>>>>
>>>>>>>
>>>> > I'm looking for a waveform generator that outputs two sine waves of
>>>> > the same frequency with 90 degree phase difference (sine and cosine).
>>>> > I need a variable frequency between 0.05 Hz and 10 Hz. Is there an
>>>> > analog design that uses a single potentiometer or perhaps is voltage
>>>> > controlled ? Low distortion is not a requirement.
>>>>
>>>> > Steve
>>>>
>>>> I've used a software DDS running on an MCU with a DAC for that sort of
>>>> thing. It's much easer to design and build than any analogue circuit.
>>>>
>>>> Leon
>>>
>>>This all depends on the frequency resolution, i.e. VCO step size. A
>>>coordic would be a better approach.
>>>
>>>I'm always amazed at how often the coordic is not suggested as a
>>>solution. I suspect it isn't taught in college courses anymore. I
>>>haven't found a good write up on the net regarding the algorithm
>>>either. It's decades old technology.
>>
>>To find it use CORDIC (COrdinate Rotation by Digital Integrating
> ^^^^
> ITYM COordinate. ;-)
>
>>Computation ). For sine / cosine DDA is simpler. At 8-bits lookup
>>tables are easier still. It will leave you time for really fancy pwm
>>methods to do error spectrum shifting / noise shaping like better CD
>>and semipro (and above) audio does.
>>
>>Besides, a uC DDS solution allows variable phase relationships that
>>most other solutions have trouble with.
>
> You only need to store one quadrant of sin(x) for both sin and cos
> eg. 0
You can use up pi/4 because cos(2x) = 2cos(x)^2 - 1
In fact you can get arbitrarily small lookup tables and use the identities to compute the rest. So one trades speed for size.