[Amps] Measuring IMD in a home workshop

[Martin Sole]

I read also that the K4 internal PA will also support pre-distortion at 
some point. Now being a software radio and being Elecraft it might not 
have this on first shipment but being a software radio and being 
Elecraft I have a high degree of confidence it will appear at some point.

Martin, HS0ZED


On 28/05/2019 04:23, Bob Wilson, N6TV wrote:
> The new Elecraft K4 XCVR announced at Dayton will support the
> pre-distortion concept when connecting the KPA1500 "TX SAMPLE" output to
> the K4 TX SMPL IN connectors.
>
> 73,
> Bob, N6TV
>
> On Mon, May 27, 2019, 6:10 PM Jim Thomson <jim.thom at telus.net> wrote:
>
>> Date: Wed, 22 May 2019 14:23:12 -0700
>> From: "Bob Wilson, N6TV" <n6tv at arrl.net>
>> To: Amps Reflector <amps at contesting.com>
>> Subject: Re: [Amps] Measuring IMD in a home workshop
>>
>> <The Elecraft K3 also has a built-in 2 TONE generator.
>>
>> <Switch to SSB mode
>> <Set *CONFIG:TECH MD ON*
>> <Set *CONFIG:2 TONE ON*
>> <Exit the menu and tap the *XMIT* button.  You can vary the amplitude of
>> one
>> <of the tones using the MIC knob; the other is fixed.
>>
>> <73,
>> <Bob, N6TV
>>
>> ##  Nice try...it wont  work !    The  IMD  from a K3  is an abysmal –24
>> db  below one tone....
>> which is on a par with  sweep  tubes from the  70s.   All  that the  2
>> tone generator thats  built into the
>> K3  will  achieve... is an easy method to measure the  K3s  IMD, thats
>> it.
>>
>> ##  IE:  to test the IMD  from a 1.5 kw linear, the driving source has to
>> be a LOT cleaner than the amp under test.
>> You cant use the  2 tone  /  100  w pep out from a K3,  to test  IMD on a
>> L4B amp with its  2 x 3-500Zs.
>>
>> ##  something like an  ANAN  7000 / 8000, using pure signal will  work.
>> They are typ  -70 for  IMD3 +
>>
>> ##  So will a yaesu  running Class  A.    Class  A  is very clean. –70  to
>> –85  for  IMD3+
>>
>> ##  another method is  to use  two xcvrs,  each outputting a CW  cxr....
>> then combine the  2 x signals
>> with a hybrid combiner.   2  tone spacing would be equal to the
>> difference between the  2 x VFOs.
>>
>> ##  My problem with the  2 tone method is  everything is In a steady
>> state.... like  B+, bias, drive,  ALC, etc, etc.
>> IE:  all loads are static, and not dynamic...like normal  ssb.
>>
>> ###  Having said all that,  I have  two  FT-1000-MK-V’s.     I TX  on one
>> of them.... while listening a bit off freq  on the
>> 2nd xcvr.   I then  toggle the  TX  between Class  AB.... and the
>> cleaner  class  A..... and the  difference in imd / splatter
>> while listening on a 2nd  xcvr is blatantly obvious.   I tune  2nd xcvr
>> just off freq enough, so no splatter is heard.... while using
>> Class  A  on  tx.    Then swap to  class  AB... and yoza.... splatter
>> galore.
>>
>> ##  Unless the xcvr is something like the  ANAN, using  Pure  signal,,,,
>> or Class  A  with the yaesu,  its all a moot point really.
>> Any decent linear  TUBE  amp  will have far better  IMD  vs the  typ
>> xcvr... like the K3..and  every other  xcvr out there.
>>
>> ##  Most  SS  xcvrs  will have cleaner  IMD  when the PO  is reduced a
>> bit, and depending on drive required for  the linear amp
>> under test, that could easily affect results.   Big difference between
>> 100w drive to a 2 x 3-500Z  amp... and  40-50  watts  into
>> a  8877
>>
>> ##  SS  1.5 kw amps  are a whole  different ballgame.   IMD is typ not
>> good, on older models..when run at max rated power.
>>
>> ##  still another method is to monitor the Tx  ssb  signal on a spectrum
>> analyzer...or sdr rx.   Assuming a semi clean xcvr,
>> you can then see how wide the  ssb  signal is...when barefoot..or with
>> external linear amp on.
>>
>> ##  yaesu, kenwood,  icom,  elecraft, all need to get with the program,
>> and implement some form of  either analog or digital
>> pre-distortion techniques in future  xcvrs.   Then the sample can be taken
>> from the  1.5 kw  amp itself... fed back to the xcvr,
>> the the  pre-distortion  concept  will correct for non linearity  in the
>> external amp as well.   Folks using  ANAN  xcvrs   driving
>> external  linear amps  are all having good success.  IMD on the external
>> amp is much improved.
>>
>> ##  Big difference between an  ANAN   7000 / 8000  with its  -80 db  imd
>> vs  -24 db  imd  from a K3.   Apples +  Oranges.
>>
>> Jim   VE7RF
>>
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