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 between 400-4000 Hz.
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 tweaking 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)
Click Here
to View Circuit
-
(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.
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