[Lf] PSK sidebands

[Andre' Kesteloot]

Vernall wrote:

> Andy G4JNT and others, Some comments on grappling with PSK
> keying clicks: >I have been considering the key click
> problem when using BPSK with switching transmitters.
> Traditionally, amplitude shaping is >used to limit the
> transmitted bandwidth of PSK signals, but this is not a
> valid solution for class E Txs . Dave's idea of going part
> >way there by ramping the PSU to the driver chips to
> generate Pulse Width Mod being only a temporary solution.
> PWM of the >drive waveform does not linearly control the
> amplitude of the RF component which varies with pulse
> width in a SIN(X) / X manner.>Has anyone any experience of
> systems that ramp the phase slowly in lieu of the
> amplitude ?    My feeling is that this will give a
> >similar result to amplitude shaping, but the text books
> don't cover PSK bandwidth control using a constant
> amplitude waveform. Class C, D or E, or any other
> nonlinear power amplifier, is fundamentally a wrong choice
> for handing a PSK signal.  There will inevitably be a
> "jump" in waveform at conduction threshold and cuttoff.
> As Class E is specifically designed to spend least time
> switching between "full on" and "cut off" then clicks
> (transient sidebands) then clicks occur at all phase
> changes.  Even if all driver stages are nice and linear,
> the PA will "digitise" it to on and off.For amateur
> operation, one way could be that key clicks are
> acceptable, for high efficiency power amplifiers, and
> weave this in to a band plan, having a segment for "dirty
> transmitters".  Those who generate key clicks should be
> happy to exist alongside other clickers.Another approach
> could be to develop a more sophisticated power amplifier,
> with a Class AB1 section to handle all small signals, and
> Class E for higher level parts of the emission.  In New
> Zealand we have enough bandwidth to run SSB on LF, so
> anyone building a big amplifier would look at including
> linear mode of operation.  I am contemplating building an
> amplifier with n+n pushpull transistors, with at least one
> pair working in Class AB1 and others in unbiased AB2
> mode.  The value of n is say 10 or 20 (no mucking around,
> at least 1 kW is a target output).  The idea is that an
> AB1 stage is active at all times and easily handles low
> level, but when a bigger level arrives it is sufficient to
> automatically commutate the AB2 devices.  I have already
> run my existing bipolar amplifier with no bias and it
> still gets readability 5 reports on SSB, even though a
> local can notice a moderate drop in quality and dynamic
> range.  When hard driven in CW mode, the efficiency is
> very high (much better than theoretical Class C).  I have
> a good stock of bipolar switching transistors, so I have a
> preference for using them rather than power MOSFETs, but
> the suggestion of having mixed classes of amplification in
> a single final unit should be able to be contemplated.
> There could even be a mixture of bipolars and MOSFETs, or
> other devices, sharing the same power supply, so long as
> an AB1 stage can operate in linear fashion.  There could
> even be different backoff in bias for clusters of devices,
> to get piecewise cut-in and cut-out over the drive
> waveform.  Some questions are:- how many of the devices
> need to be biased to give sufficiently linear operation to
> reduce PSK key clicks to a satisfactory level?- is the
> phasing of Class AB1 and Class E output signals such that
> the AB1 devices do not get interfered with, or fried?-
> does any reader have large signal modelling software to
> assess the viability of the suggestion?73, Bob ZL2CA