FUNCTIONAL TRIMMING OF THICK-FILM ACTIVE FILTERS FOR CONSUMER AND INDUSTRIAL APPLICATIONS

Active filters made on printed circuit boards require high-precision components or critical manual adjustments of resistor trimmers to tune each single stage, with many problems in reproducibility and reliability. Such disadvantages can be successfully removed by thick film technology, using automatic functional trimming by a computerized laser system. The implementation of a suitable interface box allows a laser system, originally designed for dc trimming, to adjust RC time constants, gain and phase variations of thick film active filters. In this way, filter trimming has been turned into a fast and repetitive process, with very good results in high volume production. This paper deals with the adjustment methods used in the production of two active filters for quite different applications, as electronic musical instruments and automatic controls for industrial use. The former, required above all, manufacturing costs to be reduced as much as possible, the latter required rather critical specifications to be met with great accuracy. It will be pointed out how the interface system designed for the consumer filter (already described in another paper) can be improved with simple criteria, to adjust also filters with more sophisticated characteristics.


INTRODUCTION
Active filters made on printed circuit boards require high-precision components or critical manual adjustments of resistor trimmers to tune each single stage, with many problems in reproducibility and reliability.
Such disadvantages can be successfully removed by thick film technology, using automatic functional trimming by a computerized laser system.
The implementation of a suitable interface box allows a laser system, originally designed for dc trimming, to adjust RC time constants, gain and phase variations of thick film active filters.In this way, filter trimming has been turned into a fast and repetitive process, with very good results in high volume production.This paper deals with the adjustment methods used in the production of two active filters for quite different applications, as electronic musical instruments and automatic controls for industrial use.
The former, required above all, manufacturing costs to be reduced as much as possible, the latter required rather critical specifications to be met with great accuracy.
It will be pointed out how the interface system designed for the consumer filter (already described in another paper) can be improved with simple criteria, to adjust also filters with more sophisticated characteristics.

Schematic Diagram
The manufacturers of electronic musical instruments use a large number of "octave filters" to get pure sinusoidal signals from square wave generators.Because of very tight cost constraints, filters are usually made on printed FIGURE Sallen-Key filter structure.
where polynomial coefficients have been dimensioned to give a Chebysheff low-pass characteristic of fourth order.
The frequency response is outlined in Figure 3, showing a ripple of 2 dB in the pass band and an attenuation of about 35,5 dB/octave in the stop band, considered quite satisfactory for musical instruments manufacturers.circuit boards but high-precision capacitors and a manual individual adjustment are not always sufficient to generate sounds of acceptable and constant quality from assembled instruments.
Thick film active filters replace older versions with advantages in cost and performance, thanks to a proper circuits structure, which includes general-purpose transistors and large-tolerance capacitors and is suitable for functional automatic trimming.
The filter circuit is based on the well-known Sallen-Key structure (Figure 1).The active element in each stage is a unity-gain amplifier, implemented with two transistors (Figure 2).The transfer

Functional Trimming
The actual reproduction of the calculated filter characteristic requires the trimming of frequency response in each stage, that is the polynomial coefficients of Eq. 1 must reach their theoretical values.With the first step, time constants R1C1, R C of the first stage and R a Ca, Ra Ca of the second stage are functionally adjusted, to reach the nominal values of pz coefficients.In the second step, gains K1 and K must be adjusted also to compensate for errors deriving from cross terms R C and Ra Ca and this is achieved by trimming resistors r (Figure 2), affecting the stage gains.Since RICI >>RC andRaCa >>RaC4, very small deviations of K and K: close to the unity value are sufficient to obtain the desired adjustment.
The block diagrams of the interface device are shown in Figures 4 and 5.
In the first section, a voltage generator G supplies a sinusoidal signal VG to the network RxCx under trimming and to a reference network RR CR, with nominal value time constant.
Their outputs are connected, through two peak-detectors, to the trimming system, which compares the two d.c.voltage levels and stops cutting resistor Rx when VAI VB I, that is  laerec ra j I_. ro Z AS.R xCx RR CR.In the second section, a sinusoidal signal at the stage tuning frequency, is sent to peak-detector directly and to peak-detector 2 through a divider, with ratio equal to H and through the stage to be adjusted.The d.c.voltage levels from the peak detectors are supplied to the trimming system which cuts resistor r until the stage gain reaches the nominal value H.
The most important advantage of this comparison method lies in its precision and reproducibility, not affected by precision and stability of the voltage generator G.
An electronic musical organ usually includes seven filters with seven different cut-off frequencies, ranging from 100 Hz to 8500 Hz.
The complete kit is obtained from two modules, using the same set of masks, with changes of software only.
The basic thick-film module is screen-printed on a 38.1 x 16.9 mtn.ceramic substrate.Add-on components are four general purpose transistors in SOT23 case and four multi-layer ceramic chip capacitors with a tolerance of +20%.
Figure 6 describes the frequency responses of C4 R6 FIGURE 7 Fourth order pass-band filter.
the seven filters now in production.They differ in cut-off frequencies only, which can be also changed on customers specification.
The following table gives the comparison between traditional and thick-film filters: This filter is a fourth order pass.bandtype, including two stages with multiple feed-back (Figure 7).It has been designed with a monolithic quad op.amp. of middle quality (gain x band width -3 MHz) and NPO ceramic chip capacitors, with tolerance of +5%.Its transfer function is: K2RsC'3

