JPS61105921A - Tracking circuit - Google Patents

Abstract

PURPOSE:To reduce tracking error over a wise band by adding a variable capacitor for tracking to a 3-point tracking circuit and changing the capacitor with the same control voltage as that for a tuning variable capacitor so as to avoid over-compensation by a fixed capacitor. CONSTITUTION:In order to suppress the over-compensation at high frequencies by a parallel capacitor Cp, the 3rd variable capacitor Cv3 is connected in paral lel with the Cp. A Ca is a fixed capacitor for coupling. In considering an addi tional Cp' comprising Ca, Cv3, and Cb which is a fixed capacitor connected in parallel with the cv3 as the variable part of the Cp, the correction effect of the Cp by the cv3 is obtained by setting the Ca to a small value. that is, when the control voltage is low and the Cv3 is large, the additional capacitor Cp' viewed from the Cp is limited by the Ca and when the control voltage is high and the Cv3 is decreased, the additional capacitor Cp' viewed from the Cp is decreased. Thus, as the frequency gets higher, the value of Cp+Cp' decreases and the rate of change in the frequency f0(fp) of an oscillation circuit OSC is reduced (approaches f0').

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a three-point tracking circuit that uses a voltage variable tuning element in a local oscillation circuit, and is particularly directed to improving the tracking error over a wide band. be.

[Conventional technology]

When receiving wideband signals such as MW band (530KHz to 1600KHz) with a superheterodyne receiver,
A tracking circuit is used to reduce tracking error (synchronization) over the entire band. Figure 5 shows the high frequency circuit RF and local oscillation circuit OS of a variable capacitance electronic tuner.
This is a circuit diagram showing an example of the series capacitance C of the oscillation circuit OSC.
s and the parallel capacitor Cp constitute a three-point tracking circuit TRK. The operation of this circuit will be explained with reference to FIG.

The tuning frequency raF of the high frequency circuit RF is the inductance L
It is determined by R and variable capacitance (varicap) Ctz, and changes linearly in proportion to the control voltage applied to variable capacitance CVI. The frequency fa of the oscillation circuit OSC is determined by the capacitances Cs and C
Without p, it is determined by inductance Lo and variable capacitance CV2, and changes linearly in response to the same control voltage as above.

However, even if the characteristics of variable capacitances CVI and CV2 are equal, fO and fRF cannot have the same slope.

That is, when the control voltage changes from ■1 to ■2, fRF and fO show the same frequency change ratio, so when fRF changes from fl to f2, fo changes from f3 to f4 with the following relationship.

3 fa f+ f2 Therefore, the intermediate frequency f obtained as the difference between fO and fRF
1F differs depending on the control voltage (receiving frequency fRF) (f3-f+ for V+, fa f2 for V2). This is a tracking error. fo' is an ideal curve of the oscillation frequency and has the same slope as 'RF.

If fo' changes in this way, the ideal intermediate frequency f, F
' is uniform (450 KHz) over the entire band, but as a practical matter, it is almost impossible to realize this characteristic.

Therefore, the oscillation circuit OSC is used as the tracking circuit TRK.
A parallel capacitor Cp is added to the side so that even if the tuning capacitor Cv2 becomes smaller, the overall capacitance value does not become less than Cp. As a result, the curve of fo on the high frequency side is corrected as shown by the broken line fo (Cp'). Furthermore, if a series capacitance Cs is added, even if the tuning capacitance CV2 increases, the upper limit of the overall capacitance value is limited by Cs, so the curve of fo on the low frequency side is corrected as shown by the broken line fo(Cs). Ru. As a result, the curve of fo after correction becomes S-shaped, intersecting the ideal curve fa' at three points.

This is the principle of the three-point tracking circuit. Figure 7 shows the tracking error curve when the circuit in Figure 5 is set to cover the normal MW band, and is 600KH.
It has tracking points at three points: z, 1000KHz, and 1400KHz.

[Problem that the invention seeks to solve]

By the way, according to the characteristics shown in Figure 7, the frequency range is 600KHz to 1400K.
A tracking error of about 4 KHz at maximum occurs between Hz (around 750 KH2). This value is not a particular problem when receiving normal AM broadcasts, but in recent AM stereos, such as the Harris system AM stereo, the separation required to obtain stereo depth is 20 as shown in Figure 4.
dB or more is required, and for this purpose the tuning deviation must be ±lKH.
It becomes necessary to suppress the number to about 2. Looking back at FIG. 7 from this perspective, it can be seen that in most of the receiving frequency ranges, the tuning is not within ±IKHz. The present invention attempts to further reduce tracking errors by making the above-mentioned three-point tracking circuit active.

