Absolutely correct Joe...that's why when I design a VRF-2933 based MOSFET
amp for 1500W PEP it's built for circa 2.4 kW saturated Po. This is no mean
feat; and makes the practical implementation of the TLTs, combiner, LPF
components, heatsink spreader / thermal airflow aspects, SMPS, etc bulky and
expensive. Such conservative SS amp designs are therefore not well suited
to a cost sensitive ham-radio market product.
Even without applying linearity correction a SS HF amp with this design
philosophy is remarkably clean operating at the 1200 to 1500 W PEP level
with all that headroom.
Leigh
VK5KLT
-----Original Message-----
From: Joe Subich, W4TV [mailto:lists@subich.com]
Sent: Tuesday, 17 November 2015 1:52 AM
To: Leigh Turner
Cc: amps@contesting.com
Subject: Re: [Amps] SPE 1.3K-FA and FTDX-5KMP / IMD at rated PEP
On 11/15/2015 11:09 PM, Leigh Turner wrote:
> The only other way around this intractable dilemma is the use of
> linearity correction pre-distortion techniques in either analog or
> digital form, the latter being implemented with an SDR Tx exciter
> running the requisite correction algorithm.
Even linearity correction will not work if the devices are being driven
into saturation - the TV transmitter designers know that very well.
Linearity correction works as long as there is some headroom left in
the Pin/Pout curve but once you've lost the ability to increase drive
to compensate for the compression you've reached the limit of
correction.
73,
... Joe, W4TV
On 11/15/2015 11:09 PM, Leigh Turner wrote:
Yes indeed Joe. The VRF-2933 FETS are good / arguably best of breed in
today's generation and offerings of modern RF Power semiconductors.
The key to success here is de-rating the combined module's PEP to steer well
clear of their 1 dB power compression and subsequent saturation points. Then
you will attain those desirously clean IMD numbers with good TLT output
xfmrs, diplexer style terminated harmonic output filter, etc, and FET bias
optimisation.
This has been shown to be so in practice.
The only other way around this intractable dilemma is the use of linearity
correction pre-distortion techniques in either analog or digital form, the
latter being implemented with an SDR Tx exciter running the requisite
correction algorithm.
Leigh
VK5KLT
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
> W4TV
> Sent: Monday, 16 November 2015 1:55 PM
> To: amps@contesting.com
> Subject: Re: [Amps] SPE 1.3K-FA and FTDX-5KMP / IMD at rated PEP
>
>
> On 11/13/2015 4:50 PM, Leigh Turner wrote:
>>
>> That sensible criteria Joe just about excludes all solid-sate
>> commercial amps that are currently in the ham-market...rather sadly
>> :-(
>
> Which is a shame because the Quadra and PW-1 with 8 x MRF-150 *should*
> *be* capable of -40 dB(PEP) at 800W output and -35 dB(PEP) at their
> rated 1 KW output. Given the age of those designs, it is amazing that
> the new designs are so sloppy.
>
> A pair of BLF-188A based amplifiers with splitter/combiner each running
> 800W PEP for 1600W PEP total should be able to make the -40 dB(PEP) as
> long as the designer takes care to keep the bias stable and avoid any
> saturation of the output transformers.
>
> I suspect, but have not seen the IMD curves, that the two of the old
> EB-104 boards updated with VRF-2933 VFETs and new output transformers
> could also make the grade (again 800 W (PEP) per board with transistors
> capable of 2400W total saturated power).
>
> 73,
>
> ... Joe, W4TV
>
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