/diy/ - Do It Yourself

>>1382064
You can do the phase shift with a capacitor, or more accurately, with an RC low pass filter. The thing is that you'll never get exactly 90 degree phase shift, but you'll get closer and closer as you lower the cut-off frequency. Yeah, this means increasing (and significant) attenuation as well.
If your frequency is constant, it's better idea to put two RC filters in series, each one shifting 45 degrees. You can use the normal f=1/(2*pi*R*C) to determine the proper values for R and C.

Both of these suck if your frequency varies. If you want to cover a reasonably wide range, you need fancier phase shifters. Pic related is one option (for audio, IIRC for somewhat limited range). You can also use LC filters (lead-lag filters, producing two outputs having a relative phase shift of 90 degrees over an octave or so), if you use simulated inductors for the coils.

Then there's that PLL approach the other anon mentioned and since it's 2018 now, you can also use DSP-like solutions.