[Amps] Pre-Distortion Linearizer

[Paul Christensen]

This may change as more users become accustomed to using their SDR displays 
for diagnostic purposes, especially when the SDR in use is a direct DDC 
type.  When using a device like the SDR-IQ and QS1R, res. bandwidth accuracy 
is quite good for this purpose.   Accuracy is significantly better than that 
of the simplistic displays used in the Icom series transceivers.

While tuning across the bands, searching for high-level signals shows some 
very interesting evidence.  In one instance on 20m, I observed a W6 station 
with a very sharp transmitted passband -- probably well in excess of -45 
dBc.  By contrast, an adjacent station's splatter was producing -20 dB IMD 
"wings."  It's one thing to hear IMD effects; it's another to witness it at 
the same time.

It's also become easier to differentiate between IMD and Tx filter 
sloppiness.  The SDR display also shows the effects of dynamic IMD (e.g., 
caused by a poorly-designed ALC loop) that doesn't show up when using a 
static 2-tone test.

I would like to see this type of dynamic IMD testing in future product 
reviews.  Much of the CW 'click' improvement we've seen from manufacturers 
in the last decade is directly attributed to the shame and embarrassment 
created with a damning spectral graph.

Paul


----- Original Message ----- 
From: "Ian White" <gm3sek at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Saturday, May 04, 2013 5:24 AM
Subject: Re: [Amps] Pre-Distortion Linearizer


> Tom W0IVJ wrote:
>>As long as the transceiver manufacturers give us transceivers with poor
> IMD,
>>it does little good to get the IMD on the amplifier good, unless you
> home
>>brew your own transceiver.
>>
>
> Transceiver and amplifier manufacturers will continue to give us poor
> IMD for exactly as long as the major equipment reviewers (ARRL, RSGB and
> DARC... but mainly ARRL) continue to remain silent.
>
> In the final years of tube transceivers, the reviews show 2-tone IMD3
> performance of -35dB(pep) and better. But as solid-state PAs came on the
> market, those same reviewers presided over a gradual slide into the low
> 30s, the high 20s and now even the mid-20s... with never a word said.
>
> Reviewers have consistently failed to understand is that they are not
> only writing for people who might be interested in buying that
> transceiver or amplifier. When they fail to speak out against poor TX
> performance, they are neglecting an equally important duty towards
> EVERYBODY ELSE who will have to suffer on adjacent channels.
>
> (And before the tired accusations about "commercial pressures" begin,
> the underlying story is way more complex than that. The main reason why
> nothing changes is technical and human: all of the reviewers have become
> complicit in the present situation, and none wants to be the first to
> step forward.)
>
>
> 73 from Ian GM3SEK
>
>
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