> DDS (direct digital synthesis)

If you added a DAC to make the output appear more analogue
then that could be followed by a VCO, a 1V/octave for example

The MCP482x DAC is inexpensive and easy to drive with
12-bit SPI. I use them for LFOs, up to about 300Hz, but would
do for a control voltage that takes 150ms (~ 3Hz) to rise

Even simple RC integrators to ramp up a digital 5V i/p, probably
need fiddling with

There are many ways to do what the O/P wants, some of which
will be academic exercises rather than practical

Joe


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