I disagree with the assessment of the drive for low IMD, mainly because
it does not identify low transmitter IMD, compared to realistic numbers.
There is no excuse for today's commercial rigs producing IMDs worse than
the old Collins S-line of -50 db in the 1960s, which I consider
realistic. We really need to define the line between good, poor, and
what is practical. IMD does need to be lower than most ham rigs built
since the introduction of the inexpensive rigs of the late 60s to early
70s (solid state?) and we don't need class A to get there. I see using
class A as a crutch, no, more like the walkers you see around the homes
and facilities for the physically handicapped! I see today's crowded
bands as a reason that low IMD is more necessary keeping in mind that
the noise floor on almost any HF band is due to devices outside of the
ham bands. We have to start somewhere even if it's one station at a time.
There is no legit reason for reducing the crud generation below the band
noise floor in a quiet location and with all the crud generating
appliances that floor is going to continue creeping up, possibly at an
accelerated rate. "I think" that it's quite likely that manufacturers
find it's more economical to deal with interference on an individual
basis. After all, we are such a small segment they big companies would
likely find it cheaper to sell many thousands of cheap devices, like
wall warts and PWM even if they had to pay every licensed ham many
thousands of dollars.
The general public...Or maybe I should say, most people, including many
engineers have no grasp of how things work outside of their nitch. Now
days even the percentage of hams that have little, if any understanding
of STEM subjects is getting quite small. STEM? How often do they
maladjust / set up, their rigs from the get-go? Seems they can't be
bothered following the manufacturer's set up instructions for the audio
chain consisting of mike gain, ALC, and compression. Many engineers I
worked with in the past had very little mechanical aptitude. There were
EEs that understood electronics (in their field) but no practical
knowledge on how to actually apply that to the real world outside of
their normal work. I've mentioned before about their inability to grasp
how to use four individual lines to control more that 4 valves even
though those four lines were BCD. It took less than $20 and less than a
hour to build a decoder plus optoisolators that would control 10 valves.
Sure, we can get there (relatively good reduction in IMD) with
predistortion. It will even clean up the signal (a bit) from a mediocre
rig, but many rigs produced over the last 4 or even 5 decades are beyond
the capability of even dynamic predistortion. With predistortion an
amplifier can actually put out a cleaner signal than the driving rig.
Sadly, those poor rigs will be trashing the bands for many decades to
come. I've seen a number of complaints on top end rigs with tremendous
dynamic range and steep filter skirts. The complaints blame their
megabuck receiver for hearing stations, 5 or 10 KHz up or down from a
station 20 KHz wide. They not only blame their rig, they could not
understand the explanations given to them by quite a few posters
One of the biggest problems is the new rigs that give the user access to
too many parameters, allowing them to trash an otherwise good signal. In
those cases improved IMD, or any other refinement except a power failure
will fail to improve the signals from those stations. With self driving
cars, maybe we need self configuring rigs with the only control being an
on/off switch!
73, Roger (K8RI)
On 5/8/2017 Monday 6:29 PM, Leigh Turner wrote:
Good point Peter; not an appealing scenario for your average punter...where
pragmatism and practicality rule the day.
Perhaps limiting the Class-A PEP to 10 Watts and having a PA bias mode
switch would be a good compromise for keeping the purists happy.
I think there's an unwarranted obsession with low transmitter IMD numbers in
the non-channelized HF ham bands where above a certain respectable number no
practical advantage is realized. In many cases poor Tx signals heard on air
are attributed to bad operator practice and maladjustment rather than
inherent transceiver design issues in respect of IMD; although a few
notoriously bad rigs are out there.
73
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Peter Voelpel
Sent: Tuesday, 9 May 2017 3:18 AM
To: amps@contesting.com
Subject: Re: [Amps] HV MOSFETs for RF
Its not that simple.
My class A amp does 25W and consumes 280W continuously.
Who would buy a K3 then?
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
## + 40 dbm = 10 watts. 10 watts in Class A would be the simplest way to
achieve low IMD products.
Perhaps Elecraft should think of doing just that with their K3 series of
xcvrs.
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