Notes on PA0RDT Audio Filter

Use 1% resistors for the feedback and interstage in the audio filter I
build. I buy a bag of 100 5% tolerance 0.027uF caps from the Mouser
distributor, Xicon brand. They're about 8 cents each in 100 quantity--not
something to break the bank when you buy a bag of them. It generally takes
about 20 caps to find 5 or 6 matched pairs, in the two bags I've gone
through now in building the filters.

There are various designs that can be made, all using the general circuit
diagram of this topology. But what separates them are the resistor and cap
values. One can give a broad peak or a wider bandwidth or a poorer shape
factor. The particular circuit I've been using, modified by Roelof Bakker
PA0RDT as noted on the diagram, works exceptionally well, gives very narrow
bandwidths and good shape factors.

If you want a wider bandwidth than the nominal 6-8 Hz, then you can
stagger-tune stages. Tune sections 1 and 3 to a higher frequency; tune
sections 2 and 4 to a lower frequency, with the same separation for each
pair from center frequency. For example, you can use +/- 5Hz stagger-tuning.
This will give a wider bandwidth than simply the difference (10 Hz), around
12 Hz or so. If you have a receiver that only tunes in 10Hz minimum steps,
you must use a filter with a wider bandwidth than the tuning step, otherwise
you'll skip over weak signals. The R75 is an ideal LF radio for NDB use,
with its 1 Hz minimum tuning steps, and excellent LF sensitivity.

Remember that the narrowest bandwidth will occur when you tune all filter
sections to the center frequency. Stagger-tuning will always produce a wider
bandwidth. (It may not be obvious but you cannot stagger-tune for a narrower
bandwidth than the filter will work at, with all sections tuned to center
frequency!) The two caps used in each pair basically set the minimum
bandwidth, larger caps giving a narrower bandwidth. The original PA0LQ
circuit used 0.01uF caps, producing an approximate 30 Hz bandwidth.
As noted before, this is the best audio filter I've found so far, having
started with other opamp filters back in the 1980's, then graduating to the
present DSP filters by MFJ and Timewave. The DSP filters give almost
brick-wall skirt selectivity, which this 4-section opamp filter can't
achieve, but for NDB chasing, the narrower bandwidth of the opamp filter
works better. And audio filters can be cascaded--the DSP filter(s) can be
used along with the opamp filter, for the ultimate in narrow bandwidth and
skirt steepness. But in most cases I find just using the opamp filter by
itself is all I need.

Note that you do need a narrow CW IF filter in the radio in order to get the
best from an external audio filter. A wide SSB filter will allow a lot more
noise and garbage to be presented to the audio filter, and limit how close
you can tune to strong signals. Using a 250Hz IF filter will cut out most of
that garbage. Above presented by Steve AA7U