MOTM

HI Doug,

Thanks for clearing up this FM stuff. I, for one, would appreciate any 
information about linear FM in analog and especially the through zero 
approach that you mentioned (and, for example, if someone sent me a scan of 
related material, I'd be so ecstatic as too forget to call anyone's attention 
to any possible copyright issues). On a related note, I am currently looking 
at a schematic from Juergen Habile (from his JH-3 Dual VCO board built around 
two CEM 3340s) which shows a through zero LIN FM input, so it appears to be 
possible to have through zero FM with analog.

JB

In a message dated 8/21/99 7:06:58 AM, dkraul@... writes:

>Sorry, I'm a lurker (But Paul can vouch for me)

>Analog FM's limitations are actually more basic then what was discussed.
>why all of the points are valid the most fundamental limitation of trying
>to
>accomplish "DX-7 style FM synthesis) with conventional analog VCOs is that
>such a VCO is not capable of modulating "through zero frequency".  This
>limits the modulation index to the less interesting lower range.  For
>example, using a 1:1 carrier to modulator frequency ratio the maximum
>modulation index with a conventional VCO is 1.  Unfortunately if any of
>you
>remember your DX-7 experiences the most interesting sideband evolutions
>start occurring above this figure.

>The reason for this limitation has to do with some pretty esoteric technical
>details but in a nutshell most VCOs can not "reverse on themselves".  By
>this I mean that as the modulating wave drives the carrier's frequency
>toward zero Hz it can not force it into "negative frequencies".  And yes
>these really exist, it just a 180 degree phase reversal on the waveform.

>There is a way of making a VCO that is capable of this performance and
>is
>something I discuss if anyone is interested.  or you can just scan some
>old
>Electronotes issues in the issues around #63 I think for a good write up.