[Amps] How to drive 6M 8877 with DownEast 50-28 ... IPA?

[jeff millar]

I didn't see this on the list, so here it is again.

Tom: did you get the original? ...or did my email mess up?

jeff, wa1hco

jeff millar wrote:

> Tom...
>
> IMD in almost all linear amplifiers follows the 3:1 rule...for a 1 dB
> increase in power output, the _absolute_ IMD levels increase by 3 dB.
> This rule is so reliable that it forms the basis of the IP3 intercept
> specification...That's the imaginary point where the spur levels rise to
> equal the amplifier output...but that one number allows calculation of
> all the other intermod numbers. In _relative_ IMD terms, spur levels
> improve by 2 dB for each 1 dB reduction in power level. This effect
> occurs without regard to bias level, so there's no need to rebias for
> Class A.
>
> There's no special maximum power output level in linear amplifier...it
> just depends on what IMD's you can tolerate. The DownEast transverter
> doesn't have a design problem, it just has a specification problem
> (depending you your definition of problem). For example, it's -26 dB
> and you want -40, that 14 dB more performance. Simply run 7 dB less
> power and it will work.
>
> Another example: small signal amplifiers generally have an IP3 about 15
> dB higher than the 1 dB compression level. This means that PEP is -15
> dB(IP3) and intermod therefore run about -45 dB(IP2)...for an IMD level
> of -30 dBc (relative to carrier). To maintain -40, each stage needs to
> run more than 5 dB below PEP...especically considering that all the
> intermod products add up along the way. Generally designers run the low
> level stages at lower power and let the last few higher power stages
> generate all the IMD.
>
> The usual way that a linear amplifier runs worse than this rule involves
> bad bias voltages stability. If the power bus or base/gate bias varies
> with modulation, it can really screw up the IMD performance.
>
> I really like the FET/Tube cascode design idea. But, the tube biasing
> gets a bit weird. Normal drive pushes the tube into grid current, so
> instead of a positive cathode zener to generate the idling current, the
> bottom of the the FET need to connect to a negative voltage (effectively
> allowing the grid to swing positive.
>
> hope this helps...
>
> jeff, wa1hco
>
> Tom Cathey wrote:
>
>> Hi Marv,
>>
>> Your idea about using a 4CX350J IPA is interesting. I suppose it 
>> would have
>> to be VERY physically close to the 8877 or the RF negative feedback will
>> have phase shift problems. ie, It will not work in a separate cabinet 
>> with a
>> short coaxial lead supplying feedback, I would imagine. The "J" [linear
>> version] might be an expensive tube too.
>>
>> Also, I like the idea about putting an MRF-151 in the cathode of the 
>> 8877
>> as a "gain booster", but also wonder if it would degrade the IMD of 
>> the 8877
>> significantly. Anyone have ideas about this?
>>
>> Another interesting idea was about using a ~200W triode and letting 
>> it idle
>> in class A1, giving maybe 60W output without worrying about neg 
>> feedback.
>> Maybe that would approach -40 db, or perhaps the device itself has
>> limitations - like using a triode connected 813, etc, dunno.
>>
>> The bad news: I just got the 3rd order IMD figures back from DownEast on
>> their 50-28 transverter. At 14 watts output it is NO WORSE than -26db.
>> Typically -30db. She said at a lower power it would be better. 
>> [sometimes
>> true, sometime not]. So it appears that I will have to either look into
>> idling those devices higher or modifying somehow, or tapping off the low
>> level area which should be cleaner and amplifying from 100MW? up to 
>> 60W with
>> something cleaner - as suggested above... [sigh]
>>
>> Anyone have ideas for cleaning up the 50-28 final amplifier stages? I 
>> wonder
>> what the rest of the world has been doing about this or just living 
>> with -30
>> db IMD on 6M?
>>
>> 73,
>> Tom, K1JJ
>>
>>
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>>
>>
>>
>
>
>