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From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
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Date: Wed, 18 Jun 2014 21:19:38 +0100
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Subject: Re: LF: MF CW now!
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Paul ,

> I can not find any "mirrors" below 14.4 MHz, but above that hell  breaks 
> loose...

Yes , can be  disconcerting , when  you  have  unexpected signals !

freezes. Maybe a USB-system limitation?

That's possible , there was  some  posts  round  this  , over the  10MHz 
[still in dev]  airspy version

The  input  cct  is  not  as  simple  as first  thought  as  there  is  agc 
and  gain  roll  off  to  LF , but since the  sdrsharp  gang  managed to 
acquire  the programming  'book' and  contacted  the  developer of the  main 
chip , who  was  , apparently amazed to  find his  chip  being used  for 
this , more  b/w  and  features  have  been  unlocked   ..

GL-73-G,

--------------------------------------------------
From: <phl@netikka.fi>
Sent: Wednesday, June 18, 2014 6:58 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: MF CW now!

> Hello Graham, LF
>
> I'm using the +I and -I inputs that were connected to the R820 tuner  chip 
> before.
>
> The 1:4 matching transformer is not my own idea, I just made my 
> transformer a bit beefier than the design I borrowed from. It had a 
> FT37-43 core that provided a minimum frequency of 1500 kHz at 50 ohms 
> system impedance with the given number of turns.
>
> RF transformer rule of thumb: XL at min frequency = 4 x system impedance.
>
>
> Various "sources" on the web claim that the AD input impedance is  highly 
> undefined, or at least not specified. However the article I  borrowed the 
> 1:4 XFRM idea from claimed it had some merits without  going deeper on the 
> subject.
>
>
> I can not find any "mirrors" below 14.4 MHz, but above that hell  breaks 
> loose... so I added a simple PI LPF at around 15 MHz with the  main 
> intention to make sure no strong VHF-signals make it to the  AD-chip.
>
> You can see at least 2.4 MHz chunks of spectrum although there are 
> settings above that. The higher settings above 2.4 MHz don't produce  good 
> results with my setup, signals get choppy or I get temporary  freezes. 
> Maybe a USB-system limitation? If I go below 1 MHz strange  things also 
> start to happen and mirrors appear. Running the standard  2.4 MHz seems to 
> be the best option for me.
>
>
> You can probably make a dual-purpose device from a stick that has the 
> R820 tuner that connects to the AD using only +I and -I since the +Q 
>  and -Q inputs are then free to use as the direct sampling input to the 
> AD. I wanted to try this but my eyes and hands don't cope too well  with 
> the tiny dimensions since +Q and -Q are not brought out to any  pads on my 
> stick. Therefore I just soldered away the coupling  capacitors going to +I 
> and -I and then used their pads for interfacing  my transformer.
>
>
> The article that spurred my modding interest was in the last swedish 
> magazine QTC, but very similar mods are all around the web.
>
>
> Hope I did not add confusion.
>
> BR
>
> Paul-Henrik
>
>
>
> Quoting Graham <g8fzk@g8fzk.fsnet.co.uk>:
>
>> Ok Paul,
>>
>> Its  round  the  Q  and  Q + not Q  , configuration , the  latest 
>> drivers allow  that to  be  selected  , the  single  feed seems  to 
>> work  fine , but  still  needs  some  filtering to prevent images ..
>>
>> I think,  possibly  a  pass  band limited  pre-amp  would  help  for 
>> MF/LF , the  gain drop's  past a couple of  MHz  and  the  200 ohms  z 
>> needs some matching , may  be the  mini-whip is providing this   aspect 
>> ?
>>
>> There are image responses  linked to the  sample  rate , a  pre-selector 
>> as opposed  to  a  'tuner'  would  be  'ok' for  HF , I  think .
>>
>> I have another 'now'  low band  dongle  . used to  be  high  band ,  but 
>> after  a 7 MHz session , with it connected  to a 10 mtr  Ae ,  the  vhf 
>> front  failed ..  so  that's  a  candidate for  mf/lf   conversion , 
>> these have  two life's  !
>>
>> One of  Jim's   two active  device band limited  pre-amps  , with   200 
>> ohms o/p z would be  ideal  , or  use the  toroid as  the o/p   coupling 
>> device ? that  may  be  the  most  elegant  configuration ?
>>
>> 73-G,
>>
>> --------------------------------------------------
>> From: <phl@netikka.fi>
>> Sent: Wednesday, June 18, 2014 12:23 PM
>> To: <rsgb_lf_group@blacksheep.org>
>> Subject: Re: LF: MF CW now!
>>
>>> Hello Graham, LF
>>>
>>> To be honest I have not tried other feeding methods for the AD than  a 
>>> trifilar 1:4 transformer. I used a beefier toroid (FT50-43) and  more 
>>> turns to get a XL of 200 ohms at 400 kHz for the 50 ohm  winding. The 
>>> concept of having the AD and antenna line galvanically  separated and 
>>> the antenna input DC-shorted appealed to me since  there is no other 
>>> protection against static.
>>>
>>>
>>> I will give capacitor-coupling a try at some point.
>>>
>>>
>>> A simple LPF is mandatory at least if one plans to use wire  antennas, 
>>> otherwise there is a large risk of interference from VHF  FM 
>>> broadcasting.
>>>
>>>
>>> BR
>>>
>>> Paul-Henrik, OH1LSQ
>>>
>>>
>>> Quoting Graham <g8fzk@g8fzk.fsnet.co.uk>:
>>>
>>>> Paul,
>>>>
>>>> Q , do  you find the  transformer  feed  better  than  , simply 
>>>> connecting via a  capacitor  to  the  AD ?
>>>>
>>>> 73-G,
>>>>
>>>> --------------------------------------------------
>>>> From: <phl@netikka.fi>
>>>> Sent: Tuesday, June 17, 2014 11:38 PM
>>>> To: <rsgb_lf_group@blacksheep.org>
>>>> Subject: Re: LF: MF CW now!
>>>>
>>>>> Hello Stefan, LF
>>>>>
>>>>> Your CQ's are 539 here on a Mini-Whip directly into the 
>>>>> demodulator"  -chip on a slightly hardware-hacked RTL-SDR TV 
>>>>> USB-dongle :-).
>>>>>
>>>>> The "hack" is just bypassing the VHF/UHF tuner-chip and running 
>>>>> straight into the +I and -I inputs of the RTL2832U via a 1:4 XFRMR.
>>>>>
>>>>>
>>>>>
>>>>> No TX here at the moment but receiving you so well on a hacked 10 
>>>>> euro USB-dongle and the old MiniWhip is just a tad more than I 
>>>>> expected...
>>>>>
>>>>>
>>>>> BR
>>>>>
>>>>> Paul-Henrik, OH1LSQ
>>>>> Vaasa, KP03SD
>>>>>
>>>>>
>>>>>
>>>>> Quoting DK7FC <73dk7fc@googlemail.com>:
>>>>>
>>>>>> 472.2 kHz...
>>>>>>
>>>>>> Anyone for a QSO?
>>>>>>
>>>>>> 73, Stefan
>>>>>
>>>>>
>>>>>
>>>
>>>
>>>
>
>
>