''At first glance it appears crazy''
Mike,
Welcome to the crazy world of JR !
The basic explanation is the narrow filters increase the noise power
in a limited bandwidth , ringing due to the Q , similar to CW , narrow
filters tend to round the CW signal
The DSP engine is better equipped to differentiate between carrier and
noise and has a much greater dynamic range , so optimum results are
obtained , when the signal is presented to the interface, as close to to
the original as possible , the DSP filter profiles are tailored to the
mode/speed in use
This can be noticed with the new generation of SDR support software ,
where audio/voice recovery can be superior to conventional hardware
based systems
With hardware filtering , there are transit (group) delays which can
alter the amplitude / time , either from on/off keying or with FSK,
this can be observed when sending wide band FSK , although the audio
level remains constant and 'phase continuous' , its possible some times
to see a 'am modulation' envelope on the carrier ... $$$$ can
solve this problem , but for most Ham kit its something that
'happens'
Physical constraints , if there is a very large carrier in the pass
band and this is pushing the hardware into non-linearity / A/D to
over range , giving quantising errors, then , yes filtering would
help , but its more likely its the analogue path that's causing
intermod products .....reducing the rf/if gain will provide the solution
I think that's about the picture , if Jim's about , im sure he will
fill in the gaps
73 -G..
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From: "Mike Dennison" <[email protected]>
Sent: Wednesday, July 11, 2012 10:18 AM
To: <[email protected]>; <[email protected]>
Subject: Re: LF: OPERA Question
Narrow IF filters are not desirable and reduce the
performance of the demodulator , better simply use SSB filter
GL ..73 -G..
Graham,
Why is that the case? What does the considerable extra bandwidth
achieve? Is the SSB bandwidth optimal, or would it be even better
with 10kHz bandwidth, or 100kHz?. Why is 3kHz better than perhaps
1kHz or 2kHz?
At first glance it appears crazy to let in all sorts of adjacent
channel QRM (the bandwidth is more than ten times the size of the
entire Opera window) when using a mode that occupies a fraction of
1Hz.
Is it simply that the 1.7kHz Tx tone is high enough for the SSB
filter to kill its harmonics, and on receive it is difficult to get a
1.7kHz tone out of a CW filter, even with passband shifting.
Am I missing something?
73 de Mike, G3XDV
g3xdv.blogspot.co.uk
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