----- Original Message -----
From: Soegiono, Gamal <[email protected]>
Thankyou for the very interesting summary of the KWZ30 receiver design and
performance.
My initial reaction is that it seems the analog section of the rx was
designed separately from the digital back end and no-one considered the
overall receiver performance. Deriving the AGC as you described, ahead of
the primary bandwidth determining filters in the digital IF, would appear
to be the main issue. Can you manually control the gain in the analog
section and turn off the AGC? Possibly not, as these digital designers tend
to get upset if they think the input signal range for their beloved number
crunchers is going to be exceeded. Makes them look bad! This may have been
the only design parameter that was discussed between the analog and the
digital teams involved!!! (I jest, but....)
Some form of coarse AGC in the analog section derived from a few steps
between say middle and top of the A/D converter range would surely have
helped solved the problem.
Interested in your figure of -102dBm noise level. Presume this was the
external band noise? And what bandwidth was it measured in?
A related issue with that type of broadband front end has to do with noise
blankers.
The basic receiver structure you outlined is of course very similar to many
of the amateur (and other professional) receivers on the market these days.
The DSP back end is starting to appear in amateur rigs as well.
Such designs suffer from the detrimental effect that adjacent channel
signals ( actually, signals out to the extremities of the roofing filter on
either side of the wanted signal) can have on receiver performance with
noise blankers switched on.
To preserve as much as possible the noise pulse risetimes, the drive to the
blanker is derived from the aggregate of all signals inside the roofing
filter passband. So it is easily triggered by strong close-in signals
causing intelligibility of the wanted signal to suffer, in extreme cases to
the point of 'no copy'. A strong close-in CW signal can be utterly
devastating.
With noise blankers in use more often than not on LF, a heavy price can be
paid.
For the noise blanker an answer maybe use a separate noise receiver, as per
the old Collins scheme, with a separate 40 MHz broadband rx for the blanker.
That was OK for HF but in the case of LF reception I think a 0-100 kHz noise
rx would be more useful.
Anyone tried anything like this out?
73
Dave
ZL3FJ
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