DE3314468A1 - ALLPASS FILTER CIRCUIT - Google Patents

Description

All-pass filter circuit

The invention relates to filters and, more particularly, to an all-pass filter element having superior radio frequency performance. The filter member according to the preferred embodiment is used in particular in data modems, the all-pass filter frequency ranges at 100 KHz.

Known active filters of the type discussed are subject to this

Noise capacitance effects, reducing the high frequency sensibility

scope is limited. Another problem through which limits the performance of such filters is the deviation from ideality that is caused and introduced by non-ideal coils (inductors) is by a real pole, which is generated by operational amplifiers used in such filters. If such known filters can be used as a cascade circuit,

* ^> as required in various applications, the cumulative effect of the limitations of the known circuits can greatly degrade performance.

It is therefore an object of the invention to provide an improved filter circuit to accomplish. It is a further object of the invention to eliminate deficiencies in known filter circuits, which are used in all-pass networks to compensate more precisely. It is another object of the invention to have the effect the interference capacitance to an all-pass filter circuit

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minimize. Another aim of the invention is to to provide an all-pass filter circuit which enables the above-enumerated objects to be achieved using conventional

5 operational amplifier components can be achieved.

These and other objects and advantages are achieved in accordance with the invention by providing a filter circuit is by which an adjustment of a resistance in the feedback path of an operational amplifier for elimination the effects of the non-ideality of coils is made possible and by which at the same time the effect of the 15th

Stray capacitance in the frequency band of interest is greatly reduced.

The preferred embodiment of the summarized in this way The invention illustrated is described in detail in conjunction with the drawing. In the drawing

shows:
25th

1 shows a known all-pass filter element, 2 shows the poles and zeros of the transfer function of an ideal all-pass filter,

3 shows the poles and zeros of a known all-pass filter circuit, which is subject to scattering effects,

"P. Fig. A is a schematic representation of the all-pass filter according to the preferred embodiment of the invention and

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j Fig. 5 shows an example of a resistor combination for

Design of a resistor of a current circuit according to the preferred embodiment

ρ- the invention.

A typical known all-pass filter circuit is shown in Fig. 1 can be seen. As is known in the art, it is the purpose of an all-pass filter element, a mandatory one To generate phase / frequency response with flat amplitude characteristics. For an all-pass term of the second order this is achieved when the poles and zeros of the

output function are the mirror images of each other, as from Fig. 2 can be seen.

In the practical case, inadequacies of the elements with which the circuit is physically implemented becomes, (like inductors with losses, amplifiers with decreasing gain as the frequency increases, Stray capacitances and the like) Secondary singularities introduced and the position of the ideally present Poles shifted with respect to their associated zeros.

Fig. 3 shows the case when the poles and zeros are not

are in their mirror-image positions and an additional pole has also appeared on the real part axis. The current pole zero point ^{pattern 1} 'is of course dependent

of the circuit elements currently in use and the 35

circuit structure currently used; however this is Case illustrative of the problems addressed by the invention.

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An analysis of the known circuit from FIG. 1 leads to The following:

a) · The all-pass absolute value function | H (j6j) j = 1 is no longer fulfilled for all <ύ. The real axis pole drives the amplitude / frequency characteristic down towards zero when the frequency increases.

b) The shift between the poles and zeros also provides different

Values for the quadratic factors in the

Numerator and in the denominator of the JH (JtO) J function at each value of CJ, whereby the amplitude

tudengang is further reduced. AD

c) The phase and delay curves of the known circuit deviate from the ideal case now and then not the value and the we

If the undesired elements are precisely known and compensated for, this is an error component in the desired phase and delay

gerungsgang available. Such an accurate

Knowledge and compensation are not available in the practical case.

Although it follows from life experience that shortcomings of circuit elements such as power loss cannot be eliminated in the case of inductors and stray capacitances

can, according to the invention, a circuit arrangement found, by which the elements are combined with one another in such a way that the disadvantageous

S.

ligen effects reduced to acceptably low levels are.

