I need to test the range of the CEM3340 based VCO with various pots at their min/max and also replaced with resistors or just leftout.
The pots in question is
- Osc scale trimmer, located on the board
- HF tracking trimmer, external. The Prophet 5 does not use this
- The 500k CV input offset trimmer. The reference design uses a 360k here, the P5 uses a 357k.
Now, for some reason there is a lot of noise on the board outputs (the CEM itself is clean), but even so I am able to measure the frequencies.
For all tests, unless specified, I'm running with
360k resistor instead of 500k pot
HF trim center (5k/5k)
27k CV input, meaning the CV should span 18.5 octaves if perfectly tuned (and if that's supported by the VCO, which it isn't).
A cold VCO, i.e. turned on briefly to measure but otherwise off.
PS: My resistors are to 12V, not 15V. Also, I have a 1M5 hardwired to -12V so basically the same as the Prophet 5 (though that uses -15V)
Test 1
Osc scale trimmer (10k) fully counter clockwise
Max period: 264ms (0V): 3.79HzMin period: 28.4uS (2.8V): 35.2kHz
Osc scale trimmer aprox center (so around 29k to -5 from pin 1)
Min period: 28.2uS (3.6V): 35.36kHz
Test 3
Osc scale trimmer fully clockwise: (10k top, 0k bottom, so 24k from pin1 to -5V)
Max period: 650ms (0V): 1.54Hz
Min period: 28.4uS (4.5V): 35.2kHz
Test 4
Osc scale trimmer fully counter clockwise
HF trim trimmer fully counter clockwise (0k left to summer, 10k right to HF tracking)
264ms / 3.79Hz
28.4uS at 2.78V CV
Test 5
Osc scale trimmer fully counter clockwise
HF trim trimmer fully clockwise (10k left to summer, 0k right to HF tracking)
265mS / 3.79Hz
28.7uS at 2.6V: 34.8kHz
Test 6
Osc scale trimmer fully clockwise
HF trim trimmer fully counter clockwise (10k left to summer, 0k right to HF tracking)
649mS / 1.54Hz
30uS at 4.8V: 33.3kHz
Test 7
Osc scale trimmer fully clockwise
HF trim trimmer fully clockwise (10k left to summer, 0k right to HF tracking)
656mS / 1.54Hz
31.1uS at 4.1V, not possible to get lower: 32.2kHz
Test 8
Osc scale trimmer approx center
HF input directly to GND
457mS / 2.19Hz
28.7uS at 3.8V, 28.4uS at 5V: 34.8kHz
Test 9
Osc scale trimmer fully clockwise
HF input directly to GND
VCO has been on for a bit
4V gives 20.8kHz
Then
51.8uS = 19.3kHz (not sure if I changed the CV so not very informative)
29.8uS is max = 33.5kHz
Test 10+
After staying on for at least 30min
656mS / 1.52Hz
29.2uS max
4V gives 52.27uS so 19.1kHz
Little change from intitial, cold masurements, still tunable over full range.
Then fully counter clockwise osc scale trimmer:
263mS, exact same as with cold VCO.
Conclusion
I may just go without HF tracking (connect to GND) and shorting bottom two pins of Osc scale pot - or use a 10k for higher precision tuning at the sacrifice of lowest possible note. All options seem to allow for plenty of digital tuning.
Sync
Connecting a function generator to the VCO sync inputs, I got the following:
Hard sync is only triggered on fast falling edges. This means that the input must either be a pulse or a rising saw. Double check what the DCO outputs!
Sync is triggered on the falling edge of the input square wave |
CEM Hard sync
expected input (bottom) and effect on saw wave (top) |
CEM Hard sync is currently not working very well, but I found this post http://atosynth.blogspot.com/search/label/VCO saying I had the same problem earlier and that adding a 1nF cap in series with the input may fix things. Anyway, my experience is this:
For positive going sync pulses (with PWM making the positive going part 10%), the "neutral"/ 0V part must be less than -0.66V (which is eerily similar to one diode drop).
Top is triangle output, the small dips are the classic CEM hard sync effect of positive going pulses. Zero must be around -0.7V |
If I drop the lower level to less than -1.63V it stops working again. But - if I reverse the pulse so the 10% part is the one dropping, and set the MAX value to <-0.66V, it starts working as it should again.
Negative going pulses, zero must still be about -0.7V negative for things to work |
Adding a 1nF input cap to the cem hardsync input
That fixed everything, now it looks like this, given a square wave input
Frequency modulation
Exponential FM through a 100k resistor works as expected, giving a +/-5 octave FM if VCO is trimmed properly.
Exponential FM |
Linear FM did not work out of the box. If input is < -1V through a 120k resistor the output flatlines, I had this in my design and it does not work. So what DOES work?
Linear FM, VCO flatlines when FM input is too low |
About linear FM in the CEM3340 manual:
The input resistor should be selected to produce a current equal to +/- the reference current when the max input signal is present.
The reference current is set using R_r. In the datasheet, the reference current is 15V / 1M5, or 10uA. Since I'm still using a 1M5 resistor but elected to use a 12V supply instead, I have ended up with a 12V/1M5 = 8uA reference current.
R_r is the reference-setting resistor |
In the original circuit, the linear FM input resistor is set to 1M. To get a +/-10uV input here would mean the input would have to be +/-10V.
For my oscillator, the linear FM input will be max +/-5V, and I need to match a 8uA current. U/I, or 5V / 8uA = 625kOhm, which means I should use a 470k and a 150k resistor in series.
Linear FM with a 620k input resistor works fine. |
One more thing - I want to be able to do linear modulation with CV. I therefore have to sum the input before 620kOhm input. Also, the CV is 0 to 5V when it should really be -5 to 5V. Thus, I should set CV gain to 2 and subtract 5V to center it. This, however, means that in order for FM from analog sources to work properly, the CV from the CV generator should stay at 2.5V when not in use.
Also - If I use a normal inverting summer, the CV will be reversed. This can be fixed digitally. The analog input will also be inverted, but I don't think that's an issue.