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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.
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