I have been poking around on Internet looking for a scholarly article or two
on generating SSB using switching techniques for the RF generation. It
appears that GaN MOSFETs are the promise for generating the 2 - 30 MHz RF,
and I found some hints on how combining two RF streams with amplitude
modulation on one, and phase modulation on the other can produce any
modulation type, but no detailed information on anyone who has actually done
it, or the math involved for that matter.
If anyone has a link to some information on this subject, please post
it/them...
Thanks,
-Doug Ronald
W6DSR
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Wednesday, February 22, 2017 08:44 AM
To: amps@contesting.com
Subject: Re: [Amps] SS amps watercooling - was PowerGenius XL
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/hamblade
> n/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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