JPH029725B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a crystal bandpass filter that is improved in terms of nonlinear characteristics of loss impedance of a crystal resonator.

Such crystal bandpass filters are required in communications engineering equipment, for example in the intermediate frequency stage of a heterodyne receiver.

Record of the lecture "Proceedings of the Annual Symposium on Frequency Control" (held June 6-8, 1972) pp. 180-186 ("Intermodulation in Frequency Control")
It is known from "Crystal Filters" by Stan Malinowski and Craig Smith of Motorola Corporation to consider a crystal filter as a nonlinear device and take into account the intermodulation products that occur.

According to that record, in order to obtain a crystal resonator with sufficiently small nonlinear characteristics when used under critical conditions, the crystal resonator was manufactured with special care at a high processing cost, and in relatively large quantities at a high material cost. It is recommended to select the above-mentioned crystal resonator from among the crystal resonators of .

From U.S. Pat. No. 2,266,658, a crystal bridge circuit with one crystal resonator in each bridge branch and a two-terminal network with two crystal resonators used as parallel branches,
Cascaded circuits comprising a plurality of matching circuits are known.

As shown in FIG. 2 of this U.S. patent specification,
The attenuation curve of a crystal bridge circuit configured as a bandpass filter has the following implications due to the two attenuation poles that occur in the upper and lower cutoff regions. i.e. the second edition of the publication "Siebschaertungen Mituto Schüwingkristären";
Of the three methods for selecting circuit constants for bridge branch reactances shown in W. Hertzoak, page 92, the first method for selecting circuit constants is the one that is treated there using Figure 109a. This is meant to be selected for the crystal bridge circuit shown in FIGS. 1 and 4 of this patent. Regarding this circuit parameter selection method for the bridge branch reactance of a crystal bridge circuit, the above-mentioned publication by W. Hertz-Oak, page 92, figure 109a, page 83, figure 100, and
It is characterized as follows according to Figure 1. That is, one of the two branch reactances has a zero point generated by the crystal resonator of this branch reactance almost at the center of the filter passage area, and as a result, the level within the filter passage area is changed based on the nonlinear characteristics that occur more or less. It is characterized in that fundamentally strong damping can occur depending on the Moreover, the excessively large non-linear characteristics of the crystal resonator provided in the filter circuit are considered to be a serious drawback since they cause unacceptably strong differential sound distortion. Therefore, in the case of the bridge circuit shown in FIGS. 1 and 4 of U.S. Pat. The nonlinear characteristics of crystal resonators are much more detrimental than the following crystal bridge circuits. That is, for example, in the publication "Handbook of Filter Synthesis" by Anatol Tsverev, No. 421.
It works much worse than the crystal bridge circuit with a crystal resonator in one bridge branch and a capacitor in the other bridge branch, which is described in great detail on pages 1 to 438. That is to say, in the case of a total of three possible different circuit constant selections for the course of the filter attenuation curve of this bridge circuit according to the reactance diagram given on page 429 of this publication, the zero point of the branch reactance with a crystal resonator The frequency position of is covered by the frequency position of the upper filter limit frequency. US Patent Specification No.
The two-terminal network used as a parallel branch, connected to the bridge circuit in cascade in the case of the circuit arrangement according to figure 4 of No. 22666458, has two crystal resonators connected in parallel, these crystal resonators The resonator produces a zero point in the reactance diagram of this two-terminal network, and this zero point must be located completely outside the passage area of the entire circuit arrangement, and according to the technical idea of this patent specification: It can be used to make the side edges of the filter steeper. However, this patent specification makes no mention of the problem of nonlinear characteristics of the crystal resonator.

For details, see the already mentioned "Handbook of Filter Synthesis" by Anatol Tsverev, Jon Willy & Sons, New York,
London, Sydney, 1967, pages 428-430, section 8.9 Circuit Analysis of a Simple Filter, and from page 446, figure 8.38 and page 451, figure 8.39, It is known to use bridge circuits with an oscillating crystal on the one hand and a capacitor on the other hand as bridge branches, and which, depending on the selection of the circuit constants of these bridge branches, produce an attenuation pole in the upper cutoff region.

This publication, page 426, figure 8.12 (figure 432
From page 8.17c) and page 448, table 8.2, a bridge circuit for a narrowband bandpass filter with two (identical) crystal resonators is known.
A known bridge circuit of this kind is shown in FIG.

The circuit consists of two crystal resonators Q, two bridge capacitances C1, one input impedance Re, and one output impedance Ra.
(These two impedances are based on the assumption that the output impedance of the power supply is zero and the terminal impedance of the circuit is infinite.) 2
The crystal resonators Q are shown in their equivalent circuits. They are each in a series branch, each with an inductance Lq, a series capacitance Cq,
It has a parallel capacitance Co. Furthermore, there are capacitances Cpa and Cpe resulting from the Bartlett transform at the output and input sides of this bandpass filter.

