Ok Mike
AGC , yes 'off' is the preferred mode , to enable the full range of
the dsp to be used . wide/no agc feels a bit odd but that's the
recommended setting's ...better still go SDR , but there are limits ,
I brought one of the £10 donglels , last weekend , monitoring CR
repeater on 433.150 , using HDSDR , the dongle gave better reception
than the FT-897 ... some things are better not known .. : (
I found a odd thing , with the RA1778 and the RA6790 , if the AGC
was set to 'short' then the IMD
level shown on PSK31 was something like 10 dB worse , than with
the AGC set to medium/long or off.
The agc was following the modulation envelope and flat-topping the
signal ...
This coming winter should be action packed on MF / LF , its mid
summer and 500 is full already !
73 -G.
--------------------------------------------------
From: "Mike Dennison" <[email protected]>
Sent: Wednesday, July 11, 2012 5:08 PM
To: <[email protected]>
Subject: Re: LF: OPERA Question
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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