4046 VCO experiment

Martin Czech czech at Micronas.Com
Fri Aug 18 09:21:52 CEST 2000

I checked the data sheet. The R2 (pin 12) and R1 inputs don't look very
linear, these inputs are no virtual ground summing nodes.  Only the vco
input voltage pin is said to be linear, and the error is given for some
R2 R1 examples.

I don't think that the 4046 is well suited for the application.

A XR2206 can be operated with a current source, this is quite linear
and it can run up to severall 100kHz (1MHz is the spec limit, I think,
but things are getting worse at the edge).  Been there, works. You
get the nicest sine wave of all those function generator chips as well
(unfortunately not with tri at the same time). And of course a sqa also.

I don't think it is very expensive, give it a try.

Somewhere I have a pll regulated frequency synthesizer with XR2206
schematic, well the usuall quartz, decade divider (thumbwheel), and 4046
pll. The 4046 oscillator is not used there, just the phase comparator 2.


:::X-Originating-IP: []
:::From: "Hairy Harry" <paia2720 at hotmail.com>
:::To: czech at Micronas.Com, chordman at earthlink.net
:::Cc: synth-diy at node12b53.a2000.nl
:::Subject: Re: 4046 VCO experiment
:::Date: Thu, 17 Aug 2000 16:50:39 GMT
:::X-OriginalArrivalTime: 17 Aug 2000 16:50:39.0920 (UTC) 
:::Hi everybody:
:::Two thoughts
:::1) Check out the internal VCO schematic of the 4046... its in
:::ICAN-6101. Scott's circuit is still activating the CMOS Current
:::Mirror... so basically the operation is still the same.
:::2) Think about stealing the archetecture, but built with
:::discrete CMOS chips... maybe 4007 for the capacitor reset
:::section, and 4049UB for the rest... but absolute symmetry is
:::required... each branch must be formed from devices in the same
:::package. You need symmetry along the vertical axis...
:::(matched right/left hand). Then... replace the current mirror with a
:::discrete temp compensated current source.
:::H^)  harry
:::>From: Martin Czech <czech at Micronas.Com>
:::>Reply-To: Martin Czech <czech at Micronas.Com>
:::>To: chordman at earthlink.net
:::>CC: synth-diy at node12b53.a2000.nl
:::>Subject: Re: 4046 VCO experiment
:::>Date: Thu, 17 Aug 2000 09:38:03 +0200 (MET DST)
:::>The current source should be pretty linear but with some error at the
:::>low end. (Why not use a jfet precision current source?).
:::>I think the inner VCO circuitry is not published.  Linearity is not
:::>neccessary for most of the applications.  Who will demodulate FM with that
:::>chip anyway?  It seems to be fabricated in the very same CMOS process
:::>then anything else of this logic family.  So there are limitations on
:::>matching, mirrors etc. Certainly the internal integrator has low gain
:::>(but high bandwidth).
:::>Could you be more specific on the error?
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