GB2071453A

GB2071453A - A Crystal Band-pass Filter

Info

Publication number: GB2071453A
Application number: GB8100821A
Authority: GB
Original assignee: Wandel and Golterman GmbH and Co
Current assignee: Wandel and Golterman GmbH and Co
Priority date: 1980-02-22
Filing date: 1981-01-12
Publication date: 1981-09-16
Also published as: DE3006692A1; JPH029725B2; GB2071453B; DE3006692C2; JPS56158522A

Abstract

A band-pass filter comprises a bridge circuit containing a differential transformer (U2) and a crystal resonator (Q1), the attenuation curve of the circuit having a band-pass character with an attenuation peak above the pass band, at least one dipole (Q2) being connected in parallel with the output of the bridge circuit, which bridge circuit has a matching quadripole (A1, A2) at the input and output, the dipole (Q2) is a series oscillating circuit having a parallel capacitance (Co2), the resonance frequency of the series oscillating circuit above the filter pass-band being in the stop band thereof, and a capacitance (Ca1) in parallel with the output of the bridge circuit and if required a capacitance (Ca2) in parallel at the input of the matching quadripole (A2) on the output side are reduced by an amount equal to the capacitive reactance of the dipole (Q2) at the centre frequency of the filter pass band. <IMAGE>

Description

SPECIFICATION A Crystal Band-pass Filter The invention relates to a band-pass filter comprising a bridge circuit containing a differential transformer and an oscillator, crystal, the attenuation curve of the circuit having a bandpass character with an attenuation peak above the pass band, at least one dipole being connected in parallel with the input of the circuit, which comprises a matching quadripole at the input and at the output.

Band-pass filters of the aforementioned kind are needed in telecommunications equipment, e.g. in the intermediate frequency stages of superhet receivers.

"Handbook of Filter Synthesis" by Anatol Zvever, John Wiley s Sons, New York, London, Sydney 1967, page 432, Fig. 8.1 7c discloses bridge circuits for narrow-band band-pass filters with two similar crystal resonators. A known bridge circuit of this kind is shown in Fig. 1. It comprises two crystal resonators Q, two bridge capacitors Clan input resistor Re and an output resistor Ra. (These resistors are included under the assumption of a negligible source impedance and an infinite terminating impedance in the circuit.) The two crystal resonators Q are represented by their equivalent circuits. They are in the long branches and each contain an inductance Lq, a series capacitor Cq and a parallel capacitor Co.Residual capacitors Cpa and Cpe respectively also occur at the input and output of the band-pass filter in accordance with the Bartlet transformation.

The same literature source, page 446, Fig.

8.38 and page 451, Fig. 8.39 discloses equivalent bridge circuits comprising a differential transformer and only one crystal resonator. An example of such an equivalent circuit is shown in Fig. 2. It comprises a symmetrical differential transformer Ul and only one crystal resonator which, as a result of the conversion of the circuit in Fig. 1, has different values for inductance (=2 Lq), series capacitance Cq 2 and parallel capacitance Co (= ).

2 The bridge capacitance is halved, i.e.

Cl 2 In measurement and control engineering, high requirements are frequently made on the stopband attenuation above the pass band. To meet these requirements it is known from the same literature source, page 449, Table 8.3 and page 450, Table 8.4, to contact a number of bridge circuits in series. This results in a disadvantage in that the complete band-pass filter is highly sensitive to tolerance.

The differential transformers used in these known band-pass filters have a transformation ratio of 1:1. The matching quadripoles needed at the input and output, more particularly because of the high crystal inductance, are in the form of matching transformers. Allowance must be made for the intrinsic capacitance, quality and leakage inductance of the matching transformers. Since the bridge capacitance in the circuit in Fig. 2 is only CI 2 i.e. half the value in the circuit in Fig. 1, there is a disadvantage in that the circuit is often impossible to construct, i.e. if it already has a capacitance above CI 2 The same literature source, page 483, Figs.

8.57 and 8.58, also discloses connecting a number of crystal resonators in parallel instead of one in band-pass filters according to Fig. 2.

In the known band-pass filters, the series resonance frequencies of the crystal resonators are in the filter pass-band. Since, however, the loss resistances of crystal resonators near their series resonance frequency are closely dependent on level, the known band-pass filters have a particularly serious disadvantage in that they are excessively non-linear, resulting in unacceptably high intermodulation distortion.

In order to obtain crystal resonators having sufficiently small non-linearity in critical applications, it is known from "Proceedings of the 23rd Annual Symposium on 6-8 June 1972, U.S. Army Electronics Command, Fort Monmouth, New Jersey, Frequency Control", page 180, article on "Intermodulation in Crystal Filters" by Stan Malinowski and Craig Smith, Motorola Inc., to produce crystal resonators very carefully in a laborious process and select them from a relatively large number in a manner which uses plenty of material.

The object of the invention is to construct a band-pass filter which largely obviates the aforementioned disadvantages and also meets the high requirements on stop-band attenuation.

