CGS348 VCO
ELBY Designs - Laurie Biddulph
9 Follan Close, Kariong, NSW 2250, Australia
4 of 5
There are several trim pots that need to be adjusted.
With no CV inputs connected, and the [FINE] and [FREQUENCY] controls set to their
centre position, adjust trim-pot P105 (“ZERO”) until there is 0 volts on U102_1 (the easiest
place to connect to this pin is on the left leg of R120).
The next trimmer to adjust is P106 ("SPAN").
1. Apply 0.00V to the [1V/OCTAVE] input
2. Monitor the frequency of the [SAW] output
3. Tweak [FINE] to a round number).
4. Apply 1.00 volts into the [1V/OCTAVE] input
5. Adjust [P106] until the output of the oscillator is exactly one octave higher (double the
original frequency reading)
6. Apply 3.00V to the [1V/OCATVE] input
7. The frequency output should now be 8x the original frequency reading, fine tune P106
if needed.
8. Repeat this process until the desired accuracy is achieved
1. Set [FINE] to its mid-position and [FREQUENCY] fully counterclockwise
2. Adjust P105 (ZERO) for a frequency of approximately 8Hz
1. Once the oscillator is correctly tuned, connect 0.00V to the [VC F] input
2. Monitor the frequency of the [SAW] output and adjust [FREQUENCY] and [FINE] for a
reading of 220Hz
3. Set the [VC F] pot fully clockwise
4. Apply 1.00V to the [VC F] input
5. Adjust P103 ("PRO SP") until the oscillator runs 1 octave higher (440Hz)
6. Turn the [VC F] pot fully anti-clockwise should result in an output frequency one octave
lower(110Hz)
7. Tweak P103 if necessary to bring the reading closer.
8. Repeat steps (3) to (7) until the swing is as close as possible
P401 ("WS TRIM") should be adjusted so that output J403 (“TSO”) varies between a
sawtooth and a sine wave as the [SHAPE] control (P402) is adjusted. A simpler alternative
is to adjust this trimmer until the best sine wave possible is achieved at output J402
(“SINE”). Don't expect a perfect waveform - it will most likely have a substantial glitch in it
at its best setting. Remember - this sine output is simply there to make use of a spare part
of the LM3900 - it is not a key feature of the design.
The final adjustment is to P502 ("S TRIM"). Play with it until the sub-oscillator responds the
best over the best possible sweep range. Remember this will not follow the oscillator over
its entire range. Again, it is simply there to make use of a spare part of the LM3900 - it is
not a key feature of the design.
