Message-ID: <D300C456F3F2421E913AD4C9D41EE57C@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <F2308B40167B42B486EAD4657D9D78F6@AGB> <53E3983D.9050606@gmx.net>
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Date: Thu, 7 Aug 2014 21:35:44 +0100
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Subject: Re: LF: Receivers for LF and MF    136 KHz  @ 477KHz   ???
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Status: O

Yes Tobias,

All  is not as it  seems , but as you say , adding  filtering , reduces  the 
problems  , and  with  direct conversion  , the  noise  floor  is the 
lowest possible , stability  is  at maximum , having only  1 oscillator , In 
have  good  decode  results  on 477 using  the  £5 dongle  , behind  the  TX 
atu and inv L ae

For the  £150  sdr , the  description  lists  80 msps , which  , I assume 
moves the  image  problems  out  side the  HF spectrum ? at  12 bits  , that 
starts to  provide a  reasonable  , post processed  dynamic range ?

 12- bit 80 MSPS A/D conversion

I would  not  link the  barlow-wadley , too  closely  to  lack  dynamic 
range , may  be a lack of  engineering  integrity , the  RA17  with  pentode 
rf  stage  , then  later  cascode  , with  beam deflection  mixer , was 
reasonable , though  there  was a  pre selector  for  use at  close  tx/rx 
sites ,  the  ra1771/1772  was  the  first  to  better the  ra17 , but again 
that also tends to  question the  models before ..   the  ra1772/1  is 
fitted  with  rf-pre selector , for those 'unexpected' situations ,  the 
ra6790gm , with  no  pre-amp  is the  closest  I have  seen  to  a  bullet 
proof  front  end , that  runs , where  the  ra1778 needs  the  pre-selector

But in terms  of  noise  etc , one of these  'reasonable  sdr's'  may be 
better .. I don't think I would  recommend any one  go  down the  racal 
path  these  days !

73-G,

--------------------------------------------------
From: "Tobias DG3LV" <DG3LV@gmx.net>
Sent: Thursday, August 07, 2014 4:16 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Receivers for LF and MF    136 KHz  @ 477KHz   ???

> Hi Graham !
>
> This type of device has a dynamic range of 48dB at maximum (8 Bit), so 
> they need help from a narrow-band frontend plus input-attenuators to 
> reduce the load to the A/D-converter. The conversion speed is variable 
> from 1 to 3 Megasamples/second, leading to a Nyqist-frequency of 0.5 to 
> 1.5 MHz.
>
> This filter should have a deep attenuation above the Nyqist-frequency to 
> avoid unwanted reception at harmonics/aliases of the sampling frequency (= 
> "undersampling").
>
> The advertized usage of 100kHz to 30MHz (in direct-sampling method) is 
> based on this normally unwanted harmonics/aliases, i.e. the receiver uses 
> a method that has become "famous" with the ancient "Barlow-Wadley" 
> all-band receivers. (at least equivalent to). Their 
> (large-signal-)problems of the past are reborn at the direct-sampling 
> method of these DVB-T sticks. Without narrow-band selective frontends this 
> is just a "proof of concept" and not a usable receiver.
>
> For the use at 136 kHz and 475 kHz a steep lowpass-filter (7 to 9 pole 
> Tscheby with toroids) at (e.g.) 500kHz would be mandatory. An actual 
> bandpass may not be necessary. Equipped with such filters the lack of 
> resolution (8 Bit) will become more acceptable. Using a pre-amplifier 
> without using filters will do no good.
>
> When home-brewed, such filters will not cost much, but it takes time, 
> measurement-tools and effort to build and tune them.
>
> 73 de dg3lv Tobias
>
> Am 07.08.2014 14:13, schrieb Graham:
>> Receivers for LF and MF136 KHz@ 477 KHz
>>
>> A question,
>>
>> Startingat the£5dongleriggedfordirectsample , as
>> acheapeffectivestartingpoint
>>
>> A pre amp and  pre - selector [ band pass filter ] would  help  for 
>> lf/mf
>>
>> Whatwouldbein a scaleofincreasingperformance[ notcost !]
>>
>> be seen asreasonable in terms ofconfigurationand hardware ?
>>
>> Any particular equipments  stand out   as  good cost/performance 
>> choices ?
>>
>> Tnx
>>
>> Graham
>>
>> G0NBD
>>
>