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@ifwtech.co.uk>
To: <amps@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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