I finally got some time to play with pwmout, but, when I run this code on a 18X:
'**************
main:
for b0 = 100 to 400 step 10
pwmout 3 , 99, b0
wait 2
sertxd (#b0,13,10)
next b0
high 7 'sound buzzer to signal end of loop
wait 1
low 7
goto main
'*************** end
I get this...