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Yes its a difficult area to work in ,
Op is scanning a 200 hz audio band , with
the possibility of some 60 simultaneous decodes
...
Pulse lengthening , could be defined as increasing
the noise energy in the pass band , as the
decaying ringing exists 'after' the expected
falling edge of the pulse and is therefore a
'new' energy source ? assumedly the 'new'
power is supplied by the driver amplifier
I think the bottom line is , that anything
that can change the signal parameters , before
being presented to the computational engine
is to be avoided , narrow filters may also
induce higher phase changes / Hz than wider
ones , which would degrade the matching process ,
the whole DSP side of things seems
to be bottomless pit of questions , the
deeper you dig , the more abstract things
get ...
On a practical side , testing with
differing filters would give a indication as
to what the implications are on performance
...
73 -G.
Sent: Wednesday, July 11, 2012 8:38 PM
Subject: Re: LF: OPERA Question
Hi Graham, Mike,
given the ratio of occupied signal bandwidth (a few Hz) to filter bandwidth
in question (a few 100 Hz), I don't believe there will be any
significant impact of analog filters on signal shape. Neither on background
noise, directly.
However in the presence of impulsive noise, eg lightning statics, the
effectiveness of a noiseblanker will be severely degraded if the duration
of the pulses is prolonged by narrow filters with long ringdown. The
sharper the better - especially at VLF where spherics are
dominated by the short cracks from return strokes. However strong
carriers should be excluded from the passband before noiseblanker action,
as otherwise they will be contaminated by sidebands from the cutouts.
Best 73,
MArkus (DF6NM)
-----Ursprüngliche
Mitteilung----- Von: Graham < [email protected]> An:
rsgb_lf_group < [email protected]> Cc: james.moritz
< [email protected]> Verschickt: Mi, 11 Jul 2012 4:15
pm Betreff: Re: LF: OPERA Question
''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..
--------------------------------------------------
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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