[Amps] LDMOS Solid State Amplifiers
Gary Schafer
garyschafer at comcast.net
Sat May 5 12:15:08 PDT 2012
Roger,
as I understood it he used the two independent exciters thru a combiner to
create the two tone signal for the IMD test. They would be separated in
frequency by whatever amount (a few KHz or so could be used). Equal levels
would have to be use but phase is irrelevant.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Roger (K8RI)
> Sent: Saturday, May 05, 2012 2:36 PM
> To: amps at contesting.com
> Subject: Re: [Amps] LDMOS Solid State Amplifiers
>
> On 5/5/2012 11:30 AM, Tom Thompson wrote:
> > 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.
>
> If I have followed this correctly:
> As this is a single, PP amp fed with two independent exciters how do you
> maintain proper phasing which is critical? Even if the exciters are
> synchronized a tiny difference in path length can make a difference.
>
> 73
>
> Roger (K8RI)
>
> > 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 at 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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> >>>> Amps at contesting.com
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> ing.com/mailman/listinfo/amps>
> >>>>
> >>>>
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