4046 PLL and Differential Amp question

gstopp at fibermux.com gstopp at fibermux.com
Tue Mar 11 19:18:11 CET 1997

     An interesting idea so I went and tried it...
     All's you need is a simple buffer on one of the cap pins (6 or 7, same 
     waveform on both). I used a TL082. The pin 11 resistor is 10K, no 
     resistor on pin 12, 0.1uF cap across 6 & 7. The presence of the buffer 
     did not affect the frequency.
     The waveform is a little unusual - it looks something like this:
                    /|                     /|
                  /  |                   /  |
                /    |                 /    |
     _________/      |   ____________/      |   __________ <--- ground
                     |  /                   |  /
                     |/                     |/
     It sounds somewhat like a sawtooth, but seems to have a sub-octave 
     tone along with a bright buzz. I like it!
     - Gene
     gstopp at fibermux.com

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Subject: 4046 PLL and Differential Amp question
Author:  chordman at concentric.net at ccrelayout
Date:    3/11/97 8:54 AM

One of the failings of using a 4046 PLL as a frequency multiplier is 
that it's output is square.  I have an idea to get at least one more 
waveform out of it.
The capacitor connects to two pins of the IC, neither of which is 
ground.  That means the cap floats and a single ended approach would 
probably not work.
First, I am assuming that the cap's integration voltage is not square.
If a high input impedance differential amplifier were used, it might 
be possible to amplify the capacitor's integration voltage and supply 
a single ended output.  Do any of you have part type suggestions?  I 
am sure it will need to be a FET type at least.  Are there MOSFET 
input opamps?
Or would the S/N ratio be so bad that this would be impractical?

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