[Means for solving problems]

The present invention uses a first variable capacitor as a tuning element of a high frequency circuit that determines a reception frequency, and a second variable capacitor as a tuning element of an oscillation circuit that determines a local oscillation frequency.
In a three-point tracking circuit of a receiving device that changes the values of these first and second variable capacitors using a common control voltage, the second variable capacitance is a first fixed capacitor connected in parallel to the capacitor; a second fixed capacitor connected in series to the second variable capacitor to suppress changes in the oscillation frequency in a low range;
and a third variable capacitor connected in parallel to the first or second fixed capacitor, and the value of the third variable capacitor is changed by the control voltage so as to be controlled by the first or second fixed capacitor. This is characterized in that the frequency change suppression function is limited.

[Effect]

Since the capacitances Cs and Cp of the conventional three-point tracking circuit are fixed values, it is only necessary to bring fo close to the ideal curve fo', but if the overcompensation portion beyond that is on the high frequency side (due to Cp) It appears on the low frequency side (due to Cs) (see Figure 6). Since this has the opposite effect, in the present invention, a third variable capacitor CV3 is connected in parallel with Cs or Cp, and this CV3 is varied with the same control voltage as Cvi and cv2 for tuning, so that the compensation effect of Cs in the low range is Alternatively, the compensation effect of Cp in high frequencies is limited. As a result, frequency characteristics close to the ideal curve fa' can be obtained, and trunking errors can be significantly reduced. This will be explained in detail below with reference to illustrated embodiments.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and the same parts as in FIG. 5 are given the same reference numerals. In this example, parallel capacitance C
A third variable capacitor CV3 is connected in parallel with Cp in order to suppress overcompensation in the high range due to p. Ca is a fixed capacitance for coupling for this purpose. In addition to this, if we consider the additional capacitance Cp' consisting of Ca, C70, and Cb, which is the fixed capacitance cb connected in parallel to CV3, as the variable part of Cp, then Ca
By setting the value of Cp to a certain small value, the effect of correcting Cp by CV3 appears. In other words, the control voltage is low and C
While the value of V3 is large, the additional capacitance Cp' seen from the Cp side
(, is limited by a, but as the control voltage increases and the value of cv3 decreases, the additional capacitance Cp' seen from the Cp side decreases. Therefore, as the frequency increases, the value of Cp + Cp' decreases, f o (fp
) decreases (approaches f o'). however,
If the reduction in Cp' due to CV3 becomes excessive, the effect of Cp will be reduced, so this is limited by a fixed capacitor cb connected in parallel with CV3.

FIG. 2 shows another embodiment of the present invention for correcting the series capacitance Cs, in which the additional capacitance Cs' due to the third variable capacitor ff1cv3 is
are connected in parallel as a variable part of s.

In this case, the additional capacitance C3' is controlled to increase in the low range. Therefore, according to this example, f in FIG.

(Cs) approaches fo'.

Correcting Cs in this way or correcting Cp as shown in FIG. 1 are equivalent. This is because Cp in Figure 1 is Cv
This is because if the high-frequency fo (Cp) can be brought close to the ideal curve fo' by correcting with 3, then the low-frequency [0 (Cs)] can be brought close to fo' by changing the constant of Cs. The same can be said about Figure 2.

FIG. 3 is an example of calculating the tracking error for FIG. 1, and shows that the trunking error of 600KH2-1400KHz falls within ±IKHz and can sufficiently satisfy the AM stereo separation (20dB). This characteristic is a quartic curve unlike the cubic curve in FIG. 7, but the tracking point is 600 KHz as in FIG.

It is calculated using three points: 1000KH2 and 1400KH2.

Below is the tracking point fRF&=600KHz
, f = 1000KHz, fnvc”14
RFI) Constants Cs and Cp when three points are set at 00KHz.

The calculation formula for Lo is shown below. First, let's talk about the conventional circuit shown in FIG. In the high frequency circuit RF, the reception frequency fRF and the value of the variable capacitance NCv+ have the following relationship.

RFa RFb RFc multiplied by f , f , f obtained from this formula
Let the values of VI be Cva, Cvb, Cvc, and f
.

