''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
> ================
>
>
>