From the same document, pages 446, 8.38, 451 and 8.39, an equivalent bridge circuit with only one differential transformer and one crystal resonator is known. An example of such a bridge circuit or equivalent circuit having a crystal resonator in one bridge branch and a capacitor in the other bridge branch is shown in FIG. The circuit has only one symmetrical differential transformer U¨1 and one crystal resonator, which has an inductance (=
2Lq), series capacitance (=Cq/2), and parallel capacitance (=Co/2). Bridge capacitance is reduced by half, to C1/2.

In measurement and control technology, high demands are often placed on the cut-off attenuation region above the passband. In order to meet this requirement, it is known from page 449, table 8.3, page 450, table 8.4 of the same document to connect a plurality of bridge circuits in cascade. However, such a connection has the disadvantage of large tolerances as a full-band filter.

In the case of a bandpass filter having this known bridge circuit, the differential transformer used is 1:-
It has a metamorphic ratio of 1. For matching input and output sides, the matching four-terminal network required for high crystal inductances is then constructed as a matching transformer. The transformer is provided with a separate further winding having a winding ratio different from 1:1 (which provides a matching effect).

In this case, the intrinsic capacitance, Q, and stray inductance of the matching transformer should be taken into account. In the case of the circuit in Figure 2, it has only half the bridge capacitance C1/2 as compared to the circuit in Figure 1, so this circuit is different from the existing circuit in any case, even though it uses a matching transformer. It has the disadvantage that it cannot be used when the capacitance exceeds the capacitance C1/2.

Same document, page 487, Figure 8.57, 8.
Further, from FIG. 58, in the case of the bandpass filter of FIG. 2, it is known to connect a plurality of crystal resonators in parallel instead of connecting one crystal resonator.

In the case of known bandpass filters as described above, the series resonant frequency of the crystal resonator lies within the passband of the filter. However, since the loss impedance of a crystal resonator is strongly level-dependent in the region of its series resonant frequency, a particularly significant drawback of this known bandpass filter is that it produces unacceptable differential distortion due to its highly nonlinear characteristics. It is.

The object of the invention is to sufficiently prevent the disadvantageous effects of the non-linear characteristics of the loss impedance of a crystal resonator at a relatively low cost, thereby avoiding the above-mentioned disadvantages;
At the same time, the aim is to meet high demands regarding cut-off damping.

According to the present invention, this problem can be solved by: (a) A cascade connection circuit having multiple 4-terminal matching networks is provided on the input/output side, and each of the multiple 4-terminal matching networks has a series inductance and a parallel capacitance. The cascaded circuit is composed of a crystal bridge circuit and a two-terminal network used as a parallel branch, and this two-terminal network has at least one series resonance point, and the cascaded circuit is composed of a crystal bridge circuit and a two-terminal network used as a parallel branch. (b) The attenuation pole of the entire device configuration is divided by a crystal bridge circuit and a two-terminal network, and the two-terminal network has at least one crystal resonator. It has one differential transformer, one oscillating crystal and one capacitor as a bridge branch, and the crystal bridge circuit is constructed in such a way that one attenuation pole occurs in the upper cut-off region, in which case , the loss impedance of the oscillating crystal provided for the crystal bridge circuit has only a slightly non-linear characteristic, which is resolved in that the series resonance point of the two-terminal network is likewise located in the upper cut-off region.

Next, the present invention will be explained in detail using the embodiment shown in FIG.

The embodiment of the bandpass filter of the present invention shown in FIG. 3 is composed of a Cowher type bridge circuit and a cascade connection circuit with a two-terminal network, which is used as a parallel branch and is constituted by a crystal resonator Q2. has been
A 4-terminal matching network A1 configured as a parametric low-pass filter is connected in advance to the input side of this circuit, and a 4-terminal matching network A1 configured as a parametric low-pass filter is connected to the output side of this circuit. Network A2 is downstream connected. The bridge circuit in the Kaur type has a metamorphic ratio of 1: (-u¨)
Asymmetric differential transformer U¨2
And, illustrated by the equivalent circuit Lq ₁ , Cq ₁ , Co ₁ ,
It is composed of a crystal resonator Q1 forming a bridge branch and a capacitor Cb forming another bridge branch. The crystal resonator Q2 used as a parallel branch, connected to the bridge circuit by a cascade connection, is indicated by the equivalent circuit Lq ₂ , Cq ₂ , Co ₂ . At this time, it is assumed that the value of Co ₂ is the holding capacitance of the crystal resonator Q2.

