, or a gate circuit is used as the phase comparator 32. Be sure to block the AFC path♂.

ζ Good.

To explain such an operation more specifically, if θ is the steady rod phase error in the phase comparator 13, the output of the phase comparator 13- is the recitation θ, and the output of the phase comparator 14 is the recitation (θ−π /2) Nierou θ becomes.

This state is illustrated in FIG. 10.

Since θ(1) is normally used, the output of the phase comparator 14 always has a negative potential.

Therefore, if a gate circuit 32/ is used as the phase comparator 32, which turns on when the output of the phase comparator 14 has a positive potential and turns off when the output has a negative potential, the gate circuit 32/ It is turned off and the AFC path is cut off.

The fact that such a gate circuit 32' has a function as a phase comparator will be explained with reference to the operating waveforms in FIG. 11. When ωi>ω0, the output of the phase comparator 14 is
bt - π/2), and on the other hand, the output of the low-pass p-wave generator 31 is 1/V'"TT; - 31)" (, 1u (GJ
t-1a' T31).

Therefore, the output of the gate circuit 32' has an average voltage as shown in FIG. 11, which is the same as the DC component of the ratio shearer output.

ω, kuω.

It can be seen that a similar calculation formula is also established in the case of , and that the gate circuit 32/ operates as a frequency discriminator having characteristics similar to those shown in FIG.

As is clear from the above explanation, (1) the automatic frequency control method of the present invention is carried out by comparing the phases of the input signal and the output signal of the actual pressure controlled oscillator by changing the phase of each other by tπ/2.
a low-frequency F-wave device connected to the output side of one of the two phase comparators; and another phase comparator that compares the output of this low-frequency F-wave device with the output of the other of the two phase comparators. By configuring the system using a kidney, there is no need to use a separate local oscillator, and there is no possibility of frequency errors caused by fluctuations in the central subwave number of the frequency discriminator, or restrictions on frequency control range, control sensitivity, etc. The effects obtained from Ganavi are significant in that the configuration is simple and the input signal can be reproduced completely and accurately without malfunction during use.

Furthermore, since the automatic frequency control method of the present invention can be used in combination with a general phase locked circuit, there is an advantage that the performance of the entire device can be further improved.

Brief Description of the Drawings --- Figure 1 is a block diagram showing an example of a conventional automatic frequency control system, Figure 2 is a frequency discrimination characteristic in the conventional example of Figure 1, and Figure 3 is a diagram showing the conventional automatic frequency control system. FIG. 4 is a block diagram showing another example of the control method; FIG. 4 is a block diagram showing an embodiment of the automatic frequency control method according to the present invention; FIG. 5 is a frequency discrimination characteristic in the embodiment of FIG. 4; FIG. 1 is a block diagram showing the basic form of the automatic frequency control system according to the present invention, FIG. 1 is a block diagram showing another embodiment of the present invention, FIG. 8 is a frequency discrimination characteristic in the embodiment of FIG. 7, and FIG. The figure is a block diagram showing still another embodiment of the present invention, FIG. 10 is a comparison diagram of the output waveforms of the phase comparators 13 and 14 with phase errors, and FIG. 11 is a waveform comparison diagram showing the operation of the gate circuit. It is.

In the figure, 11 is vCO112 is a π/2 phase shifter, 13
, 14° 32 is a phase comparator, 31 is a low-frequency p-wave filter for discrimination,
33 is a smoothing low-pass P-wave filter, 34 is an AC coupler, 35 is a frequency discriminator, 36.44 is an adder, and 42 is a smoothing low-pass filter.

Claims (1)

[Claims] 1. An oscillator whose frequency can be changed by voltage control. The output of the oscillator is split into two, and the phase of one of the signals is π.
a phase shifter that shifts by /2, and a first and second phase shifter that compares and detects the phases of the output signal of the phase shifter and the other signal branched into two, respectively, and the phase of the separately applied input signal. a second phase comparator and a first and second phase comparator;
a second low-frequency wave transducer connected to the output side of one of the aml phase comparators; and a phase of the output signal of the first low-frequency p-wave transducer and the first and second phase comparators. a third phase comparator that compares and detects the phase of the other output signal, and a second low-frequency tear wave detector that removes an alternating current component from the output of the third phase comparator, the second low frequency filter. An automatic frequency control method in which the frequency of the oscillator is controlled by the output of the oscillator.