On 4/21/2014 8:51 PM, Sam Morgan wrote:
> don't forget the K3's tunable Notch, I have found it works quite well
> to attenuate those offending S9 +30 signals.
The Manual notch is outside the AGC loop - it can not protect the AGC
system from pumping and as such only serves to protect the sound card
from overload. It does not prevent blocking of the AGC.
73,
... Joe, W4TV
On 4/21/2014 8:51 PM, Sam Morgan wrote:
don't forget the K3's tunable Notch, I have found it works quite well to
attenuate those offending S9 +30 signals. The width is just shy of a
JT65 signal, but if you make sure to cover the low end it works. You can
watch it working in the waterfall. I always run with the K3's AGC
off...ymmv
sm
On 4/21/2014 7:39 PM, Don Wilhelm wrote:
The situation on HF is different. If there is a station within the
passband at S-9+30, it is on the borderline for activating the Hardware
AGC. That signal should be moved outside the roofing filter passband to
reduce its response in the K3 first IF (and DAC input).
Signals with strength less that S-9+30 can be easily handled by the DSP
in the K3 and likely also by the K3 DSP of the JT65HF algorithms without
overload.
The situation being addressed is the case where there is a strong
undesired signal that will activate the hardware AGC and cause 'pumping'
in the receiver response. That is where the roofing filters come into
play (and consideration for their phase shift). The narrow filters will
have greater group delay and greater phase shift - that is just a 'fact
of life'.
If there are no S-9+30 signals in the receiver passband, the combination
of the K3 DSP and application DSP can handle it quite nicely.
In other words, for best JT65HF, keep your filters wide so the JT65
software can do its job. If you find an extremely strong signal inside
the K3 IF passband, narrowing the filter width may help, it all depends
on the filters you have installed and the frequency difference between
the offending strong signal and your desired station.
Bottom line - when operating JTx modes (or other digital modes) learn to
recognize those conditions where the K3 Hardware AGC is being
activated. If Hardware AGC is not an issue, the DSP in either the K3 or
the JTx software/firmware should be able to handle it.
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