First post and very new in the dsp world but I'm learning.
For a small project I need two sine waves, e.g. 10 and 20khz. I placed a sine tone of 10khz and used the multiplication block to get the 20khz tone. Obeserving both waves on a oscillioscope I discovered that they seemed to be phase locked (prob are) but the sine waves don't cross the zero axis at the same point. My guess is that they are 45 deg shifted. Of course I can use a delay or all pass filter to correct this but rather not. Is there any simple solution to this? Why are they shifted?