From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Tue, 23 Nov 2004 15:14:38 -0000
Message-ID: <000001c4d16f$260c1e80$e6a4c593@RD40002>
MIME-Version: 1.0
Importance: Normal
In-Reply-To: <41A3248D.19617.9BEB5D@localhost>
Subject: LF: RE: Linear amplifiers
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

Dear Mike, LF Group,

The amplifier does not have to be very linear for PSK modes - just so long
as you have some sort of ramp up and down to zero amplitude, rather than an
abrupt on-off transition, the spectrum of the signal will be reasonably
narrow. When I used a linear, I found the best bet was to bias the MOSFETs
so they were slightly cut-off, and then over-drive them a bit - more or less
class C operation. This improved the efficiency somewhat, but it was still
only about 60% at best.

For PSK generation at the moment, I amplitude-modulate the HT supply to the
Decca class D PA, whilst separately phase modulating the drive carrier. This
is much the same as the "envelope elimination and restoration" techniques
which now seem to be standard for big AM broadcast transmitters. I keep
meaning to write this up, but it is the usual problem of having too many
other things to do!

The advantages of PSK31 are that there is readily available software, the
bandwidth is narrow enough, and people are familiar with it - but it isn't
particularly well suited to LF operation; it works fine when the band is
very quiet, but QRN impulses corrupt characters each time they occur, so the
band does not have to be very noisy to render the mode unusable except for
stations very close together. I think there is a case for a similar mode to
PSK31, but incorporating error correction, so that a moderate level of QRN
corrupting a small proportion of bits does not cause a big problem. VE2IQ's
"Coherent" mode did this quite well, but requires MS-DOS and dedicated
interface hardware, making it less attractive.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Mike Dennison
Sent: 23 November 2004 11:53
To: rsgb_lf_group@blacksheep.org
Subject: LF: Linear amplifiers

When I was compiling the book LF Today, I could not find a 
reliable published linear amplifier design. Several people 
have mentioned recently that they are using linear amps. 
What circuits do they use? How do they ensure they are 
linear? What efficiency do they get?

Mike, G3XDV
http://www.apersonalguide.co.uk/lf