[Amps] HV MOSFETs for RF

Wed May 10 01:59:50 EDT 2017

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 at contesting.com] On Behalf Of Peter Voelpel
> Sent: Tuesday, 9 May 2017 3:18 AM
> To: amps at 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 at 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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