Tom
Interesting. Thanks.
With your brass tube transformer, how was Vdd fed to the drains?
The 10db reduction in H3 & H5 is a significant improvement but I am
wondering how much is due to the bifilar choke feed of Vdd and how much due
to transition to a TLT? It would have been interesting to see what change
resulted from adding the bifilar choke feed to your brass tube transformer
set up.
H3 & H5 at -22dBc or better is easily good enough. After that, it's all
about IMD performance.
73 Bob, 5B4AGN
On 4 May 2012 23:43, Tom Thompson <tlthompson@qwest.net> wrote:
> Bob,
>
> I am experimenting with the 300 W Freescale part. Using the brass tube
> output transformer with a single turn on the primary and 2 turns on the
> secondary without a harmonic filter I measured the following:
> Vdd = 50 V
> Id = 10.5 A
> Po = 200 W
> 3H = -11.8 dBc
> 5H = -20.5 dBc
> I then followed Manfred's suggestions and used a 4:1 transmission line
> transformer wound with 30 ohm coax, a bifilar wound power combiner to
> supply drain voltage, and a choke balun on the output of the transmission
> line transformer. I then measured the following with no harmonic filter:
> Vdd = 50 V
> Id = 7.5 A
> Po = 200 W
> 3H = -22dBc
> 5H = -30 dBc
> When I reduced the power output to 100 W, 3H went to -30 dBc. In all
> cases the total Idq was 1.5 A.
> I hope this helps.
>
> 73, Tom W0IVJ
>
>
>
> On 5/3/2012 8:21 PM, Manfred Mornhinweg wrote:
>
>> Bob,
>>
>> My problem area was the extent of harmonics generated within the
>>> device. H3 was within a dB or two of fundamental energy levels and
>>> H5 only marginally better.
>>>
>> That typically happens when your output network isn't correctly done.
>> There is an incredible amount of equipment, including HF ham
>> transceivers comemrcially made today, that have incorrectly implemented
>> power amplifiers, due to their designers not understanding of the basic
>> principles under which transformers operate.
>>
>> A serious problem. My output arrangement focused largely upon a 1:9
>>> coax wound RF2000 from RF Parts as used in the Granberg designs at
>>> the 1kW level.
>>>
>> Granberg apparently was the one who "invented", or at least popularized,
>> the wrong output network. Several of his papers contain the mistake,
>> but others do not. It seems to me that he really didn't understand this
>> issue, at least not when he published those old papers.
>>
>> How are you feeding the drains? If you are using a bifiliar choke,
>> designed in such a way that it can act as a balancing autotransformer,
>> then that should be fine, and you have to look elsewhere for the reason
>> of the high harmonics. But if you are using two individual chokes, then
>> that's wrong, and if you are feeding the drains through some sort of
>> center point on the transformer, then there is a pretty good chance that
>> it's wrong too!
>>
>> Typical symptoms of the incorrect output configuration are: Extremely
>> high distortion (harmonics, IMD), horrible waveform at the drains, that
>> includes peaks well above twice Vdd, low efficiency, low gain, and a
>> sort of gain breakpoint: Up to a certain power the amp is easy to drive,
>> and from that point up it gets suddenly very hard to drive further.
>>
>> Harmonics were not a consequence of transformer saturation
>>>
>> That could hardly ever happen at HF. Before you saturate a ferrite core
>> at HF, you will melt it down with the losses!
>>
>> But DC saturation can happen, in very tricky situations, specially if
>> you have hugely more inductance than needed.
>>
>> No problem in a single frequency amp but I am way short of clever
>>> enough to figure out a scheme which will handle that over 5 octaves.
>>>
>> Use either an output transformer that has a true center point, or a
>> bifiliar choke to supply power. Note that the typical RF power
>> transformers made from two ferrite tubes, with a single-turn primary, DO
>> NOT HAVE A CENTER POINT. The junction of the two metal tubes is NOT a
>> center point! Using this junction as a makeshift center point causes
>> endless trouble, and many amplifiers, based on some of Granberg's
>> designs, contain exactly this mistake.
>> With transmission line transformers, a center point is usually also
>> unavailable, but some transmission line configurations can have one.
>>
>> The basic point is this: Class B or class AB push-pull amps MUST, I
>> repeat _MUST_ have something that provides balance around a true center
>> point. It cannot work in pure differential mode, because each FET
>> conducts for half of each cycle, and is in high impedance during the
>> other half cycle. You cannot draw current between one transistor that is
>> on and another that is off! That's why balun or balbal type output
>> transformers only work correctly in conjunction with a bifiliar feed
>> choke that provides the center point.
>>
>> Class A push pull amps do not have this restriction, and can work well
>> in pure balanced mode.
>>
>> So, check your feed arrangement, maybe that's where your problem is!
>>
>> Manfred
>>
>> ========================
>> Visit my hobby homepage!
>> http://ludens.cl
>> ========================
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>>
>
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