[Amps] Tubes vs. Solid State
Larry Benko
xxw0qe at comcast.net
Thu May 3 11:16:22 PDT 2012
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 at comcast.net
> <mailto:xxw0qe at 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 at rogerhalstead.com
> <mailto:sub1 at rogerhalstead.com>> wrote:
> >
> >> On 5/1/2012 11:01 PM, donroden at hiwaay.net
> <mailto:donroden at 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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