On 5/3/2013 8:11 PM, Tom Thompson wrote:
As long as the transceiver manufacturers give us transceivers with poor
IMD, it does little good to get the IMD on the amplifier good, unless
you home brew your own transceiver.
Tom W0IVJ
On 5/3/2013 8:39 PM, Leigh Turner wrote:
That's right Bob, complex indeed. Such linearization and intermodulation
distortion correction techniques are not the panacea they might first
seem;
their practical implementation in ham-radio gear is not trivial and the
costs generally outweigh the meagre gains and benefit. As Paul points out
the other impediment for commercial equipment manufacturers is the
complex
IP and licensing minefield.
It's my understanding that the analogue version is relatively simple and
good for about 10bd decrease in IMD, beyond that it gets complicated and
expensive.
With exciters running on the order of 30-35 db a 10 db increase to 40 or
45 db would certainly seem worth while. With the better the exciter the
better the amp output so the FTDX 5000 in the 40 db range (AB1) as well
as the new Kenwood would produce amp outputs with around 50 db IMD which
sure sounds like a worthwhile goal. Admittedly we are talking expensive
rigs in the $5000 to $800 range to drive an almost equally expensive amp
but QRO amps with 50 db IMD would be practically unnoticed except for
strong, clean signals.
More than 10 db requires complex digital circuitry, but sooner or later
some one is bound to develop a system on a chip or 2, or 3 with
relatively low cost.
Am I missing something?
73
Roger (K8RI)
For the average ham it's far easier to run an amplifier conservatively to
steer well clear of the amplifier's compression transfer curve where the
gain and Po depart from a linear relationship with input power, i.e.
practice power and drive back-off to attain maximum linearity.
The majority of common ham-radio amplifier tubes respond nicely to this
simple approach and yield sufficiently adequate IMD characteristics when
deployed in a well designed amp.
Compensation techniques to linearize an imperfect amp abound, but are not
simple to implement in a 1500 Watt PEP Tx system.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Robert
Carroll
Sent: Saturday, 4 May 2013 11:37 AM
To: amps@contesting.com
Subject: Re: [Amps] Pre-Distortion Linearizer
As a former Bell Labs design supervisor in the cellular wideband linear
amplifier design area, I can say that predistortion schemes as well as
feedforward linearization were being studied intensely and resulted in
many
patents at BTL and elsewhere. It was a very complex problem both
practically and theoretically. The simpler means of incorporating
predistortion quickly became inadequate to meet FCC requirements as
mobile
telephony moved away from its FM roots.
Bob W2WG
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Paul
Christensen
Sent: Friday, May 03, 2013 8:35 PM
To: amps@contesting.com
Subject: Re: [Amps] Pre-Distortion Linearizer
Check out US Patent 4588958 by Allen Katz K2UYH. It's a common way to
linearize solid state PAs for the cellular market back in the day.
Great info, Jeff. Looks like K2UYH has multiple, related linearization
patents. Also, thanks for passing on the references.
In looking at his patent citations, work really accelerated around the
time
of the cellular industry explosion. I imagine AT&T/Bell Labs was faced
with similar spectral efficiency issues with its Long Lines carrier
microwave systems, but they had more usable licensed spectrum way up
at 6 &
11 GHz where fixed, point-to-point communication is possible at those
wavelengths -- but not roving communications.
Often when the subject of pre-distortion comes up, the usual questions is
"why doesn't manufacturer X have this technology?" Peering through the
patent data is one reason why. It's easy for us to want the
technology --
not so easy for the manufacturers to wade through the intellectual
property
landmines or secure licensing agreements.
Paul, W9AC
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