DE730124C - Crystal bridge filter with adjustable bandwidth - Google Patents

Description

The invention is based on the object of reducing the bandwidth of a so-called crystal bridge filter to make it gradually or continuously adjustable. The previously known adjustable crystal filters (quartz filters) generally exist from oscillating circles or resistances located between two tubes, which are connected by a crystal are coupled. The bandwidth regulation takes place here in that the crystal is attenuated to different degrees.

In contrast, the invention is based on crystal bridge filters. Such filters are not coupling filters, but filters in the sense of the four-pole theory and are characterized by a particularly large slope. They are terminated by terminating resistors or matched to the resistance of the receiver and the internal resistance of the transmitter and contain at least two oscillating crystals, but in practical use rarely more than eight oscillating crystals in the four branches of the bridge. In general, a filter contains four oscillating crystals, which are distributed over the four branches of the bridge and opposite to one another are equal. - If one considers such a filter with a simple design with four crystals, the series resonance frequency of the crystals in the adjoining branches must be shifted so far that the parallel resonance frequency of one crystal (with the lower series resonance frequency) matches the series resonance frequency of the other crystal (with the higher Series resonance frequency). This is shown in Fig. 1, in which the alternating current resistances W as a function of the frequency / for the

both crystals O ₁ and O _{2 are} applied and the last-mentioned common frequency is denoted by f _m. Of the two other resonances, the series resonance of the crystal O ₁ (with the lower series resonance frequency) represents the lower limit frequency Z _{1 of} the pass band and the parallel _{resonance of the other crystal O 2} (with the higher series resonance frequency) represents the upper limit frequency f * Dimensioning are referred to below as narrowband bridge filters.

Broadband bridge filters are obtained in a known manner by connecting inductances upstream or parallel to the oars. You can z. B. shift the series resonance of a quartz by connecting an inductor. One then speaks of a pulling of the quartz frequency. The new alternating current resistance curve f (apparent resistance curve) is obtained according to Fig. 2 by adding the original quartz curve and the inductance curve I. The series resonance S ₁ is thereby shifted to vS " ₂ , while the parallel resonance P is retained. At a higher frequency, it also occurs A series resonance part S _a . In a broadband back filter with four oars, two pairs of oars are used, which are dimensioned so that the two curves of the drawn crystals as shown in Fig. 3 coincide.

The invention is not limited to this bridge filter alone, but also to the equivalent circuits of the differential bridge filter, in which two branches through a differential transformer are replaced, and the bridged T-link can be used.

The invention consists in that the series resonance frequencies are used to regulate the bandwidth of the crystals by additional inductors connected in series with the crystals or capacities can be shifted.

R_

and that at the same time the capacities and the Termination resistors are variable in such a way that they are within the pass band • lying series resonance frequencies with the 'parallel resonance frequencies of the other Crystals remain collapsed during the regulation (Fig. 1 and 3). and that in the case of a broadband bridge filter with in series or parallel to the crystals placed coils these can also be changed at the same time so that the product from the Inductance of these coils and the associated parallel capacitance of the crystal in the Regulation is retained.

FIGS. 4 and 5 show exemplary embodiments for a controllable narrow-band filter and shown in Fig. 6 and 7 for a controllable broadband filter.

To explain this method according to the invention for bandwidth control, the formulas for the narrow-band bridge filter consisting of four oars, as shown in FIG. 4, are now considered, the inductance L being omitted in the initially non-controllable state. A ₁ and L ₂ denote the inductances of the crystals Oi and O ₂ in Fig. 4, C ₁ and C ₂ the series capacitances of the same, C _lp and C _{2 /)} the parallel capacitances, while R in Fig. 4 is the same on both sides Terminating resistor is.

The following simplifications are made to make the formulas more clear introduced:

C ₁ = C ₂ C ₁₁ = C ₁₁₁ L ₁ : L ₃ = fr. f _s ,

where j \ and / ₂ mean the cut-off frequencies of the pass band. For the data of the bridge branches one obtains the formulas _{in this simplified form from the given frequencies Z 1} and f.2 and the terminating resistor R:

105 / 2- / 1

f *

C, = C ₁ , ^Ci "~

- Γ

- L]

TZRf,

The bandwidth control according to the invention results on the basis of these equations as follows. If L _{1 is} constant, as is the case with an oscillating crystal _{, a change in the bandwidth Z 2 -Z 1} initially requires, according to the first of these equations, a corresponding change from R. Da f _t · Z ₂ for the bandwidth change possible with a narrowband filter is constant, C ₁ and C ₂ do not change here. Cj ₇ , and C ₂ ,, must be changed inversely proportional to R and thus also inversely proportional to the bandwidth, the small change in Z _{2 being} negligible. L ₂ must also remain constant. The influence of the change from / ₂ - Z ₁ on L ₂ must therefore be eliminated. This is done in a simple manner in that to increase the bandwidth in series with the quartz O ₁ with the lower natural resonance Z ₁ an inductance L in Fig. 4 and for reducing the same in series with the crystal capacitance C

is placed in Fig. 5. The inductance L is directly proportional to the change in bandwidth d (f ₂ - f _± ) , while the capacitance C is the same

- "- - / 2 A

The terminating resistors R are therefore to be controlled in direct proportion to the bandwidth, while an inversely proportional control is to be carried out for the parallel capacitances. In addition, the series resonance frequency of the quartz with the lower series resonance frequency must be adjusted in the manner mentioned.