RI. ++ .p .p+l
The frequency response is outlined in Figure 8 and is expressed, in normalized form, by the equation: Frequency response of pass-band filter.
The two filter stages are defined by their own frequency responses, plotted in Figures 9 and 10.
Tuning frequencies, practically coinciding with maximum points, may be written as" [o

2 Functional Trimming: Principle of Operation
In each stage, at its tuning frequency, there is no phase deviation between input and output signals.
Eq. ( 2) and (4) show that frequencies and phase- deviations of the two stages depend on a and a2, p:-coefficients in the transfer function.Thus we can tune each stage at its own frequency, by trimming resistors R2 and R, stopping the laser trimmer when phase deviation becomes zero and p2.coefficients reach the nominal values.We can then proceed to adjust gains Hi, according to Eq. ( 5).Coefficients b l, c l, b: and c2 could not be exactly equal to their respective theoretical values but computer analysis shows that deviations are negligible and cause a small vertical translation of the curve in Figure 8, without affecting filter selectivity.

Implementation
The interface system is designed to use the standard d-c type computer controlled laser trimmer.
The block diagram of the phase-control section is shown in Figure 11, where Vi is a sinusoidal signal, supplied to this circuit and to the filter stage under trimming at the same time.Figure 12a shows Vi and Vo before trimming, which produce the waveforms V1, V: and V3 of Figures 12b, c, d in the interface network.When the phases of V0 and Vi became equal, we have at the same instant V2 V3 "1" and V4 "0".At this moment, the output Vs from the memory unit goes up to "1" and stops the laser, trimming resistor R2 (or R).
Note that the one-shot no 2 is connected to similarly operate the AND-gate, to have the same time-delay for signals in the two paths towards the laser control input and, in this way, to improve the system precision.
This simple solution offers remarkable accuracy in operation, with response times in the order of a few nanoseconds.The interface section to control gain adjustments is the same previously described for the consumer filter   7), directly affecting K1 and K: parameters.
Actually, phase and gain adjustments are not independent at all but affect each other in a small amount, because of effective limitations in op.amp.characteristics, particularly gain bandwidth product.To compensate for the consequent error, the adjustment of each stage is achieved through four steps, the first two of which are a coarse calibration and the second two are a fine calibration.This is described in Figure 13, concerning the first filter stage.First of all, the circuit is tuned at a frequency FIGURE 13 Diagram showing stages of the first filter adjustment.
foi" < foi, then the gain is brought to a value H1" < H1 and finally, by two other similar steps, final values fo and Ho are reached.

Hybrid Module Characteristics
The circuit is assembled on a ceramic substrate of 22,4 x 38,1 mm., screen-printed on both sides.
Functional trimming resistors have rather large dimensions, to get final values with several cuts without generating problems in noise and stability.
After functional trimming, modules are packaged into a plastic case filled with epoxy resin and then tested with an automatic computerized process.

CONCLUSION
The thick film module of the pass-band filter described in this paper is a good compromise in performances, accuracy, dimensions and price.
Other electrical circuits would be more suitable for adjustment and accuracy but would require one amplifier more in each stage.
The above-mentioned method of functional trimming combines a system of simple implemen- tation with very good operation in the production process and, moreover, can be extended to other kinds of filters.
The opportunity of performing functional adjustment with several steps of increasing accuracy allows more critical circuits to be tuned.

FIGURE 3
FIGURE 3 Frequency response of filter.FIGURE 4 Block diagram of interface to laser (No. 1).

FIGURE 4
FIGURE 3 Frequency response of filter.FIGURE 4 Block diagram of interface to laser (No. 1).

FIGURE 5 FIGURE 6
FIGURE 5 Block diagram of interface to laser (No. 2).
FIGURE 11 Block diagram of phase control section.
FIGURE 12   control circuit.