Does output IMD always directly add to input IMD contribution or is this a
worst case figure?
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Tom Thompson
> Sent: Saturday, May 05, 2012 11:31 AM
> To: Bob Henderson
> Cc: James ( Pete) Gaddie; amps@contesting.com
> Subject: Re: [Amps] LDMOS Solid State Amplifiers
>
> 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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> ing.com/mailman/listinfo/amps>
> >>>
> >>>
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