OK, Graham, Many thanks for the helpful reply. Of course it helps if
you have a bomb-proof radio that doesn't mind having close-in strong
QRM. The main problem with my old IC-706 is the inability to switch
off the AGC.
de 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..
>
>
>
>
> --------------------------------------------------
> 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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