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Re: [Amps] Tubes vs. Solid State

To: amps@contesting.com
Subject: Re: [Amps] Tubes vs. Solid State
From: Bob Henderson <bob@5b4agn.net>
Date: Thu, 3 May 2012 18:37:06 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
On 3 May 2012 18:16, Larry Benko <xxw0qe@comcast.net> wrote:

> Bob,
>
> I am currently experimenting with one of the smaller Freescale parts.  I
> do not see any reference to the -7dBc spec in any of the datasheets.  On
> the other hand I have seen the output waveform look like a rounded
> squareish wave with the center part depressed.  This occurred when the
> coefficient of coupling in output power combiner was too low.  A
> waveform of this type would have the lower fundamental amplitude
> relative to the harmonics which is what you are and I are seeing.
> Simulations I have done show this same phenomenon.  Change the
> coefficient of coupling from 1 to .9 and voila the waveform has the
> sunken squarish wave look.  Are you using any feedback or source
> degeneration in you amp?
>
> For the 300W Freescale part the reference design of the part showing
> very good IMD3 and IMD5 numbers is for a parallel (not push-pull) amp
> which operates at a single frequency of 230MHz.  The larger parts do not
> include the IMD tables in their datasheets.
>
> 73,
> Larry, W0QE
>
> On 5/3/2012 11:55 AM, Bob Henderson wrote:
> > Larry
> >
> > I agree.  But Freescale themselves state H3 at -7dBc isn't unusual.
> >
> > 73 Bob, 5B4AGN
> >
> > On 3 May 2012 17:41, Larry Benko <xxw0qe@comcast.net
> > <mailto:xxw0qe@comcast.net>> wrote:
> >
> >     Bob,
> >
> >     Something doesn't sound right here.  Even if your amp produced a
> >     perfect
> >     square wave output, the harmonics of the square wave would be down
> >     more
> >     than what you saw.  The Fourier series of a square wave with
> amplitude
> >     +/-1V has the amplitude of the fundamental freq. = 4/PI, the 3rd
> >     harmonic = 4/(3*PI), the 5th harmonic = 4/(5*PI) etc.  This means
> that
> >     the 3rd harmonic is 9.54dB below the fundamental and the 5th
> >     harmonic is
> >     13.98dB below the fundamental.  I don't know what the problem might
> be
> >     other than something seems incorrect.
> >
> >     73,
> >     Larry, W0QE
> >
> >     On 5/3/2012 11:22 AM, Bob Henderson wrote:
> >     > The BLF578XR is rated for use between 10-500MHz, which is
> >     something of a
> >     > limitation for ham use.  A pity because it appears it might be a
> >     lot more
> >     > rugged than the Freescale MRFE6VP61K25H which is rated at a
> >     similar power
> >     > level from 2-600MHz.
> >     >
> >     > I looked at this type of device in my pursuit of a small low
> >     weight 1kW
> >     > "air travel friendly" amp for 1.8-54MHz.
> >     >
> >     > Towards the end of last year my experiments with one of the
> >     above Freescale
> >     > devices ended badly when it self destructed due to a moment of
> >     > instability.  At the time I was evaluating different output
> >     arrangements.
> >     > Since then I haven't summed up the courage or foolishness needed
> >     to invest
> >     > in a second one.
> >     >
> >     > I used a Fischer Elektonik forced air cooled heat sink which
> >     dealt with
> >     > heat disposal rather well.  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.  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.  Harmonics were not
> a
> >     > consequence of transformer saturation nor of fundamental overdrive.
> >     > Harmonics were at a hostile level even when just tickling the
> >     device with
> >     > enough drive to produce a few watts output.
> >     >
> >     > I am told others have had better experience through use of a
> >     broadband TLT
> >     > at the output but I haven't been able to persuade myself that such
> a
> >     > broadband device will magic away hostile odd harmonics within its
> >     > bandwidth.  At GBP200+ per device I'd like to see hard evidence of
> >     > harmonics reduced to a manageable level in a 5 octave amplifier
> >     before I
> >     > put up another 200 quid plus for a second throw of the dice.
> >     >
> >     > Most designs published on the web using these devices are single
> >     band
> >     > VHF/UHF amps which are a whole lot simpler proposition when it
> >     comes to
> >     > considering harmonics.
> >     >
> >     > I engaged in several exchanges with Freescale over the harmonic
> >     issue which
> >     > they acknowledged was a problem, though in their view H3 should
> >     be no worse
> >     > than -7dBc.  My experience was seriously worse at more like -2
> >     or -3dBc.
> >     > Freescale suggested the best approach would be to adopt class F
> >     filtering.
> >     > 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.
> >      Their back-up
> >     > suggestion was to use absorptive filtering.  Diplexers and dummy
> >     loads are
> >     > easy but between H3 and H5 I'd be dumping half the output as
> >     heat.  Not for
> >     > me.
> >     >
> >     > Freescale told me they were scheduled to publish a core design
> >     for a 4 or 5
> >     > octave 1250W amp based on the MRFE6VP61k25H in February this
> >     year but it's
> >     > now May and nothing has yet appeared.  I guess they are still
> >     working on it.
> >     >
> >     > FWIW
> >     >
> >     > 73 Bob, 5B4AGN
> >     >
> >     >
> >     >
> >     > On 2 May 2012 03:11, Roger<sub1@rogerhalstead.com
> >     <mailto:sub1@rogerhalstead.com>>  wrote:
> >     >
> >     >> On 5/1/2012 11:01 PM, donroden@hiwaay.net
> >     <mailto:donroden@hiwaay.net> wrote:
> >     >>>> IF and I have to emphasize the IF these transistors were
> >     rugged enough
> >     >>>> for prime time and I'm quite willing to take your word on the
> >     power you
> >     >>>> are getting out, the manufacturers would be jumping on them
> >     like flys
> >     >>>> on...er... honey because they could run them PP/parallel for
> >     the legal
> >     >>>> limit out plus comfortable orverhead and couldn't build them
> >     fast enough
> >     >>>> to meet the demand.    There has to be a reason they are not
> >     doing so.
> >     >>> http://www.youtube.com/watch?v=8ziYqjMQGEQ
> >     >> But what is the price and where do I get them?  I still wonder
> >     why we
> >     >> aren't seeing them in amps on the market.
> >     >>
> >     >> 73
> >     >>
> >     >> Roger (K8RI)
> >     >>
> >     >>
> >     >>> Don W4DNR
> >     >>> _______________________________________________
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> >     >>>
> >     >>>
> >     >>
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