Rik and other readers,

For those who want to calibrate the soundcard there is also a simple
hardware solution :
Take a CMOS4060 (14-stage divider with internal oscillator) and let it run
with a xtal in the 8-12MHz range. At the end of the divider chain (pin 3)
you will get the xtal frequency divided by 16384 (will be in the 500-700Hz
range).

I have had such an accessory for a few years, delivering an output of 1000
Hz.  The squarewave output has much harmonic content, but it can be
filtered.  I also have a distortion measuring instrument that shows that
most audio oscillators have "rude" harmonic content, but my home brew unit
with suitable filtering gives about 70 dB of purity to the 1000 Hz
fundamental.  So it is very good for both frequency accuracy, and purity.

The 1000 Hz tone, to millihertz accuracy, is very handy for checking the
dial accuracy of amateur transceivers, in SSB mode.  With a frequency
counter on the output, the sideband should be either +1000 Hz for USB,
or -1000 Hz for LSB, and any error is in the transceiver RF department.
1000 Hz is the best frequency for least mental arithmetic by the operator
;o)

I have more recently made considerable use of the accessory for calibrating
PC sound cards.

72, Bob ZL2CA