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
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> >
>
>
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