> I believe the external carrier version of the Serge FSOne thing I always wondered about the external carrier
> is more expensive because a second PDN is needed in
> order to generate a quadrature signal from the single
> carrier input signal.
version is the lowest possible frequency shift.
PDNs have a lower corner frequency by definition. The
ideal hilbert transform is non-causal, so you need a delay
to get the desired phase shift for a certain bandwidth, and for
the lower corner frequency approaching zero, you'd need
infinite delay.
With an internal quadrature VCO it's possible to go to very low
frequencies (or even thru zero), but an external (non-quadrature)
signal needs a PDN and is therefore limited in lower range.
(The PDN does not hurt on the "program" input, because the
audio frequency to be shifted is limited by the range of the human
ear anyway.)
The fact that Serge's FS with external input is more expensive than
the internal VCO version gives hope that they designed a PDN with
very low corner frequency for the external input. I have no idea
where this corner frequency actually is located, however.
Any information ? Just audio range (16Hz) or 1Hz (difficult ...)
or even lower ?
JH.