- A good active linearization circuit can produce far better linearity
than class AB with 10dB of negative feedback, and even better than that of
tetrodes.
With a linearizer running as an adaptive predistorter, we can largely
dispense with amp class type at least within reasonable limits. Warren
Pratt has shown that with APD active in the RF exciter, even class B
amplification can produce better linearity than class AB amps, and many
class A amplifiers. A linearizer in the exciter with power sampling fed
back from the external amp produces maximum linearity as a complete system.
We have this available today, at least with the Apache ANAN series of
transceivers. The Zeus ZS-1 uses a manual linearizer but the end result is
the same. IMD reduction is not improved by some trivial amount. The
improvement is in the order of 30 dB for third-order products.
Under strong signal conditions, I've observed AC2IQ and WA1OXT with
completely vertical transmit passband edges. AC2IQ uses an analog
linearizer (see his QRZ page) while WA1OXT uses a firmware-based linearizer
in his ANAN 100D transceiver. Maximum improvement typically occurs near Fc
then gently degrades evenly on both sides of Fc. But maximum reduction
occurs where it's needed most: within +/- 10 kHz of Fc. Warren Pratt is
working on methods to minimize IMD spectra well away from Fc.
I own a SPE 1KFA SS amp and do not foresee adding another SS amp until: (1)
better cooling + noise reduction solutions are developed; and (2) we see
better output power regulation with changes in PA deck temperature. This
should be something easily managed with internal gain control, but yet I see
this power/heat variation with several SS amps.
Paul, W9AC
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