Jan-Martin,

very interesting, and it is still allowed to use an additional notch

filter, or did I misundertand?<

At this point, no extra filtering should be required. The quadrature outputs
from the sound card are run through a TL074 to provide 0, 90, 180 and 270
degree audio sources (the 180 and 270 are just inverted from the 0 and 90).
That is applied to the four switches in the 74HC4066. The switches are
driven from a 4X signal that goes through a ring counter and decoder to
provide 0, 90, 180 and 270 degree outputs. The outputs of the switches are
summed into an opamp that provides 0 dBm into 50 ohms. The only conventional
filter is a simple half-wave filter at the 50 ohm output.

I'll have a schematic this weekend, but there are still a few problems to
solve.

John Andrews