Nterated Circuit Audio Filter

The development of complicated multi-inductor filters and multi-transistor rc filters is culminated in a new type of integrated curcuit audio filter. The filter should have many-applications in amateur radio equipment designs.

One item that has been used in innumerable pieces of amateur equipment and accessories over the years is an audio filter. Such filters, particularly if they were used for audio selectivity purposes, could get to be very elaborate and large with multiple section designs. It was probably only inevitable that the current stream of progress toward the micro-miniaturization of electronic components would also reach audio filters. However, the miniaturization of audio filters that has been achieved is not just simply a miniaturization of inductors and transformers. None of these components are used in the audio filters to be described, and these integrated circuit filters offer adjustment versitiiity that couid never be achieved with inductors. Such filters open up the possibility for the construction of numerous compact pieces of accessory equipment that can be used to improve the operation of receivers and transceivers.

Fig. 1, Basic rc networks allow filter circuits to be built without the use of Inductors. High-pass circuit (A), low-pass circuit (B), and notching and peaking circuits (C).

Fig. 1, Basic rc networks allow filter circuits to be built without the use of Inductors. High-pass circuit (A), low-pass circuit (B), and notching and peaking circuits (C).

Background

Many attempts have been made to do away with the use of inductors in filters both because of size and cost factors. Some of these attempts date back quite a few years and all revolve about the use of rc networks in place of inductors. For instance Fig. 1(A) shows an rc high-pass filter. As the frequency of the input signal increases, the reactance of the capacitors decrease and more voltage appears across the output. Fig. 1(B) shows a low-pass filter that works in a similar manner. If you combined the filters, using an amplifier for each, you could form a bandpass filter. You could also combine various forms of rc networks to form notching or peaking filters, as shown in Fig. 1(C). Such filters by themselves, of course, are crude and provide poor selectivity. Usually such filters are used together with amplifier stages to compensate for the filter attenuation and also in feedback arrangements so the filter

Fig. 3. The WM3 filter can be used 3lone [(A) and (B)] to function as a low-pass or hugh-pass filter or in conjunction with a postamplifier [(C) and (D)J to produce a steeper slope at the cutoff frepuencv.

Fig. 2. Block diagram of the Western Microwave WM3 filter. ALL of the blocks shown are contained in the single filter unit shown in the photograph. Each amplifier is, in fact, a separate integrated circuit amplifier. The numbers refer to the terminal connections.

Fig, 4. The extremely sharp peaking and notching characteristics of the filter make it ideal for use as an cw selectivity device. The peaking and notching frequencies can be tuned from 500 to 1500 cycles.

circuits are not loaded down. Unfortunately, by the time you combine sufficient discrete component re networks and transistor amplifier stages to have the rc filter duplicate the performance of an inductor network, the rc' filter can be as large and as costly as the latter. The advent of integrated circuits has changed all that, however. High gain amplifier circuits and multiple rc networks can be incorporated in one physically compact unit.

The block diagram of the integrated circuit filter is shown in Fig. 2. Three multi-transistor operational amplifiers and the necessary rc networks in a feedback arrangement are combined in the hybrid filter unit. Three external resistors are used and can be manipulated to change the operational characteristics of the filter. One can see some of the filter components in the unencased view of the filter in the photograph. The encased filter measures 0.8 inches x 0.65 inches x .15 inches thick. The surface area is about that of a 25 cent piece, and it is hardly any thicker. Performance

The filter really begins to shine when one investigates its performance possibilities. It can be used as a high-pass filter, low-pass filter, peaking filter, notching filter, etc. The center frequency can be adjusted as desired by an external potentiometer as well as the Q if desired. The unit can also be set up so you can switch select a variety of different filter effect outputs.

Graphs portray the performance of this type of filter best. Fig. 3 illustrates the output versus frequency characteristic of the filter in several low-pass and high-pass circuits. The filter can be used alone for these functions, or its output used to drive another ic operational amplifier power stage to further increase the slope of the frequency response at the cut-off frequencies.

Fig. 4 shows the Filter used as a peaking or notching filter. Note the extreme

Fig, 4. The extremely sharp peaking and notching characteristics of the filter make it ideal for use as an cw selectivity device. The peaking and notching frequencies can be tuned from 500 to 1500 cycles.

Fig. S. The above graphs indicate how by simple external potentiometer control, the frequency as well as the Q of the filter can be varied.

sharpness of the response at the nominal center frequency of 1 khz. Actually, both the center frequency as well as the sharpness of the filter response can be tuned by making various of the external resistors variable as shown in Fig. 5. Thus, the center frequency of a nominal Ikhz filter can be tuned from about 500 to 1500 cycles. The Q can be varied from about 1 to a maximum of 100.

As might be imagined from looking at the arrangement of the external resistors and the output terminals used for each specific application of the filter, you can devise various switching arrangements to select different outputs, different specific center frequencies, etc. The possibilities in this direction are pretty well only limited

FRÜH FïF AMPLIFIER

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