JPS6049371B2 - low pass filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-pass filter with a simple configuration and improved attenuation characteristics.

A conventional polarized LC low-pass filter has, for example, inductances L1 to L4 and capacitors C1 to C1, as shown in FIG.
C9, and as a series arm, inductance L1
A parallel resonant circuit consisting of ~L4 and capacitors C1 to C4, and a parallel arm consisting of capacitors C5 to C9.

Figure 2 shows the attenuation characteristics of this low-pass filter, f1 to f4.
is the parallel resonant circuit resonant frequency, f1=1/(2πV
L1・Cl), f2=1/(2πVL2・C2),
f3=1/(2π, /L3・Cs), f4=1/(2π
It is expressed as /L4・C4). Since the transmission characteristics of such a low-pass filter are generally influenced by the Q of the coil, it is desired to realize a coil with a high Q.

In general, the higher the signal frequency of the circuit, the smaller the desired inductance value can be, and the smaller the inductance value of the coil, the lower the Q value. It has not been easy to improve the transmission characteristics of the filter. Conventionally, it has been considered to use a tapped coil to realize a high-Q coil. Figure 3 shows the case where a tapped coil is applied to the inductance L2 in Figure 1. The turns ratio of the coil between the primary side and the secondary side due to the tap is n
Then, L2' = d-L2 (1) c2. = c2/n (2).

Also, the resonant frequency f2' of this parallel resonant circuit is f2'=1
/(2π, /L2'·C2') ...(3).

However, when a coil as described above is used, the leakage inductance becomes relatively large, thereby deteriorating the damping characteristics.

If this leakage inductance is LO, then the equivalent circuit of FIG. 3 is as shown in FIG. 4. In this case, the parallel resonant frequency of the parallel resonant circuit formed by the inductance L2 and the capacitor C2, and the leakage inductance LO
Because of the series-connected configuration, there is a series resonance frequency, etc., so at a certain frequency, as shown in FIG. 5, a drop occurs in which the amount of attenuation becomes smaller in the attenuation range of the attenuation characteristic. In order to compensate for such a drop in attenuation due to leakage inductance, the following measures have been considered in the past.

(1) Predict the drop in the amount of attenuation and design the amount of attenuation to be larger in advance by that amount.

(2) Cascade-connecting door transducers with attenuation characteristics that compensate for the drop in attenuation.

In the case of the method (1), for example, in the attenuation characteristics shown in FIG. 6, even if there is a drop in the attenuation due to leakage inductance as shown by the dotted line, the desired attenuation can be ensured. Although the amount of attenuation is designed in advance to be large as shown in the solid line, it is necessary to accurately predict the amount of drop in attenuation due to leakage inductance, and in frequencies other than those where the amount of attenuation falls, it is necessary to design Since the structure is configured to obtain a sufficient amount of attenuation, it has the drawback of increasing the circuit scale.

In addition, in the case of the method (2) above, for example, in the attenuation characteristics shown in Fig. 7, if the attenuation characteristics of the main offshore wave device are as shown by the solid line a, an attenuation amount that is greater than the amount of drop in attenuation amount can be obtained. A door wave device, for example, a deep wave device with the attenuation characteristic shown by the dotted line b, is connected in cascade to obtain the overall characteristic as shown by the dotted line c, but an auxiliary p wave device is connected to the main wave device. Because they are connected in cascade, they have the disadvantage of being bulky and uneconomical.

The present invention improves the conventional drawbacks as described above, and aims to compensate for the drop in attenuation due to leakage inductance with a simple configuration.

Examples will be described in detail below. FIG. 8 is a circuit diagram of an embodiment of the present invention, which corresponds to the configuration of the conventional example shown in FIG. Since the capacitance values are different, Lbi~L
! 4'' and Cbi-C9''.

Therefore, the resonant frequencies f1 to F4 of the parallel resonant circuit can be expressed as follows in relation to the circuit of FIG. 1J
1- ~'1 ノ
``'River''l) Also, parallel arm capacitor C8'' and inductance L
Since the impedance 4 of the series circuit with FO is J when its internal resistance is R2, by selecting L5, the impedance 4 becomes R2 at the series resonant frequency of the FO.

7 Usually, in a circuit normalized by characteristic impedance, R2 times 1, and R2 times (1/(2π
Since C')), at the frequency FO, it is grounded by the minute resistance only 2, and the amount of attenuation at this frequency point increases compared to before the inductor nonce L5 is inserted.

Therefore, by making the series resonant frequency FO coincide with the frequency at which a drop in attenuation due to the television inductance occurs, it is possible to compensate for the drop in attenuation. Figure 9 shows the attenuation characteristics and impedance characteristics (mismatched attenuation characteristics).If a drop in attenuation occurs at frequency ■ due to leakage inductance as shown by the dotted line A'', then As in the example,
By connecting the inductance L5, the frequency F.

The amount of attenuation becomes large, and the attenuation characteristic shown by solid line A is obtained. Note that the inductance L5 causes a drop in the amount of attenuation in the used band, but this is often a band that does not affect the used band, so there is no problem as a low frequency filter. Also, by connecting the inductance L5, the impedance characteristics in the transmission band will deteriorate, but by changing the constants of each circuit element due to the connection of the inductance L5, the deterioration of the impedance characteristics can be prevented. can do.

For example, if the dotted line B'' in FIG. 9 is the impedance characteristic before connecting the inductance L5, then by connecting the inductance L5 and performing element approximation, the impedance characteristic shown by the solid line B was obtained.Inductance L5
In addition to being connected in series to capacitor C8'', it can also be connected in series to any parallel arm capacitors C5'' to C9''. In that case, select the capacitor with the largest capacitance value. It is preferable to connect the inductance L5 because the value of the inductance can be reduced.

It is also possible to connect inductances to each of the plurality of capacitors in parallel arms. As explained above, the present invention provides a low-frequency p-wave device in which coil taps in a parallel resonant circuit are connected in cascade to obtain a high-Q coil. This method compensates for the drop in the amount of power by connecting one micro-inductance coil in series with the capacitor in the parallel arm.It has a simple and economical configuration and provides a low-frequency wave generator with good transmission characteristics. There is an advantage that it can be done.

[Brief explanation of the drawing]

Is Figure 1 the conventional basic low range? Wave device, Figure 2 is the attenuation characteristic curve diagram of Figure 1, Figure 3 is the conventional low frequency? Figure 4 is the equivalent circuit of Figure 3, Figure 5 is the attenuation characteristic curve diagram of Figure 4,
6 and 7 are explanatory characteristic curve diagrams for compensating for the deterioration of attenuation characteristics due to leakage inductance in the prior art, FIG. It is an attenuation characteristic and impedance characteristic curve diagram of the figure. L1 to L5, Ll'' to L4'' are inductances, JCl
~C9, Cbi~C9'' are capacitors.

Claims (1)

[Claims] 1. In a low-pass filter having a parallel resonant circuit in the series arm and a capacitor in the parallel arm, and in which at least one of the parallel resonant circuits is cascade-connected using the taps of the coil, leakage of the coil having the taps. A low-pass filter characterized in that a coil for compensating for a drop in attenuation due to inductance is connected in series to the capacitor in the parallel arm.