A Notch Filter
Fig. 1 Circuit diagram of the notch filter and output stage.
The circuit diagram of a notch filter and output stage appears in Fig. 1. The notch filter is based on IC11 and IC12, which are used in a conventional state variable filter. In this case it is only the notch output at pin 7 of IC12 that .is utilized. Ganged potentiometers R49 and R53 form the tuning control, and R56 is the balance control. The latter is adjusted to optimize the attenuation at the centre of the notch, and around 40dB to 60dB of attenuation can be achieved.
The resistor R52 enables the Q of the filter to be varied. A high Q value gives a relatively narrow notch, with little attenuation well away from the notch frequency. This type of response is best for dealing with heterodyne tones, as it enables the tone to be dealt with effectively while leaving the wanted signal largely intact. A Low Q gives a somewhat wider notch, and also tends to give significant losses well away from the notch frequency. This obviously has a more detrimental affect on the main signal, but it is more effective at combating interference that covers a small range of frequencies (such as an RTTY signal with a shift of 170Hz).
Switch S1 enables the notch filter to be bypassed when it is not required. From S1 the signal is coupled to an attenuator and then to the output amplifier. This amplifier is based on an LM386N-1, which requires few discrete components. In this case it is operated at minimum gain, and the only discrete components required are a DC blocking capacitor at the output (C32) and a supply decoupling capacitor (C33). Resistor R59 is used to reduce the maximum drive current to a level that suits most headphones. For sensitive headphones a higher value of about 390 will give better results. If the unit is used with insensitive headphones, or to drive a loudspeaker having an impedance of 8D or more, R59 should be replaced with a shorting link.