Narrow band audio bandpass filter

Posted by Author makecircuits

Narrow band audio bandpass filter

Introduction

This audio bandpass filter is useful for amplification and filtering of weak AM TV video carriers. For example, a DFM (digital frequency audio multimeter) may have insufficient input sensitivity for measuring extremely weak SSB TV video audio signals. By using the 20 Hz filter to peak the wanted carrier, the DFM will display the carrier frequency.

Another possible application for this filter is increased amplification and reduced bandwidth of weak BCB heterodyne AM carriers. The filter is also very useful for separating video carriers that are in close proximity of each other.

By definition, a bandpass filter is usually a low-pass and high-pass filter in series, allowing only a certain range of frequencies through. Because the cut-off frequencies are close to one another, the effect will be similar to that of a peaking filter.

The bandwidth of the filter, when peaked is approximately 20 Hz. This is much narrower than the typical 2.4 KHz SSB bandwidth of most communications receivers. The advantage of this filter is a constant 20 Hz bandwidth, regardless of the resonant frequency, when peaked betw1een 400-4000 Hz.

Typical set up

The audio line-out or headphone output from a VHF/UHF scanning or communications receiver is connected to the input of the bandpass filter. The output of the bandpass filter is connected to a digital frequency meter (DFM), and/or audio monitor speaker. A monitor speaker is used when tw1eaking the filter`s resonant frequency. If a DFM is not used, a PC program, such as Spectrum Lab could be used for spectral display of TV video carriers on a computer screen.

The tuning range of the filter is from ~ 400 Hz to 4 KHz, when using a 50 KHz potentiometer. The writer typically tunes the filter to resonate around ~ 1000-1300 Hz. This frequency range corresponds to the maximum output level of 2.4 KHz USB mode.

A 10K fine tuning potentiometer has also been added (not shown on the schematic). This can be included in series with the main 50K pot. I did this on my filter, and the centre of the tuning range moved from 300 Hz to 1000 Hz and improved the bandspread by a factor of about 4 times up to 2 kHz. A fine tune pot is useful for use with receivers that have 100 Hz minimum tuning steps, for example, Icom R7000/7100/8500, etc.

Initially, a LM348 op-amp was used in the circut. Although this worked ok, it was found that by replacing the LM348 with a LM6134BIN IC, improved high frequency response was obtained.

Please note that only one single IC is used in the band pass circuit. The four op-amps indicated on the circuit diagaram are all contained within one IC.

The author has also fitted a by-pass switch, to enable audio frequencies above the filters resonant frequency.

Bandpass filter Schematic (all four op-amps are used in the LM-348)

Part list:

  • (1) 50K linear dual-gang potentiometer.

  • (1) 10K linear dual-gang potentiometer (optional fine tune control).

  • (2) .1 uf ceramic capacitors.

  • (1) .01 uf ceramic.

  • (2) .01 uf greencap capicitors.

  • (2) .1 uf monoceramic capacitors.

  • (2) 5.6K resistors.

  • (2) 10K resistors.

  • (3) 11K resistors.

  • (1) 120K resistor.

  • (1) 1M ohm resistor.

  • (1) LM6134BIN op-amp IC,

OR:

(1) LM348 (or similar) op-amp.

Note: all resistors are 1/4 watt, metal film 1% tolerence.

Bandpass filter Schematic without extra amplification (only three op-amps used)

Biquad filter. A close relative of the state variable filter, is showm below. This circut uses three op-amps. It has the interesting property that you can tune its frequency (via the single double-gang pot) while maintaining constant bandwidth (rather than constant Q).

The audio output is from pin 7 of the LM348 IC.