[Lowfer] Phasing exciters

[Johan Bodin]

Stewart,

> Generating a 90 degree phase shift (mathematically) is easy.
> Google for "Hilbert transformers" and you'll see lots of

Another alternative is a complex filter. It takes care of the 90 degree =
shift
and does bandpass filtering at the same time. My favorite.

Example:

1.
Design a simple FIR 1350 Hz lowpass filter.

2.
Make two sets of FIR coefficients by multiplying the original 1350 Hz =
LPF
coeff's by "sin 1650 Hz" and "cos 1650 Hz" respectively (taking Fs into
account). To preserve the phase linearity of the FIR, place the 1650Hz
phase so that the coeff' sets become symmetrical or "antisymmetrical"
with respect to the center of the filter (peak or zero crossing at the =
middle
coefficient - or around the two middle coeff's if the FIRs have an even =
number
of taps).

3.
Put the "sin FIR" in the "I" line and the "cos FIR" in the "Q" line and
you have a single sided 300-3000 Hz response. Just swap I and Q
to change sideband.

73
Johan SM6LKM