The preferred embodiment of the invention is shown in Fig. 4 can be seen. It contains an operational amplifier 11, resistors R ,, R "and R-, and an inductor L and one Capacitance C. The resistance R "is in the feedback path of the amplifier 11 between the output 13 and the 15th

inverting input 15 switched on. The resistors R, and R -, are connected to a common connection 19 and with their opposite connection ends to the inverting

2Q the or the non-inverting input 15, 17 of the amplifier 11 connected. The inductor L and the capacitance C are connected in series between the non-inverting input 17 and ground.

The filter circuit is driven by a source V with a series impedance Z connected to the common terminal 19 of the resistors is connected. With the scarf

device according to the preferred embodiment can have the effect of the non-ideal Q-factor of the inductor L can be compensated by adjusting the resistance R ". this

nr can be calculated mathematically by the equation for the transmittance to be shown:

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T (s) =

^(K -

of the inductor, L and K =

When K = J_2 equals 1 + 2 ^R l

Qo Q,

= Q factor

is set, is

the absolute value of the linear expression in the numerator and in the The denominator of T (s) is the same, which results in an all-pass characteristic which can be achieved by adjusting the resistance R "can be completed.

Furthermore, the impedance level at the non-inverting input 17 is very low because R, is of the order of a few hundred ohms. Acceptably good estimates of the value of the stray capacitance, C, are 10 to 30 picofarads (pf). For frequencies in the 50-100 KHz band of interest, L and C have values in the range of 2 millihenry and 1000 pf. It can be seen that the effect of C in which

band of interest is minimal.
30th

An additional consideration should be shown. The circuit is typically driven by another operational amplifier, represented by \ l, Z.

The impedance Z should be as small as possible for the best effect. Z £ 5SX can be achieved with commercially available standard amplifiers.

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For a typical 100 KHz all-pass filter according to the preferred one Embodiment of the invention can be used for V and the Operational amplifier 11 LM 318 operational amplifier used will. The resistor R i is a 1000 ohm resistor and R "is designed by a variable 500 ohm resistor in parallel with 124 ohms, the parallel combination being in series with 953 ohms. This R “shape can be seen from FIG. 5.

Exemplary values for R are 113 ohms and 41 ohms for Q values of 14 and 40 with C = 0.001 microfarads, respectively.

Deviations and adaptations of the subject matter of the preferred embodiment are based on the person skilled in the art which is apparent from the explanation of the invention. Hence can within the scope of the claims, the invention can be implemented differently than specifically described here.

Claims (4)

VIERING & JENTSCHURA admitted to the European Patent Office European Patent Attorneys - Mandataires en Brevets Europoens Dipl.-Ing. Hans-Martin Viering · Dipl.-Ing. Rolf Jentschura · Steinsdorfstrasse 6 ■ D-8000 Munich 22 Attorney File 4273 Racal-Milqo, Ine; Miami, Florida / USA All-pass filter switching Claims All-pass filter circuit with an operational amplifier, characterized in that on the non-inverting Input (17) of the operational amplifier (11) a frequency-dependent impedance (R- ,, L, C) connected which shows a finite Q factor that on the 25th inverting input (15) of the operational amplifier (11) a resistor (R,) is connected and that in the feedback weight of the operational amplifier (11) a device (R ") is switched on, through which essentially the frequency-dependent Impedar.z caused the non-ideality of the Frequency characteristic can be compensated. 35 I / w -2- Telephone (089) 293413 and 293414 Fax (089) 222066 Telex 5212306 jepa d Telegram Steinpat Munich 2. All-pass filter circuit according to claim 1, characterized in that that the frequency dependent impedance means to minimize the effects of stray capacitance having. 3. All-pass filter circuit according to claim 1 or 2, characterized characterized in that the frequency-dependent impedance has a second resistor (R,) connected to the non-inverting input (17) of the operational amplifier (11) and to the input of the all-pass filter circuit connected, and one between the non-inverting input (17) and earth switched on Has a series combination of an inductance (L) and a capacitance (C). 4. All-pass filter circuit according to claim 3, characterized in that the second resistor (R,) one Has a value of 20 to 300il and the capacitance (C) is of the order of 0.001 microfarads.