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Message-ID: <DA9B6256FF94488393A5CC1BFE6CFAE8@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <00c301cbcb04$b57989a0$0401a8c0@xphd97xgq27nyf><AANLkTim2VEtkiobqX=dgj_y8bkLG=MV4hg9u0+p7j21d@mail.gmail.com><00dd01cbcb0a$b7f1f4f0$0401a8c0@xphd97xgq27nyf><AANLkTimQd7=M6O-uLrTP+ECOxXiitE4DKCYmEn5a7vxJ@mail.gmail.com><00eb01cbcb0d$6a82ee10$0401a8c0@xphd97xgq27nyf><AANLkTim+Zz_0xum_b=rnBhcfuOKDSt-AQ7k8Q1Sor5h8@mail.gmail.com><012001cbcb15$e63c14c0$0401a8c0@xphd97xgq27nyf><433CFC410770450186BB307A8AD28484@JimPC> <AANLkTin7PuZ=MUJz+U89W=y4zwioXC+rxfC6PnsBcqh+@mail.gmail.com>
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Date: Sun, 13 Feb 2011 16:39:16 -0000
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Subject: LF: Re: CQ WW / Multiple spectrograms
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Dear Warren, LF Group,

The difference between your signal and the "locals" here is about 50 - 60dB 
I guess; G3XDV is more of a challenge, being only about 11km from here. But 
previous tests have shown that this kind of signal range should not be 
impossible.

Part of the idea of this test was to find a way round the problem of people 
wanting to use different sub-bands for different locations. With modern PCs 
it is not difficult to display a number of spectrograms from a single 
sound-card input. The whole 136kHz band can be accomodated in a normal SSB 
filter bandwidth. So several sub-bands can be monitored simultaneously, 
using different modes if desired, using a normal RX and PC. The main 
difficulty of course is distortion effects caused by very strong signals in 
the same passband as weak ones. This is probably best handled by directly 
digitising receivers like SDR-IQ and Perseus, or "Softrock" type IQ 
downconverters, where there are fewer non-linearities in AGC IF stages, 
product detectors or audio stages to contend with. But it also seems to be 
possible with ordinary rigs like the IC718 I used here. I selected a carrier 
frequency of 135.6kHz USB, adjusted the passband shift to minimise the 
levels of DCF39 and HGA22 as much as possible, and backed off the RF gain so 
that hardly any sound comes out of the speaker apart from these utilities' 
carrier tones at low level. I set the sound card "volume" so that the audio 
was well below the clipping level on the Spec Lab input monitor. 
Spectrograms can then be configured for audio frequencies centred on 577Hz 
and 2180Hz. The noise level seems to be acceptably low, and the distortion 
products are not a serious problem, it would seem.

I think using receivers more flexibly is part of the answer to the different 
frequency ranges needed for various reasons. Also, I think receiving weak 
amateur signals in the presence of strong locals is not as great a problem 
as is sometimes made out.

Cheers, Jim Moritz
73 de M0BMU