-1 I2AX7


-1 I2AX7


fine tune

multiplier decreased in popularity. The bandwidth and steep skirts of the filters reached an optimum point for SSB reception and use of a peaking Q multiplier simply introduced distortion in selectivity, it was at this point that at least some operators discovered the very useful and somewhat forgotten notch function of a Q multiplier. Although it was no longer possible (o eliminate QRM on SSB by reducing the if bandwidth in a receiver without severely losing voice identity and intelligibility, it was possible to eliminate narrow frequency bands where QRM was present without affecting voice intelligibility to any great degree. The situation is very similar to various combined voice/teletype transmission systems where a 80 to 150 cycle slot is placed in the 300 to 3000 cycle total bandwidth for teletype transmission.

The usefulness of a notching device across

Multiplier 455 Notch

Fig. 1. Bridged-T network (A) is basis of most notch filters. Q multiplier in notch function uses network in feedback circuit. (C) varies notch frequency approximately ¿6 khz from 455 khz center frequency, the if bandwidth is indicated by the fact that many of the better and more costly receivers which already incorporate optimized if SSB filters also incorporate notch filters, since it is the only means left to eliminate QRM without causing distortion. Notch filters have been improved and simplified considerably in recent years. They can be a very useful accessory to add to a receiver or transceiver which already has a good if filter. They are a particularly appealing accessory to add to a transceiver which is used on both SSB and CW but which has only a SSB filter since they provide that extra bit of QRM rejection so necessary when a SSB transceiver is used on a crowded CW band.

This article surveys various notch filter configurations used in amateur receivers. The reader who wishes to should be able to adopt the various circuits to his own needs. The parts needed can all be purchased or fabricated. In place of ihe-latter, some of the more critical components might be available direct from receiver manufacturers.

Basic Notch Circuits

Most notch filters are based upon the Bridged-T network shown in Fig. 1(A), The network is balanced at the frequency at which it resonates and theoretically offers infinite attenuation between input and output terminals. At other frequencies, it is unbalanced and these frequencies pass through. The degree of attenuation at the notch frequency depends, in practice, upon the Q of the components used but can approach as high as 60 db.

The old Q multiplier notching amplifier (Fig. IB) made use of a network in a feedback arrangement in order to enhance o-

IH 455kt\z the Q of relatively simple components. At the resonant frequency of the network a high negative feedback occurs to the first section of the 12Ax7. The feedback drops its plate resistance to a very low value and, in effect, at the one frequency it shunts the if line to ground. It acts similarly to a frequency selective switch, Although the general tendency in notch filters has been to eliminate the need for active networks by using passive components of sufficiently high Q, the single point connection of the old Q multiplier circuit to the if circuit still gives it an unique advantage.

Notch filters can be built for any if frequency within the limits of achieving frequency stable circuits. With care they can probably be used up to frequencies of a few megacycles, although most designs have been made for frequencies below 1 megacycle. Fig, 1(C) is a good example of a transistorized notch filter design in a recent receiver design-

Typical Circuits

Fig, 2 shows tiiree notch filter circuits which utilize only passive components. The filters may be placed at almost anyplace in the if chain but after the main selectivity (crystal or mechanical filler). The loss they introduce (at other than the selected notch frequency) is usually low enough so that no additional amplification need be provided as compensation.

Fig. 2(A) shows a tightly coupled if transformer used between the filter terminal. Instead of the common point of the transformer going to ground, however, it is coupled to the bridged-! network n slightly modified form. The impedance transfer is o-

IH 455kt\z

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