Larry,
EME is about weak signal reception, and all the signals on the band
segment are weak.
So there are no strong adjacent signals to worry about.
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.
73,
Don W3FPR
On 4/21/2014 4:36 PM, Larry Lopez wrote:
But what about EME.
You want a narrow filter and you want constant group delay.
I'll research that as a separate topic.
Let's close this out.
In a pinch the 5 pole filter which comes with the rig is ok.
6K AM, 12K FM are recommended making up selectivity
in the DSP.
Narrow filters recommended when in filter band signals
are producing overload.
Narrow filters may degrade digital signals.
Signals near the skirts of filters may be degrade digital signals.
constant group delay filters are recommended.
I haven't found any.
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