Jim,
## which broadcasters are using SSB ???
Only a few do, on short wave, most of them just outside ham bands. Most
broadcasters use AM or FM, of course.
I do take your point. High efficiency amplification is trivially simple
on FM, and on AM it's still simpler than on SSB. But high efficiency
amplification of SSB signals is far from impossible, and when using
modern technology it's actually practical. By "practical" I mean that
the added circuit complexity is more than outweighed by the simpler
cooling and lower power consumption, and that the added cost for
additional small signal components is _far_ outweighed by the lower cost
for the high power components, which have less demanding specs in high
efficiency amps! That applies to the required power dissipation of the
RF devices, the entire cooling setup, and the power supplies.
David,
Are you speaking of Class-E amps?
Indeed class E is one of the circuit configurations that can be used,
and in various different ways.
A basic, bare-bones class E amplifier without any additions is only
suitable for constant-amplitude modes, such as FM or RTTY. With a simple
shaping circuit it could be used in CW too. But with either
predistortion or with envelope elimination and restoration it can be
used in SSB. The former approach is very simple these days, using a
Software Defined Radio as the driver, and results in improved but not
spectacular efficiency, while the latter is somewhat more complex and
can produce nasty wideband noise if poorly implemented, but results in
best efficiency.
http://www.classeradio.com/sokal2corrected.pdf
Yes, that's a good article on basic class-E amplification.
Here's a switching design from the late 80's
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/radcom/1990/02/page30/index.html
The second circuit on that page actually operates as class E amplifier,
even if it's not explicitly mentioned. The term "class-E" became widely
used only a relative short time ago, but class E amplifiers have been in
widespread use for a long time. For example many VHF radio final stages
actually operate in class E, even if it's said that they are running
class C. As soon as you drive a class C amplifier into saturation, and
this amplifier has the proper output configuration (enough capacitance
in parallel with the transistor, and a matching circuit that presents a
high impedance to harmonics) you get class-E operation.
Such an amplifier is the basic building block. It can be used directly
for FM or RTTY, and stuff can be added around it to make it usable for
SSB. That "stuff" is either a simple RF feedback to the driving SDR that
implements predistortion, or it's a modulated power supply controlled by
the SDR.
It's also possible in principle to take a standard drive signal from any
100W transceiver and split it into phase and envelope components, then
time-delay the phase signal and drive the amplifier from it, while using
the envelope signal to control the switching power supply. But this
method is prone to various distortions that can't be easily corrected by
analog circuitry, and result in a low quality transmitted signal.
Because of the higher circuit complexity and the lower quality result,
this method really isn't convenient these days, with the SDR-based
methods being far better.
Fascinating ...
I agree. At this time I'm eagerly awaiting my SDR board, already
ordered, to finally start working on my pet project: A high efficiency,
legal limit Software Defined Radio!
Manfred
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http://ludens.cl
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