[sdiy] CMOS mystery

[mark verbos]

David,

I have encountered brand dependence in CMOS before. For me, it was with 4016 analog switches. Turns out the Motorola ones can handle a bigger power supply range, and it needed to be an A series one for this particular circuit. I can’t help you with why it should be any different in actual digital signals. But, I’m not sure you need to go through so much trouble to condition your opamp output going in i.e. using that zener. I just run the signal through a resistor and let the CMOS input stage handle it. 10k should do.

Mark




> On Jul 5, 2018, at 9:01 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> Hey Team,
> 
> I built a couple of "Perfect fifth generators" for someone.  They are pretty
> simple: you feed a 10Vpp triangle wave and an in-phase square wave in, and
> you get a square wave out which is a perfect fifth higher in pitch.
> 
> Here's how it works (note, this is not my idea): The triangle wave is sent
> to a comparator with a threshold voltage of about 1.67V, and this generates
> a pulse wave of 1/3 duty cycle.  The square wave is sent to a ripple
> counter, and this generates a suboctave square wave.  The 1/3 pulse wave and
> the suboctave square wave are fed to either an XOR or an XNOR gate, and this
> gives the perfect fifth.  Easy peasy.
> 
> I used a TL072 as the comparator, with a 10k output resistor dropping across
> a 5.1V grounded zener to generate a 0-5V 1/3 duty cycle pulse.  The ripple
> counter is of the 4024 variety.  For the XOR, I wired up a quad NAND gate
> (4011) in the time-honored way.  For XNOR, a quad NOR gate (4001) may be
> used instead.  I have used both and they both work well.  Ultimately I went
> with the quad NOR XNOR gate because I had more than one 4001 chip.  Both the
> 4024 and the 4001/4011 are powered from a 78L05.  Finally, the output of the
> XOR/XNOR is fed to the other side of the TL072, which is wired as a
> non-inverting amp with a gain of 2, and with the output level-shifted -5V
> from the 78L05 to give a perfect fifth square output which is 10Vpp and
> centred around 0V.
> 
> So, here's my problem: If I use either an MC14024 or a CD4024BCN for the
> counter, the circuit works perfectly.  If I use a CD4024BE, the circuit
> doesn't work.  The outputs of all three counters look the same on my scope,
> but the 4001 or 4011 outputs are all wrong for the one counter, and
> perfectly correct for the other two.
> 
> Do any of you "gurus" have any insight as to why this might be?  I thought
> that perhaps the 4001/4011 didn't like the zener-derived square wave,
> because it would be perhaps slightly higher than 5V.  However, the
> datasheets suggest that input voltages between -0.5V and VDD + 0.5V (which
> would be 5.5V) are OK.  Could it be that the zener is only limiting the
> "zero" voltage to about a diode drop below ground, and this is slightly
> below -0.5V?  If so, then why would the choice of counter chip make any
> difference?  The output of the counter is determined by the 5V supply (the
> 78L05), which is common to both CMOS chips.  Could the 4001/4011 be confused
> by the two slightly different levels of the 1/3 duty cycle square wave and
> the suboctave square wave?  If so, why would the counter chips be giving
> different output voltages if they are all supplied by the same 5V supply?
> 
> I'm stumped.
> 
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