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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).<br>
<br>
Scott Bernardi wrote:<br>
<blockquote type="cite" cite="mid400FE096.6000600@comcast.net">
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If you were interested in turning this circuit into an exponential
through-zero VCO: <br>
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<br>
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?<br>
<br>
Daniel Araya wrote:<br>
<blockquote type="cite" cite="mids00e8ad7.042@gwia.sr.se">
<pre wrap="">Nice one!
<a class="moz-txt-link-freetext"
href="http://www.reed-electronics.com/ednmag/article/CA341451">http://www.reed-electronics.com/ednmag/article/CA341451</a>
/d
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<pre wrap="">Peter Blackett <a class="moz-txt-link-rfc2396E"
href="mailto:dragon.servicing@virgin.net"><dragon.servicing@virgin.net></a> 2004-01-21 14:12:21
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<pre wrap=""><!---->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 <a class="moz-txt-link-abbreviated"
href="http://www.edn-europe.com">www.edn-europe.com</a>
but I don't know if they have the article on line .
regards Peter
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<br>
<pre class="moz-signature" cols="72">--
Scott Bernardi
<a class="moz-txt-link-abbreviated" href="mailto:sbernardi@comcast.net">sbernardi@comcast.net</a></pre>
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<br>
<pre class="moz-signature" cols="72">--
Scott Bernardi
<a class="moz-txt-link-abbreviated" href="mailto:sbernardi@comcast.net">sbernardi@comcast.net</a></pre>
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