A radio performance testing item that I think has been ignored for far too long
is its “in-band” or recovered audio cleanliness. Nobody is or has been
consistently testing this that aspect of radios that I'm aware of.
I have long believed that the cleanliness of the RX audio is a major component
of what people are actually describing when they say one radio is less
fatiguing than another.
And I'm quite convinced that the cleanliness of the LO, or phase noise in
general inside of 1 kHz distance from the carrier is a major component of that
listener “fatigue factor”. Because that phase noise that close to the carrier
has to be mixing with the desired audio and degrading it. Take notice of some
of the radios which have good phase noise traits outside of 1 kHz, but look
closely at the slope of their phase noise plots as it approaches the carrier,
they rise very steeply. Often much more steeply than some radios criticized for
having poorer wider spaced phase noise.
And also AGC behaviors certainly play a big role too. Not exactly sure how that
kind of recovered audio quality testing could be achieved with consistency over
the years and over different generations of radio technology. And the different
signal path architectures.
So how would or could the audio quality test process or protocol deal with the
radios that have a lot of AGC parameter adjustments? And then layered on top of
that, the interactions of RF or IF gain settings relative to the AGC setting.
Remember that not all radio's analog IF or RF gain is directly tied into the
AGC system, the Corsair's come to mind, as do most PC-based SDRs. So then there
is the aspect of testing for the radios that only invoke the analog AGC when
the AGC in the DSP reaches a predefined limit. So the DSP AGC interactions with
analog AGC aspects would also needed to be looked at. The testing matrix for
all of those adjustment variables would be huge. And as such it would be easy
for the tester person completely and inadvertently get results that may not be
completely valid due to not fully understanding the details of the radio's
overall AGC implementation.
And then there's the issue of IMD contributions by passive components in the
signal chain. It is interesting to note that some radio's close-spaced IMD DR
performance is now limited by the IMD contributions of the roofing filter
itself.
In addition to the cleanliness of the RX audio, these same basic factors and
components where they overlap in the TX signal chain would also apply to the
in-band cleanliness of the TX audio as well.
Then addition to this audio cleanliness aspects there are the AGC interactions
with short duration impulse noise that Rob Sherwood has been calling attention
to. That short impulse response testing is complimentary aspect to a radio's RX
audio cleanliness in general, even when there is no impulse type noises in the
mix.
Duane
N9DG
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