[hpsdr] ALEX - Call for Comments - II - Clarification

[Murray Lang]

What about strong signals within the same band? The impression I get 
(from criticisms I've read) is that the wideband ADC approach is 
particularly susceptible to cross-modulation from such signals, and that 
it's too late to apply filters in software. Is this fair? If so can ALEX 
step in and help with this problem?

Murray VK6HL

pvharman at arach.net.au wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Graham,
>
> Thanks for taking the Project Leader role on for Alex.  Re your request 
> below, based on my tests with the prototype Mercury setup the native 
> performance should be very acceptable without any additional input filters. The 
> Mercury board contains a switchable  1.5MHz HPF and 55MHz LPF to remove high 
> level MW broadcast signals and prevent VHF signals from aliasing into the 
> passband. 
>
> I live in an area close to multiple MW broadcast stations and with the HPF in 
> circuit have no problems on any HF band. If I want to listen on MW then I will 
> bybass the HPF and if necessary turn off the preamps and use the 0-30dB 
> attenuator. 
>
> I used the Mercury prototype on 40m during the recent ARRL CW contest. As I 
> test I mixed an S9+60 db signal in with the antenna and at 1kHz spacing could 
> still copy a signal at the noise floor. The limit of the test was the sideband 
> noise of my HB8640B not the receiver strong signal performance. 
>
> Since we have little practical experience in using direct digital conversions 
> receivers (and having a frontend fully open from 1.5 to 55MHz does not feel 
> right to an analogue receiver designer!) Alex is there in case folks live in a 
> high signal environment, require additional front end selectivity for contest 
> or DXperdtion operations etc.  I suspect that in many cases with the natural 
> selectivity offered by antennas, ATUs, HF beams etc then Alex may not be 
> required. 
>
> 73's Phil...VK6APH
>
>
>
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>
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>
>   
>> I assume the majority of the users want good ham band performance
>> and casual SWL, WWV, and experimental capability.  The
>> native performance of the underlying SDR should be good
>> enough for the "casual" and incidental uses with the preselectors
>> bypassed. [ Phil H, please comment. ]  Any serious or focused
>> other uses can put a dedicated filter section in the user
>> defined spaces, or rework the whole filter pack.  The PCB
>> layout will accommodate third order (three inductor)
>> bandpass or low-pass filters in each section.
>>
>>     
>
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