Ang: [sdiy] Moog/Bode type frequency shifter design in EDN europe magazine

[Scott Bernardi]

To answer my own question .... I thought about this on the way to work 
and realized that although it may be a practical design for a frequency 
shifter, it wouldn't work as a through-zero dynamic depth FM VCO. The 
reason is that normally you have an exponential converter running around 
a DC operating point which is the reference current, and you inject an 
AC signal to perturb it from that operating point for FM. With DC 
coupling and no "operating point", the reference current depends 
directly on the modulating signal amplitude. As the modulating signal 
decreased in amplitude, your VCO frequency would go down also, rather 
than just the deviations from the center frequency. You would need a 
precise constant amplitude modulation signal (so no dynamic depth).

Scott Bernardi wrote:

> If you were interested in turning this circuit into an exponential 
> through-zero VCO:
> Instead of driving linear voltage to current converters, you could 
> drive the linear FM inputs of two expnential converters; i.e., on the 
> servo opamp that usually drives the emitters of the exponential 
> transistor pair, you would DC couple the modulation signals, one 
> inverted and one non-inverted, into the inputs of the servo opamps 
> where the voltage references usually go. If you used a quad pnp array 
> like the THAT120 for the exponential pairs, the exponential tracking 
> should be pretty good between the two current sources. You would have 
> a single CV summing amp that would drive both of the exponential 
> pairs.  The rest of the quadrature circuit would be the same as in the 
> article
> I think this would give you the benefit of the smooth through-zero 
> transition that the article purports, but with an exponential CV 
> response.  Does this make sense or am I missing something?
>
> Daniel Araya wrote:
>
>>Nice one!
>>
>>http://www.reed-electronics.com/ednmag/article/CA341451
>>
>>/d
>>
>>  
>>
>>>>>Peter Blackett <dragon.servicing at virgin.net> 2004-01-21 14:12:21
>>>>>
>>>>>        
>>>>>
>>Hi,
>>I just thought you might be interested to know that in the latest
>>issue
>>of EDN Europe , Jan 2004 ,
>>there is an article in the design idea's section for a VCO that
>>produces
>>positive and negative output frequencies.the article mentions that
>>this
>>is traditionally done in analog music effect units such as they
>>Bode/Moog frequency shifters.
>>Article is written by Henry Walmsley from Farnborough .
>>edn's web address is www.edn-europe.com 
>>but I don't know if they have the article on line .
>>regards Peter
>>
>>
>>  
>>
>
>-- 
>Scott Bernardi
>sbernardi at comcast.net
>

-- 
Scott Bernardi
sbernardi at comcast.net

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20040122/2362714a/attachment.htm>