[Amps] HV MOSFETs for RF

[Jim Thomson]

Date: Tue, 9 May 2017 07:59:45 +0930
From: "Leigh Turner" <invertech at frontierisp.net.au>
To: "'AMPS'" <amps at contesting.com>
Subject: Re: [Amps] HV MOSFETs for RF


<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

##   leigh, no class A  amp runs a  lousy 8.9%  eff.   I see that the new OM-600  SS
600watt amp is the only ss amp on the market without an ALC jack on the back of
the amp.  The average operator will have fun times trying to get his 100-200 watt xcvr..
.....down to just a few watts to drive it.   Dunno if henry radio is still selling their pads
or not.   Ideally, you find the imd sweet spot on the xcvr..then insert the correct pad
for the SS amp drive level.   A k3s  sweet spot for imd is 30 w pep out.  A 3- 6db pad is
then used, so 7.5 - 15 watts to drive the ss amp. 

##  what do you call acceptable IMD ??   I mean well away from the other fellows freq. 
And lets not make the classic mistake of adding the RX BW to the TX BW.   IE:
Tx on say  3800  LSB.  RX station listens on  3803 + 3797..and thinks the TX station is  3 khz up,
and  3 khz down...using his 3 khz wide ssb filter. If the RX station had used a 200 hz filter,
he would hear the TX station down 3 khz....  but only 200 hz up. 

##  baffles me why I / K / Y cant produce ham gear with IMD thats as good as their marine gear.
Sure, the marine ssb gear does not have a mic gain control + processor,  but its still tested  with
a  2 tone signal, same as the ham gear.   Its not like its gonna cost them more $. 

## The old  yaesu 767  ham xcvr is a magnitude better than modern ham gear.  50%  eff, and
100 w pep out from a pair of MRF-422s.  My 1000-D uses the same pair of MRF-422s..and runs
200w pep out at 50%.    And  100w pep out  at 35%  eff.   So what happened there ? 
The 767 was designed to operate at 100w pep out + 50%..and real low IMD, like –46db pep IMD3 
The 200w yaesus  have good imd...at 100w out.... but then eff sucks..at 35%. 
They already had it right the 1st time..with the 767.   If they wanted a 200w xcvr, they should used
4 x MRF-422s. 

Jim    VE7RF