Larry
Freescale make no reference to expected H3,H5 in their spec sheets. The
-7dBc reference came as a result of a direct enquiry to the Freescale
technical folks. I avoided use of any kind of reactive feedback as I was
concerned this might tempt fate in a device with a 600MHz b/w. I simply
used a 9:1 transformer at the input and held the gate impedance down
resistively. Aside from the fatal slip up in output arrangement it
appeared extremely stable.
I didn't get far enough to make any IMD measurements.
Bob, 5B4AGN
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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