[Amps] IMD and driving power

[Rob & Terri Sherwood]

The Yaesu Mk V in class A, on the other hand is astoundingly clean. 
http://nc0b.com/documents/NC0B-Contest-U-2008-9.pdf

Joe Subich, W4TV wrote:
>
>   
>> As a general rule if you can reduce your amp driving power in 
>> half you gain a 3db reduction in the transceiver IMD.
>>     
>
> This is one of those "general rules" that is very wrong when it 
> comes to solid state transceivers.  Almost all modern power 
> transistors have a "saddle shaped" IMD curve.  When power output 
> is reduced significantly below the optimum point IMD increases.  
> If power is increased above optimum IMD increases rapidly.  
>
> For example, look at the 2SC2879 used in the Yaesu Mark V Field: 
> http://www.datasheet4u.com/download.php?id=71199.  The IMD vs. Power 
> Output curves show a "sweet spot" around 55 to 65 watts per transistor 
> before filter and transformer losses of about -38 dB.  With the losses 
> in the transformer, lowpass filter and antenna tuner, this represents 
> just about the transceiver's rated 100 watt output.  Decreasing power 
> to 40 watts per transistor (1.4 dB) increases IMD by more than 6 dB! 
> Increasing power output to 80 watts per transistor (~1 dB) results in 
> an 8 dB IMD increase and pushing to 100 watts (~2 dB) per transistor 
> results in a nearly 14 dB increase in IMD.  
>
> If your amplifier requires substantially less drive than your 
> transceiver produces you are better off using an attenuator on 
> its input rather than reducing the drive in the transceiver. 
>
> 73, 
>
>    ... Joe, W4TV 
>  
>
>
>  
>
>   
>> -----Original Message-----
>> From: amps-bounces at contesting.com 
>> [mailto:amps-bounces at contesting.com] On Behalf Of Robert Kirkland
>> Sent: Sunday, August 31, 2008 9:04 AM
>> To: amps
>> Subject: [Amps] IMD and driving power
>>
>>
>> As a general rule if you can reduce your amp driving power in 
>> half you gain 
>> a 3db reduction in the transceiver IMD. Less in, less out. Bob,w5pvr 
>>
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>>     
>
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