The output side matching 4-terminal network A2 configured as a parametric low-pass filter sets the terminal impedance Ra of the cascaded circuit of the bridge circuit and the 2-terminal network in the form of a crystal resonator Q2 to a desired value in the pass region of the bandpass filter. Convert to value Ra′. For information on such parametric low-pass filters, see p.
Allemandou's article ``Filter symmetlique patu seva u patu seau, des desgres 5, incere ontre du résistance.
1963, Vol. 17, No. 1, pp. 56-69 (1963)
A basic explanation is given in 2013. The output-side matching four-terminal network A2 in the form of a parametric low-pass filter is composed of a capacitance Ca ₁ , a calculated input-side capacitance Ca ₂ (indicated by a broken line in the figure), and an inductance La ₁ . The capacitance value of the actual input side capacitor K (shown as a solid line in the figure) of the output side matching 4-terminal network A2 is determined by the formula a + b + c - d consisting of each component described below.
, where a is the output side matching 4-terminal network A
The above calculated input side capacitance Ca2 of 2,
b is the calculated circuit capacitance Cpa caused by the Bartlett transformation (see Figures 1 and 2), and c is the crystal bridge circuit U 2, Q 1, as a result of introducing the asymmetric differential transformer U 2,
The calculated capacitance C1(u-1)/2u, d that occurs in parallel on the output side of Cb is the calculated capacitance corresponding to the capacitance of the capacitive reactance component of the crystal resonator Q2 at the band center frequency of the bandpass filter. . A parametric matching low-pass filter A1 is connected in cascade to the bridge circuit on the input side, and this filter adjusts the input resistance Re of the cascade circuit between the bridge circuit and a two-terminal network in the form of a crystal resonator Q2 to a desired value. It is converted into a resistance value Re', and is composed of capacitance Ce ₁ , Ce ₂ and inductance Le ₁ . The capacitance _Ce2 is determined from the sum of the calculated capacitance of the low-pass filter, the circuit capacitance Cpe (FIGS. 1 and 2), and the capacitance C1.(1-u)/2). This capacitance C1.(1-u/2) needs to be connected as a result of introducing an asymmetric differential transformer U2 in parallel on the input side of the bridge circuit.

The bandpass filter of the present invention illustrated in FIG.
By using two crystal resonators for the bandpass filter shown, it has a significantly higher cut-off damping above the pass region. However, in this case, only one of the two crystal resonators (Q1) needs to be selected so that the loss impedance is as independent as possible of the level.

The transformer U¨2 has a turns ratio of three windings of 1:m:
Similar to the known bridge circuit differential transformer in the asymmetric form of n, it has a transformation ratio 1:(-u¨), so that for the bridge capacitance Cb the value C1/2u is can get. In this case, C1/ is the capacitance obtained in the case of a commonly used conventional bridge circuit with a transformation ratio of 1:(-1), as shown in Figure 2, and is often very small in order to be practically configurable. Selected. Since u¨<1 for the asymmetrical transformer U¨2, the bridge capacitance Cb has a realizable capacitance value. The capacitance Co ₁ is determined by Bartlett transformation from the cascaded circuit and is always greater than C 1/2.

The level-dependent nonlinear characteristic occurring at the series resonant frequency of crystal resonator Q2 is the upper cutoff frequency (cutoff region) of the bridge circuit with respect to its frequency position.
, and therefore has virtually no influence on the distortion characteristics of the filter or the entire device. The equivalent circuit diagram of crystal resonator Q2 is inductance Lq ₂ ,
Series capacitance Cq ₂ , parallel capacitance
Cq ₂ , consisting of parallel capacitance Co ₂ . Since the matching networks A1 and A2 are configured as low-pass filters, they can be configured as bandpass filters or
In addition to obtaining the desired terminal impedance in the passage region of the cascade connection circuit with the two-terminal network constituted by the crystal resonator Q2, which is used as a parallel branch with the bridge circuit in the Cower type, the desired terminal impedance can be obtained in the cut-off region of the filter. You can also obtain a similar attenuation.

[Brief explanation of drawings]

1 and 2 are equivalent circuit diagrams of a known crystal filter for explaining the present invention, and FIG. 3 is an equivalent circuit diagram of a crystal filter according to the present invention. Q, Q1, Q2...Crystal resonator, U¨1, U¨2...
…Differential transformer.

Claims (1)

[Claims] 1 (a) A plurality of four-terminal matching networks A1 on the input/output side;
A cascade connection circuit comprising A2 is provided, and each of the plurality of four-terminal matching networks A1 and A2 has a series inductance Le1; La1 and a parallel capacitance Ce1, Ce2; Ca1, Ca2, and the cascade connection The circuit is a crystal bridge circuit.
U¨2, Q1, Cb and 2 used as parallel branches
The two-terminal network Q2 has at least one series resonance point. Q1,
Cb and the two-terminal network Q2, the two-terminal network having at least one crystal resonator Q2, (b) the crystal bridge circuit U¨2, Q1, Cb has one differential transformer U¨2, one oscillating crystal Q1, and one capacitor Cb as a bridge branch, and the crystal bridge circuit U¨2, Q1, Cb has one attenuation pole. The loss impedance of the oscillating crystal Q1 provided for the crystal bridge circuit U¨2, Q1, Cb has only a slight non-linear characteristic. A crystal bandpass filter characterized in that the series resonance point of the two-terminal network Q2 is also within the upper cutoff region. 2. The crystal bandpass filter according to claim 1, wherein the transformer U¨2 of the crystal bridge circuit has a transformation ratio U¨ smaller than 1. 3. The crystal bandpass filter according to claim 1, wherein the plurality of matching circuits A1 and A2 are configured as parametric low-pass filters.