DE3048099C2 - Active RC filter - Google Patents

Description

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The invention relates to an active RC filter according to the preamble of claim 1.

Such active filters are particularly suitable for filtering out audio frequencies and are therefore .'ti Remote control receivers and especially in ripple control receivers that send their control signals in the form of Received audio-frequency pulses via the electrical supply network, applied.

In FR-PS 22 79 258 an active RC filter of the type mentioned above is described, which has several stages having. At least one of these circuit stages contains a capacitor with a pronounced Temperature drift to be compensated. There are two types of thermal compensation in this one Patent specification described. A first type, which does not form the subject of the invention of this patent specification is that the temperature response of capacitors in one or more integrator stages through compensates for the equally large but opposite temperature curve of the input resistances of these stages will. The invention according to this PS aims at a different type of compensation of the Temperature response of the capacitors in that the capacitors in each integrator stage and a individual resistance have an equally large and equal temperature response, this resistance angeord in an inverter stage connected to the feedback branch of an integrator stage

On page 3, lines 14 to 20 of this FR-PS 22 79 258 it is described that these compensation resistors of the first type generally consist of nickel-chromium alloy, not in any required form Sizes are available and must be individually tailored to the critical values.

The invention is based on the object of creating a filter of the type mentioned at the outset, which can be easily used in of the execution and an improved compensation of the temperature change by means of easily removable Has resistors made of inexpensive material

The task is in the case of an object according to the preamble of claim 1 by its Features solved in the identifying part

The filter according to the invention has the advantage that all the properties of the filter in a simple manner from Temperature response can be made independent and that common and for compensation Any material that is available anywhere can be used, which is also inexpensive and easy to use required dimensioning can be provided.

The invention is explained in more detail with reference to a drawing

The only figure in the drawing shows two active band filters connected in series.

A single one of the preferably identically constructed and identically dimensioned band filters 1 and 2 second Order, i.e. with less negative feedback, is based on this principle, including the basic formulas Calculation, however, without information about its behavior with changing temperatures, to Buch von U. Tietze, Ch. Schenk "Semiconductor circuit technology", Springer-Verlag (1975), pages 276 and 277, described.

In each filter 1 and 2 the active part consists of an operational amplifier 3 with an inverting (-) and a non-inverting (+) input. A voltage divider consisting of the two resistors Ri and Rj is arranged between the respective input terminals 4 and 5. The resistor Rj is made up of the series connection of zve ^: partial resistors 5 and 6. The partial resistor 5 is a fixed resistor and the part against was 6, a preferably bifilar winding made of copper wire. The ratio of the resistances Ri to Rj is large, as is known from the literature mentioned above.

A common point 7 of the two resistors Ri and Rj is coupled on the one hand to the output via a capacitor C \ and on the other hand to the inverting (-) input of the operational amplifier 3 _{via a second capacitor C 2.} Another resistor R ₂ connects the inverting (-) input to the output of the operational amplifier 3. The two capacitors G and C ₂ preferably have the same capacitance.

The capacitors G and C ₂ , as frequency-determining elements, may only have a slight age-related change in capacitance. This is the property of capacitors with a polystyrene dielectric. This material also has the advantageous properties of having a low dielectric constant and very low dielectric losses. It has a negative temperature coefficient of about 120 ± 50 x 10- ⁶ per K. This temperature-dependent change of the capacitance of the capacitors C and G ₂ must be countered by suitable thermal compensation.

This is done in that the partial resistance 6 of the

The voltage divider on the input side is formed from the resistors Ri and R ₃ as an induction-free winding made of copper wire. It is well known that copper has a fairly high positive temperature coefficient of approx. +353 · ΙΟ ^-3 . Since the resistance Rj is relatively small and is also divided into the two partial resistances 5 and 6, a very finely adjustable compensation of the significantly lower negative temperature response of the polystyrene dielectric of the capacitors G and C _{2 can be achieved with the help of the resistivity of the copper of the partial resistance 6} can be achieved.

In addition, by encapsulating the partial resistor 6 in plastic, a high aging resistance of the thermal compensation between the dielectric and the copper can be achieved. This effect is supported by the fact that for the resistors Ri and R ₂ a resistance material with a low temperature response and low aging is used.

With an active band filter 1, 2, a bandwidth of ± 5 Hz with a center frequency of 175 Hz can be achieved. If a filter with a higher slope and a slightly lower bandwidth If desired, two band filters 1 and 2 can be connected in series, which are located in the western part of the gkiich are dimensioned and do not differ in structure. This is how the production takes place the filter is very easy and inexpensive.

The quality can also be compensated in the same way as the frequency of the filter. A little Any remaining temperature drift can be accepted, since the one following a filter Threshold circuit can easily be dimensioned so that it is at a lower level of the resonance frequency curve appeals to

The filter according to the invention can replace an electromechanical filter and on the other hand sufficient be made broad that it is also with fluctuating network frequency and with it synchronized transmission frequency can be operated safely. If necessary, the series-connected filter stages 1 and 2 can be slightly detuned (sta, ger tuned) with respect to one another in the known manner in terms of the response frequency.

1 sheet of drawings

Claims (4)

Patent claims: 1. Temperature-compensated active RC filter with multiple negative feedback and bandpass characteristic, with a resistor-voltage divider arranged between two input terminals, the center tap of which via a capacitor (Q) to the output of the operational amplifier (3) and via a second capacitor (C2) to the inverting input of the operational amplifier is connected, in which the inverting input and the output of the operational amplifier are connected via a resistor, the non-inverting input and the center tap of the voltage divider via the series connection of two partial resistors to ground, one of which is a partial resistor for setting Filter sizes is used, and the temperature response of capacitors is compensated by means of resistors with opposite temperature elements, characterized in that the partial resistor (6) used for setting compensates for the negative temperature responses of both capacitors (Q, C2) , wire-shaped a is formed and made of copper. Z RC filter according to Claim 1, characterized in that the partial resistance (6) consists of a bifilar winding consists 3. /? C filter according to claim 2, characterized in that that the winding is embedded in plastic. 4. RC filur according to one of the preceding claims, characterized in that the capacitors (Q. C2) have a polystyrene dielectric.