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 <[email protected]>:
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: <[email protected]>
Sent: Wednesday, June 18, 2014 12:23 PM
To: <[email protected]>
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 <[email protected]>:
Paul,
Q , do you find the transformer feed better than , simply
connecting via a capacitor to the AD ?
73-G,
--------------------------------------------------
From: <[email protected]>
Sent: Tuesday, June 17, 2014 11:38 PM
To: <[email protected]>
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 <[email protected]>:
472.2 kHz...
Anyone for a QSO?
73, Stefan
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