It is advisable to calculate the crystal for the widest desired position and then to set the series resonance frequency Z ₁ using a capacitor (as in Fig. 5 1. «Ο All variables to be controlled can be attached to one axis. Regulation in stages is simpler Since the sizes can be compared more precisely, if the simplifications in the filter data are dropped, there are no restrictions whatsoever in terms of controllability.

The invention can also be viewed a little differently:

When applying the invention is in the narrow band filters, for. B. to broaden the bandwidth, the series resonance frequency Z ₁ (Fig. I) of the crystals Q ₁ (Fig. 4)

(AA) ³

is inversely proportional. The following conditions therefore apply to bandwidth control:

/1

HC

(A-A) -

by connecting the adjustable inductance L in the direction of the lower frequency (in Fig. 1 7 « to the left). (The inductances L can also be within the bridging by the parallel capacitances C ₁₁₁ when their values change, which also applies to the other figures.) The series resonance (f _m in Fig. 1) of the crystals 0 « , however, remains unchanged. The parallel resonance frequency of the quartz Q ₁ , which has shifted as a result of the change in the terminating resistances R required at the same time to keep L ₁ constant, must now be brought back into agreement with the series resonance frequency of the quartz Q ₂ , which is done by regulating the parallel _capacitances C lp. Furthermore, the parallel resonance frequency f _{2 of} the crystal Q ₂ must be shifted so far in the direction of higher frequencies that the frequency f _{m is} again in the middle of the filter, which is done _{by changing the parallel capacitances C 2p.}

A corresponding calculation leads to the following conditions for broadband filters, for Fig. 6:

(AA) ²

V ₁ C)

For Fig. 7, in which the additional inductances J ₁ and J ₂ required for a broadband bridge filter, as is known, lie parallel to the quartz crystals, one obtains

" ^R ~ (f * - fi)

and otherwise the same values as for Fig. 6.

When applying the invention to the broadband bridge filter, one proceeds as follows: First, the series resonance frequency of the Ouarze Q ₁ in Fig. 6, if one wants to regulate to a narrower bandwidth, must be shifted upwards, for which the two capacitances C are provided, which are reduced . At the same time, the series resonance frequency of the two quartz Q ₂ must be shifted downward, for which purpose the inductances L are switched on or inductances that have already been switched on are increased. Furthermore, the parallel capacitances of all four oars must be increased and the four inductances L ₁ and L ₂ already present in the known broadband bridge filters must be shifted so far that the product of a capacitance and associated inductance remains constant. Finally, the terminating resistors R are changed. All of these changes must be chosen to match each other so that the series resonance points lying within the pass band still coincide with the parallel resonance points of the other quartz (P ₁ XmU P ₂ in Fig. 3).

In Fig. 6 and 7 you can save one control element in each branch by adding two Rule elements combined into a single one.

The above statements can be generalized in such a way that the same applies control method according to the invention the bandwidth of bridge filters of any Ouarzzahl and arbitrary circles can be regulated, whereby the Ouarze itself is not

be replaced. The bandwidth control according to the invention here requires a Adjustment of the terminating resistors, adjustment of those in the bridge branches Circles to the relationships created by changing the terminating resistances with simultaneous suitable relocation of the oarz series resonance points.

Claims (2)

Patent claims: i. Crystal bridge filters with terminators, including a bridged one T-filter and a differential bridge filter, characterized in that that the series resonance frequencies of the crystals to regulate the bandwidth by additional inductances or capacitances connected in series with the crystals are displaceable and that at the same time the capacitances and the terminating resistances lying parallel to the crystals can be changed in such a way that the series resonance frequencies lying within the pass band coincide with the parallel resonance frequencies of the other crystal remain coincident during the regulation (Figs. 4 and 5), and that in the case of a broadband bridge filter with in series or parallel to the Coils placed on crystals (Fig. 6 and 7) can also be changed at the same time are that the product of the inductance of these coils and the corresponding parallel capacitance of the crystal the scheme is retained. 2. crystal bridge filter according to claim 1, characterized in that the bridge branches contain any number of crystals with otherwise any suitably selected apparent resistances. 1 sheet of drawings