On Wed, 2006-08-16 at 06:58 -0700, John R wrote:
> I'm a little late coming to this subject, but QST ran a fix in its
> "Hints and Kinks" for the AGC "pop" problem back in 1982 or l983.
> Someone may have more info on this.
>
> 73,
> John, W3ULS
The fundamental cause of thump in analogue Tentec radios is that the AGC
is derived from audio sometimes after some audio filtration. In any case
the audio wave rises more slowly than the IF wave and that delays the
attack of the AGC, in the mean time there is time delay for signals
passing through the crystal filters and post detection filters that is
already past the gain controlled stages when the AGC finally acts. Hence
the thump or click. The filter time delay is the most of the problem,
most cures work at speeding the AGC attack with changed time constants.
It may be that there needs to be an audio AGC after the detector to
address that thump, though I've not seen it. DSP based radios need an
audio AGC too after the hardware AGC protected the A/D from wide band
signals, but sometimes a strong post processing rejected signal can make
the desired signal "pump" from that AGC action. That pumping is the bane
of post detection based filtration.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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