Question about calibrating the vco

stilz · Joined: Jun 22, 2010 Posts: 13 Location: Germany Eschwege

Hi,

i have a question. When i build my vco from yusynth i didn't remember that there are more then one way (hfe, vbe, etc.) to match transistor. So I only match them with my integreted hfe dc-meter. Now I have the problem that I can' t get a acceptable sine wave. I change the value of R23 like Yves descripes it. But it doesn't help Sad

Is it possible that the "bad" matched bc547b's cause a bad expotential converter which makes it impossible to get a good sine wave?

I tried the calibrating seqeunze so many times ...

Many thanks stilz

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

The matching of the BC547 only have an impact on the VCO tracking, not the waveshape. Can post oscilligrams of the sawtooth, and triangular wave you obtain ? (indicate the voltage scale as well).

The sinewave shape depends on the good matching of the diodes and on the symmetry of the triangle. It also depends on the power supply, are you using -15V/+15V ?

stilz · Joined: Jun 22, 2010 Posts: 13 Location: Germany Eschwege

Good Morning,

i use two regulated power supplies like in the image this to get the + / - 15 V.
Voltages are -14.98 and + 14.98
I can't get a real "edge" with the triangle and saw wave it alwase like \_/ instead of \/
I removed the 82K as the wave looks better.
Any ideas?

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yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

From what I see (oscillogram) the overall swing of the triangle wave and sawtooth wave is 0.6Vpp while it should be something like between 8 to 10Vpp.
Are you sure you scope system is correctly calibrated ? Do you use a special hardware or the sound board of your PC ?
The sawtooth should not show such overshoot at this frequency are you using a probe (is it calibrated ?)

stilz · Joined: Jun 22, 2010 Posts: 13 Location: Germany Eschwege

You are right I use my soundcard to adjust the wave shapes. I use a multimeter do measure the voltages (-5V, +5V). I think I need to get a better multimeter or better a oscilloskope. mh...

stilz · Joined: Jun 22, 2010 Posts: 13 Location: Germany Eschwege

Hello,
I upgraded my test environment. I buy this old scope from the ddr for almost 30 bucks Smile

And a new dc-meter with auto range and with 0.6% error tolerance.
I order a new probe, when the probe is arrived I will continue here...

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new multimeter with autorange, hfe, cap...
and my *new* old scope

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