This is a continuation of the 50kHz VCO thread I started last week. I tried the stepped sine wave idea as suggested by James A, and Phil H. The circuit clocks a MC14017 at 10x(F) to make a stepped sine wave at frequency (F). The ten outputs from the 4017 are sent through appropriate resistors and into the summing junction of an opamp. Here=92s a =91scope shot of the stepped output overlaid with a sine wave.
The resistor values were chosen to intersect the sine wave at each new phase. (R(n) =3D 1/sin^2(n*18degrees))
Approximate values, R0=3Dopen, R1=3DR9=3D105k, R2=3DR8=3D28.9k, R3=3DR7=3D1=
5.3k, R4=3DR6=3D11k, R5=3D10k. all 1% resistors.Here=92s the spectrum as recorded by an SRS770 spectrum analyzer.
The 2nd harmonic is only down by 50dB. I don=92t understand why it=92s so big. Is there some way to do better than this? The 9th and 11th harmonics are big and then the 19th and 21st.
Thanks George H.