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<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>Hello SDIY
Superfriends,</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>Yesterday, I
finished up my first bought-and-paid-for Sketchy Labs 5U Frequency Shifter, and
upon testing it, found that the dual-core TZFM VCO was generating sine and
cosine waves with very different amplitudes.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>The amplitude
control is achieved in each core with a single zener diode inside a "zener
bridge" consisting of four 1N4148s. Since the 4148s drop approximately
0.5V, a 3.9V zener (1N4370) should give a total drop of roughly 5V, which should
guarantee waveforms which are 10Vpp.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>One was about right,
but the other was way low. I replaced the zener of the low one, and it got
worse. So then I decided to actually measure some zeners. After much
grumbling, the bottom line is that I had to delve into my bag of 1N4372s (4.7V)
before I found two zeners that read almost exactly 3.9V. They were all
very low relative to their nominal values. (I tested them by putting a
3.3k resistor between +15V and the cathode, and connecting the anode to
ground. I use 3.3k resistors on the comparator in my VCO
core.)</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>So now I have
decided that I have to measure each and every zener diode, and separate them
into plastic bags based on their actual voltages rather than their
denominations. In this way, after a certain amount of tedium, I will have
a collection of precision zener diodes (within +/- 0.05V). However, I
shouldn't have to do this. I would expect zeners to be off by 0.2 (or, at
most, 0.3V) from their nominal values, but to cross over one or two (or
even three) denominations? That is unacceptable. What are these
zener diode manufacturers doing wrong?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=693325219-30062020>Cheers,</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=693325219-30062020>Dave
Dixon</SPAN></FONT></DIV></BODY></HTML>