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Synthesizer Chips Relived
Here's my short preliminary review of recently made VCO chip from Alfa semiconductor fab in Latvia, which aparently is redesigned functional equivalent of CEM3340.
First let's look at the waveforms and compare them to (also very recent) CEM3340 Revision G from Onchip. All tests were performed on the same PCB, by swapping the chip in socket, and here is the test circuit:

The circuit adds an amplifier on triangle output to match the amplitude to saw output. Also square output is a bit attenuated to go down to the same level, but as it turned out, not enough. Scopeshots below are outcome of quick test made with DIY power supply capable only of about 11.2V, so the amplitudes are considerably lower. At regular 15V supply the saw and triangle are 10Vpp, and square is 12Vpp. This is to be fixed later of course to have square wave at 10V too.
low frequency triangle
low frequency saw
low frequency square
please ignore kilohertz reading here, the scope was fooled by ringing instability of fallng edge of the square wave, obviously this is the same 10Hz and 5Hz accordingly as above
high frequency triangle
high frequency saw
high frequency square
saw falling edge
saw mid-hill bump
Because of how tri-to-saw converter works, there is inevitable bump in the middle of saw rising slope. Here it is shown in details
saw mid-hill bump zoomed
triangle low corner at low frequency
Above screenshots of triangle were made after the opamp, so it hides some of the minor artefacts of how internal triangle looks. Here are 3 examples of direct triangle output from the chip. This is the most noticeable difference in waveforms between the chips, however the glitch is only 120mV at worst case.
triangle low corner at medium frequency
triangle low corner at high frequency
triangle top corner

Now let's focus on frequency stability. This is just preliminary and non-scientific measurement, as temperature was not actively controlled, but was slowly rising due to self heating of the circuit in closed thermally-isolated cage, and also heated by warm power supply under the box. The temperature profile was measured by logging thermometer and then superimposed on frequency measurements based on timestamps. The VCO board was literally laying on the termometer. However, as it is ambient temperature measurement and not chip temperature, the gradiend had to be very low in order to give it a chance to track temperature with a glimpse of assumption that it is somewhat acurate. Which also means that it was enough to log temperature reading as rarely as once per 5 minutes. Only Alfa chip was tested this way to get the impression of general stability.
During those tests power supply (not the same as before, so it was supplying +15V and -5V this time) was monitored at the beginning and end of each cycle and it drifted only by about 300uV, same applies to the CV input. So basicaly the influence of power supply drift was below 1 cent.
219Hz drift during 3 hours and 7.5C temperature change
27Hz drift during 30 minutes and 1C temperature change
4kHz drift during 30 minutes almost 1C temperature change
temperature dependence of frequency measured during 2 hours test
So it looks like AS3340 works pretty well, with only slight waveform differences to CEM3340(G) that only purist geeks could notice.
A little disapointment about temperature stability - roughly 550ppm, which is rather poor and unacceptable by today's standards.

The next day I took CEM2240(G) for a ride and here is the result. Only one graph, because it looks terribly bad - calculated tempco is about 1500ppm, hard to believe! So the measurements need to be repeated in more controlled conditions.
Anyway, this time I took less frequency measurements, only 6 per minute, but more temperature readings - one per minute. It makes the diagram less steppy and noisy.
220Hz drift of CEM3340(G) during 3 hours and 14C temperature change

During proper test I hope to achieve better results using different selection of components.
Till then I have to do a thousand other things, but let's hope this is going to be continued soon.
Hope you enjoyed it the way I did!
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© 2017 Roman Sowa
Last revised November 28, 2017