Friday, 3 February 2012

LFO Prototype on the Bench

Here's a shot for those who like to see prototypes on the bench:
The donor VCLFO PCB is on the left, with various components and wires hanging off it.  The breadboard on the right has a FET follower/buffer for the sync input.

LFO Auxiliary Sine and Sync

Just for the record, here are scope shots of the Aux LFO sine output.  The first shot shows the basic diode-shaped sine wave:
Clearly not a perfect sine, but the distortion is down to around 1% THD.  Not hi-fi grade but certainly useful for modulation purposes.

The second shot shows the same sine wave but sync'ed to the positive edge of an external square wave:
The sync circuit is designed to reset the auxiliary oscillator to the 0 degree starting point.  With the switch in the NEG position the oscillator is reset by the falling edge of the square wave.

Wednesday, 1 February 2012

LFO Prototype Finished!

At last the prototype of the first module - a dual LFO based on my earlier design - is complete.  The schematic will be finalised in the next day or so and then on to PCB layout.  Specification is firming up, key features including:
  • Main LFO:
    • Three sine outputs at 0, 120 and 240 degrees
    • Frequency range 100Hz down to around 1mHz (16 minutes)
    • CV control of frequency, link-selectable normalised to either internal bus or Aux LFO sine
  • Auxiliary LFO: 
    • Triangle and sine (diode-shaper) outputs
    • Frequency range same as for Main LFO
    • CV control of frequency
    • Sync input, link selectable for positive or negative edges
    • Shape control to adjust triangle to rising or falling sawtooth, also CV controllable
  • Eurorack format
  • Standard 16-pin IDC header power/bus connection, with reverse voltage protection and RFI filtering
  • Blinky lights for both LFOs :)
The frequency range of these LFOs is subject to some further thought.  The prototype is currently configured to go from about 22Hz down to about an hour.  The advantage I can see of going for the higher range quoted above is that it makes for a nicer control range (100-10-1-100m-10m-1mHz) for the front panel.  But if I leave off any real-world markings and just have tick marks then the actual range isn't so important, and can be set to something more useful.
Another option would be a range switch as a lot of other LFOs have.  But I prefer having the full range available in one sweep rather than split into overlapping segments.

Target size is 14HP.  Although I'm considering a wider option so I can bring out the sync edge selection to a front panel switch. If I go with the 14HP panel then the five knobs will be in a single vertical column, and with 20mm between the centres I have identified some sexy rubbery 11mm knobs from Sifam.
The wider panel option would allow me to use larger 15mm knobs.  We shall see how this works out when the PCB is finished.  I'll probably knock up a couple of prototype panels in both formats so I can compare the two and get other opinions.

For now, on with the PCB layout!