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A project log for CEM3340 module

all the parts needed to use the voltage controlled oscillator CEM3340

Frank BussFrank Buss 09/30/2018 at 21:291 Comment

I got the boards and parts from Aisler for revision B. After soldering everything, first I measured the voltage regulators. The -5 V was fine, but there was no +10 V. Turns out that I forgot to connect the SHDN pin to VIN. Usually such pins can be left floated, but as the datasheet says, you have to connect this pin for the MCP1804.

After this I tried the sawtooth output again, which had the high frequency noise in the last revision. And the noise was still there.

I thought maybe that the trimmers could introduce some noise, if they were wirewound or something like this. So I desoldered them. But the construction looks pretty good, with just two small stripes:

Detail image of the slider:

Without the trimmers, the noise was still there. So I lifted the pins of the CEM3340 one by one, starting with pin 1:

When I reached pin 9, the noise was gone! I guess the layout could be a problem, because the triangle output at pin 10 is right besides pin 9:

For revision C I moved the capacitor a bit to the right for better distance.

But the datasheet says also, that you should ground pin 9 with a 100 nF capacitor, if not used. So after resoldering everything, I just connected J6 to GND, and the noise was gone as well! Lesson learnt: always read the full datasheet, especially if there are some pins left unconnected.

This is how the final version looks like. Top side:

Bottom side, with the small patch for U3:

Now the sawtooth looks perfect:

It goes down to 0.5 Hz:

and up to more than 20 kHz, but then the waveform gets a bit distorted:

But you can't hear this anyway unless you are a bat :-)

Discussions

naught101 wrote 02/05/2019 at 12:34 point

Damn, this is super nice. I would be keen to make up a couple. Is the current kicad stuff all working?

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