RF Passive Notch Filters for 40-70 MHz

The notch filters described on this page proved a great deal of success when Band 1 BBC-TV was transmitting since it was capable of high attenuation with a very steep skirt response, allowing reception on closely adjacent frequencies. With the current interference problems, for example, cordless phones, baby alarms, 50 MHz radio amateurs, etc, a notch filter may offer considerable improvement to weak DX TV signals.

The circuit in Fig 1 is illustrated, the response is such to provide an attenuation in the order of 40dB or greater at resonance and allowing signals some 250 KHz to pass with a low order of attenuation.

The adjustment of the notch is extremely critical, the tuning must therefore be adkusted very slowely so as not to pass the notch. When the notch has been found, the variable resistor is adjusted to give a further increase of attenuation, there being one critical value for this adjustment.

The design shown in Fig.1 needs varnish or glue over the close spaced coil windings. The filter is tuned with the ferrite core. Adjust the core for the maximum desired resonant frequency, when C1 is at minimum capacitance. The preset potentiometer peaks the notch and enables an extremely sharp characteristic to be achieved in conjunction with the main tuning capacitor.

Fig1: Band 1 (40-70 MHz) notch filter.

The design of a double notch filter that features an insertion loss of only 1 dB with notch depths typically of 50 dB is shown in Fig 2. This filter is able to notch two different frequencies at the same time.

Fig2: Band 1 (40-70 MHz) double notch filter.

Fig3: Band 1 (40-70 MHz) double notch filter designed by Graham Deaves.

The double-frequency notch filter devised by Graham Deaves, is shown in Fig 3. At the sound and vision frequencies, it gives a notch of roughly 50 dB, with an insertion loss of about 1 dB.

To set the filter up, first adjust the variable resistors to their maximum value, then tune the ferrite cores to the vision and sound frequencies respectively. The notch is very sharp indeed, and ideally, a non-metallic(plastic) alignment tool should be used to adjust the cores, otherwise some detuning will be noted on removing the screwdriver. After the notch has been set, slowly adjust the resistors. A deeper notch will appear at about half resistance. The cores and resistors should then be glued firmly into place otherwise detuning may occur after a period of time. Some may prefer not to glue the cores and ferrites, if various frequencies are likely to need notching.

References and Acknowledgements

The material in this article was based on Roger Bunney's book A TV DXers Hanbook, published in 1986 by Bernard Babini BP176.

Roger Bunney has a monthly DXTV column in TELEVISION magazine. Notch filter designs are included on a periodical basis.

A tunable 48-70 MHz notch filter is also available from HS Publications.