Bob,
Vdd was fed in at the U point on the brass tube, single turn, point. If
I switch back to that transformer, I'll try the bifilar choke feed and
report. I did some IMD measurements this morning. I fed the amp with
two Norcal 40 QRP transceivers on 40 m. The input IMD due to the
combiner isolation was -44 dBt where dBt means below one of the two tone
peaks instead of the carrier which if present would be 6 dB higher. The
contribution from the input IMD is given by B = 20Log(1+10^(-A/20))
where A is difference in dB between the input IMD and the output IMD. I
measured an output IMD3 of -32 dBt at 100 watts on the amp under test.
That makes A = 12dB therefore B = 2dB which makes IMD3 = -34dBt. IMD5
was -60dBt and IMD7 was -52dBt.
73 Tom W0IVJ
On 5/4/2012 11:59 PM, Bob Henderson wrote:
> 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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