STAR LO Buffer Amplifier
The PICASTAR uses a single transistor stage to buffer the DDS
Reconstruction Filter and feed the required signal level to the 'Magic
In this section will eventually be an analysis of the amplifier,
particularly with a view to achieving an input impedance much greater
than the original 20R or so, to determine whether it is possible to
reach a reliable figure of 200R for more directly terminating the
original reconstruction filter and avoiding the signal loss in the
original terminating and matching resistor.
If successful, this could be an alternative to the reduction to 50R of
the filter impedance. However, the original filter characteristic, with
reducing attenuation of DDS image frequencies, would not change. Since
those who are likely to be in a position to re-work LO components can
just as easily change the filter impedance, they may as well build the
50R filter and kill two birds with one filter.
The PICASTAR buffer amplifier is a clone of the design which originally
appeared in QST of November 1981, Hayward & Lawson's "Progressive
Communications Receiver", with identical topology and component values.
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