= fRF + fiF Assuming that the variable capacitor CV2 on the OSC side of the circuit that oscillates at a frequency fo has the same capacitance change as CVl, the oscillation frequencies f oa, f ob corresponding to fRFc are f ,'+tFb'Fa
, f oc can be expressed as shown below.

However, Cot(i=a, b, c) is. Solving the above simultaneous equations (2) yields x-Cy. The curve of fa obtained by applying the constants Cs+Cp, Lo of equations (4) to (6) thus obtained to the circuit osC in FIG. By determining how far the difference f1F from the fRF curve at that time deviates from 450 KHz, the tracking error frequency fE can be obtained.

fB =f o-f -450KHz ・=”
(7) RF The curve in Figure 7 was drawn in this way.

On the other hand, in the circuit of the present invention, in FIG. 1(b), C
V2. CV3. Ca, Cb (7) part as one variable capacitor 1
Consider it as tcv'.

However, in the above equation, it is assumed that CV2=CV3""CV. And this Cv' is 'RFa, fRFb
.

f-ya. Find out what value it will be. In other words,
As mentioned above, Cv alone is Cva, Cvb, Cv
c, but according to equation (8), CV' is Cva', Cv
b′.

Take each value of Cvc'. Therefore, (41-(Cva of formula 61).

Cva-, Cvb', Cv instead of Cvb, Cvc
By substituting c', fRFa in Figure 1 (bl)
= 600KH2, fapb = 1000KHz
, fR,c = 1400 KHz, and the values of Cs, Cp, and Lo are obtained using the three points as tracking points. Applying this value to the peak in Figure 1) and changing CV', we get (7)
The curve of the tracking error frequency fE obtained from the equation is shown in FIG.

On the other hand, in the circuit shown in Fig. 2, the value of variable capacitance cv3 is
In the previous explanation, it is assumed to be Cs').

This is a value obtained by appropriately correcting the value 'Cv of Cv+, Cv2, or has a different type of characteristic. In this case (
The simultaneous equations in equation (2) can be solved by setting Cs in equation (3) to (Cs + Cy"). The error frequency fE obtained as a result shows the same tendency as in Fig. 3, As expected, tracking error is small.

〔Effect of the invention〕

As described above, according to the present invention, three-point tracking is provided in which a local oscillator of a receiving device that uses a variable capacitor as a tuning element is provided with parallel and series fixed capacitors that suppress the frequency change rate in high and low frequencies. A variable capacitor for tracking is added to the circuit, and this is varied with the same control voltage as the variable capacitor for tuning to prevent overcompensation due to the fixed capacitor, so tracking errors can be reduced over a wide band. . Therefore, there are advantages such as the separation characteristics in AM stereo being stable over the entire band and the difference in sensitivity depending on the reception frequency being reduced.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, Fig. 2 is a circuit diagram showing another embodiment of the invention, Fig. 3 is a characteristic diagram of the tracking circuit of Fig. 1, and Fig. 4 is an AM Characteristic diagram showing the relationship between stereo separation and detuning frequency, Figure 5 is a circuit diagram showing an example of a conventional three-point trunking circuit, Figure 6 is an explanatory diagram of three-point tracking, and Figures 7 and 5 are diagrams showing the relationship between stereo separation and detuning frequency. FIG. 3 is a characteristic diagram of the tracking circuit shown in FIG. In the figure, RF is a high frequency circuit, OSC is a local oscillation circuit, and TR
K is a tracking circuit, CVI""CV3 is a variable capacitance,
Cs is a series capacitance, and Cp is a parallel capacitance. Applicant Fujitsu Ten Ltd. Representative Patent Attorney Minoru Aoyagi
lI. Raiko 1-san *r'

Claims (1)

[Claims] The first variable capacitor is used as a tuning element of a high frequency circuit that determines the reception frequency, and the second variable capacitor is used as a tuning element of an oscillation circuit that determines a local oscillation frequency.
In a three-point tracking circuit of a receiving device that changes the value of a second variable capacitor with a common control voltage, the second variable capacitor is
a first fixed capacitor connected in parallel to the variable capacitor; a second fixed capacitor connected in series to the second variable capacitor to suppress changes in the low range of the oscillation frequency; a third variable capacitor connected in parallel to the first or second fixed capacitor, and the value of the third variable capacitor is changed by the control voltage to increase the frequency of the first or second fixed capacitor. A three-point tracking circuit characterized in that it limits a change suppression function.