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
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|