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> 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> wrote:
> >
> >> On 5/1/2012 11:01 PM, 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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