The invention solves this problem, in the case of a band-pass filter of the initially-mentioned kind.

Thus, according to the present invention, there is provided a band-pass filter comprising a bridge circuit containing a differential transformer and an oscillator crystal, the attenuation curve of the circuit having a band-pass character with an attenuation peak above the pass band, at least one dipole being connected in parallel with the output of the bridge circuit, which has a matching quadripole at the input and at the output, characterised in that the dipole in parallel with the output of the bridge circuit is a series oscillating circuit having a parallel capacitance, the resonance frequency of the series oscillating circuit above the filter pass band is in the stop band thereof, and a capacitance in parallel with the output of the bridge circuit and, if required, a capacitance in parallel at the input of the matching quadripole on the output side are reduced by an amount equal to the capacitive reactance of the dipole at the centre frequency of the filter pass band.

The invention will now be described by way of example only with particular reference to Figure 3, which shows a band-pass filter circuit of the invention.

The band-pass filter comprises an asymmetrical differential transformer U2, a crystal resonator Q1, represented by its equivalent circuit Lq1, Cq 1 and Col, and a bridge capacitance Cb.

A crystal resonator Q2, represented by the equivalent circuit Lq2, Cq2 and the mounting capacitance Co2, is in parallel with the output of the bridge circuit. A matching quadripole A2, in the form of a parametric low-pass filter, is disposed at the output side in series and converts the terminating impedance Ra of the bridge circuit to a desired value Ra' in the pass band. It is formed by capacitances Ca 1 and Ca2 and inductance La 1. Capacitance Ca2 is the sum of the calculated capacitance of the low-pass filter A2, the residual capacitance Cpa (Figures 1 and 2) and a capacitance C1.

()-1 2u which must be connected parallel to the bridge circuit output as a result of introducing the asymmetrical transformer U2, minus the capacitive reactance component of the crystal resonator 02 at the centre frequency of the band.

On the input side a parametric matching low-pass filter Al is connected in series with the bridge circuit and converts the input resistance Re of the bridge circuit (Figures 1 and 2) to a desired resistance Re'.

The low-pass filter Al is made up of capacitances Cue1, Ce2, and an inductance Lel.

Capacitance Ce2 is equal to the sum of the calculated capacitance of the low-pass, the residual capacitance Cpe (Figures 1 and 2) and a capacitance C1.

1 -u 2 which, as a result of the asymmetrical construction of the differential transformer U2, must be connected in parallel to the input of the bridge circuit.

Owing to the use of two crystal resonators, the band-pass filter according to the invention shown in Fig. 3 has a much higher stopband attenuation above the pass-band than the band-pass filter in Fig. 2, but only one (01) of the two crystal resonators has to be selected to obtain a loss resistance substantially independent of level.

The transformer U2 has the transmission ratio 1: (-u), giving the value C1 2u for the bridge capacitance Cb. In this expression, C1 is the capacitance in a conventional bridge circuit as shown in Fig. 1. In the case of the asymmetrical transformer, ü < 1, so that the bridge capacitance Cb has a value which can be achieved in practice. The capacitance Col, as a result of Bartlet's transformation from the chain circuit, is always greater than C1 2 Consequently the non-linearity of resonator Q2, depending on level, which occurs at its series resonance frequency, is in the stop band of the filter and has practically no effect on the distortion properties of the band-pass filter. The equivalent circuit diagram of resonator Q2 comprises inductance Lq2, series capacitance Cq2 and parallel capacitance Co2. Since the matching networks Al, A2 are low-pass filters, the terminating impedances in the filter pass band have the desired value, with additional attenuation in the stop band.

Claims

1. A band-pass filter comprising a bridge circuit containing a differential transformer and an oscillator crystal, the attenuation curve of the circuit having a band-pass character with an attenuation peak above the pass band, at least one dipole being connected in parallel with the output of the bridge circuit, which has a matching quadripole at the input and at the output, characterised in that the dipole in parallel with the output of the bridge circuit is a series oscillating circuit having a parallel capacitance, the resonance frequency of the series oscillating circuit above the filter pass band is in the stop band thereof, and a capacitance in parallel with the output of the bridge circuit and, if required, a capacitance in parallel at the input of the matching quadripole on the output side are reduced by an amount equal to the capacitive reactance of the dipole at the centre frequency of the filter pass band.

2. A band-pass filter as claimed in claim 1, wherein the dipole connected in parallel to the output of the bridge circuit is a crystal resonator.

3. A band-pass filter as claimed in claim 1, wherein the dipole comprises a number of crystal resonators connected in parallel to the output of the bridge circuit.

4. A band-pass filter as claimed in any preceding claim, wherein the differential transformer has a transformation ratio smaller than unity.

5. A band-pass filter as claimed in any preceding claim, wherein the matching quadripoles are in the form of parametric lowpass filters.

6. A band-pass filter substantially as hereinbefore described and as shown in Figure 3 of the accompanying drawing.