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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