AW: EMS VCS3 Ring Modulator - how does it work?
Juergen.Haible at nbgm.siemens.de
Fri Jul 17 12:27:06 CEST 1998
> Maybe Juergen could comment on that?
Almost everything was said already.
The circuit is very similar to the standard AC coupled 1496 application,
and I have used a 1496 in my clone.
Bias currents for the VCS3 are far smaller than suggested for the 1496,
but the 1496 works with these values, too.
The two additional trimmers are used to cancel bleedthrough at the
*double* carrier and modulator frequency. (In addition to the usual trimmers
that provide carrier and modulator suppression.) I'm not sure if this was
to compensate unsymmetries of the modulator, or crosstalk from the matrix.
You can implement these additional trimmers in a 1496 circuit, too.
The output is single ended, i.e. not the usual differential amp after the
cell. Only a single transistor to amplify one of the two collector paths.
Works fine. May be chosen because of economy, and may affect PSR.
If you look at the EMS diagram and the 1496's internal circuit, it's clear
how to to the replacement. Keep the dc bias levels from the original, and
everything is fine. The bias current generation, and the bias current offset
for the two legs is different, but it should make no difference. Just head
the same overall current thru the gilbert cell.
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