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Re: LF: MF CW now!

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







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