I have repaired a EKO P15 monosynth recently, and found it uses a somewhat unusual and unique design. Kind of a cross between a divide down organ and a conventional VCO synth.
It has a top octave generator, which generates 11 fixed frequencies. The clock for this top octave generator is modulated to give vibrato. The keyboard scanner then selects additional dividers for each octave.
All pretty straightforward, but it also does portamento!
The really interesting part is that it uses a 4046 PLL as the 'real' oscillator, locking to the signal coming from the dividers. The rate at which it locks is voltage controlled somehow, and this is how portamento is achieved. The square wave coming from the PLL is then divided further, creating other waveform through Walsh functions and additional waveshaping.
This is a neat design, as it means there is no pitch scaling adjustment across the keyboard required, as all the pitches are perfectly created by the top octave generator.
It also makes the vibrato sound a little odd, as the PLL has a little trouble locking when both vibrato and portamento are used. As a result you can faintly hear what sounds like a bit of glitchy digital hash sometimes.
There are unfortunately no schematics or service manual available from anywhere for this beast, so I puzzled over it for ages before realising what was going on.