PRECISION THICKFILM ACTIVE RC FILTERS

The application of hybrid thickfilm technology to the realization of precise active RC filters with passband 
frequencies up to 3.5 kHz is described and it is shown that relatively expensive thin-film technology can often 
be replaced by the simpler thickfilm process.


INTRODUCTION
In this paper the application of hybrid thickfilm technology for realizing precise active RC filters with passband frequencies up to 3.5 kHz is described.
The following examples of filters used in com- munication and audio-frequency measurement systems show that relatively expensive thinfilm technology can be replaced by the simpler thickfilm process in several cases.
The circuit design procedure has to take the thickfilm properties into account and a great amount of effort is required to reduce the sensitivities of the attenuation with respect to component variations.Because of their low-sensitivity properties doubly- terminated LC-prototypes are used for the active simulation.

GYRATOR PCM LOWPASS FILTER
In order to simulate the grounded or floating inductors of the 5th-order elliptic LC lowpass (LP) FIGURE LC-prototype for the gyrator PCM LP filter.137 filter shown in Figure by gyrators and capacitors, two gyrators are realized on 0.33 in x in substrates by blocks.These circuits consist of two or three op-amps and eight or ten resistors, respectively.Figure 2 shows these building blocks.The capacitors are to be connected externally in the case of a CCITT PCM LP filter because of the high price and the required volume of +_.1% chip-capacitors with element values from 2.5 nF to 11.2 nF.Only one resistor ink (10 kohm/r) is used.The cuts were trimmed automatically by a YAG-laser system 2 and, by a proper selection of the geometries, an R-accuracy of +2%0 can be achieved, a For Rmin 3.16 kohm and Rmax 27.2 kohm an R-accuracy of +5% (including the long-time stability) is obtained.The entire active filter circuit (see Figure 3) consists of five op-amps, eighteen resistors and six capacitors.The measured attenuation curves are shown in Figure 4 together with the attenuation restrictions.

PCM LP FILTER SIMULATING THE SIGNAL FLOW GRAPH (SFG) OF AN LC-PROTOTYPE
To obtain an active circuit with a minimum number of capacitors, the SFG of the elliptic LC filter in Figure 5 (refer to Saal and Entenmann4) is simulated by a canonical circuit, In order to reduce the number of printing processes a universal layout (0.5 in x in) is used for two subcircuits.By laser cutting of conductive paths the layout is adapted to the special requirements.Seven op-amps are ultra- sonic wire-bonded and the five chip-capacitors are connected with a printed solder paste.One of the advantages of this filter concept is that only the RC- product of each integrator influences the frequency FIGURE 2 Gyrator circuits for the simulation of grounded (a) and floating inductors (b).response of the filter.Thus roughly tolerated capacitors and adapted laser-trimmed resistors yield the required time constants T RiCi.Two resistor inks (10 kohm/D, 50 kohm/D) and five chip-capacitors with equal values of C 680 pF _+ 10% are used.
Figure 6 shows the photograph of one subcircuit of the complete PCM LP filter circuit is shown in Figure 7, while the measured attenuation curves of this filter are shown in Figure 8.

VARIABLE OCTAVE FILTER
A variable 6th-order bandpass filter with specially approximated attenuation curves is designed to satisfy the requirements of DIN 45 651 (see Figure 9) for octave filters with centre frequencies in the range from 10 Hz < fm < kHz.The lowpass- bandpass transformation is applied to the SFG of  the reference LC lowpass filter.The resulting second- c, rder transfer functions are realized separately on 0.5 in x in substrates using the analogue-computer principle.A voltage-controlled network containing resistors and pn junction diodes (connected in odd symmetrical pairs) allows the variation of the centre frequency.The bonding and soldering processes are the same as in the preceding examples.Additional layout problems caused by the fixed pin sequence requires a two-layer technique.Two prints of the insulating material separate the layers.Figure 10 shows a second-order section with four pn junction diodes.The realization of a second-order bandpass filter section is shown in Figure 11.Measured attenuation curves with the control voltage Us as a parameter are shown in Figure 12.

CONCLUSION
In all cases mentioned in the preceding sections functional trimming can be avoided because the accuracy of thickfilm resistors (AR/R 5%0 TKR 100 ppm/K) yields very stable attenuation curves in view of the low-sensitivity properties of active- simulated terminated LC filter circuits.Within the temperature range from 0C to +70C, both the gyrator filter and the SFG filter meet the requirements very well.Their excellent dynamic range of more than 100 dB (power consumption 250 mW Vcc --+ 15 V) should be mentioned especially.The proposed octave filter allows a continuous variation of the centre frequency by a factor of 100 without vklating the tolerance requirements.
Circuit of the gyrator PCM LP filter (R in kohm, C in nF, used op-amps uA 741).