[sdiy] VCO reference voltages

[George Hearn]

Voltage references can be very cheap and I like using them instead of the
power rails when I can for critical VCO voltages (threshold for reset, expo
current bias etc.).  In my VCO modules I use an LM4040, 39p for peace of
mind and independence to the V+ power rail over a range of 12-15V.  You can
use an LM4030 which is even better, but I found it not necessary.  

The module; http://sites.google.com/site/hearnmorley/products/hmvco1b


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Simon Brouwer
Sent: 16 April 2010 11:31
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] VCO reference voltages


Oren Leavitt schreef:
>
>
> David G. Dixon wrote:
>> Does anyone here have any strong views on deriving reference voltages
>> from
>> the power supply rails in VCOs for comparator or schmitt trigger
>> thresholds,
>> etc (as opposed to using precision references)?  I know that most "DIY"
>> VCO
>> designs do so.  I'm wondering whether most commercial VCOs also do so,
>> and
>> whether it is considered "kosher", even in more expensive VCOs.
>>
>
>
> Me-self:
> I greatly prefer on-board voltage refs for the "critical" voltages - the
> Schmitt trigger threshold, expo converter reference current,

Note that if these both are derived from a non-stable reference voltage
the effect on frequency stability on the one will be compensated by that
on the other. You will only get a small difference in amplitude.

> the "rails"
> for any coarse/fine tuning controls,

A quick calculation: the voltage regulation of an LM7815 for output
current variation between 250..750 mA is 50mV (max) which is 0.33% of 15V.
Assuming 0V=middle of the audible VCO range then this corresponds to ca.
20 cents error at the top or bottom.

Using a 78L12 for local voltage reference could be a cheap option. Lightly
loaded and supplied by a stabilized voltage, line and load regulation are
hardly an issue; remains a ca. 0.008%/degC voltage drift.

-- 
Vriendelijke groet,

Simon Brouwer

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