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Hi Gerhard, Jim,
so this would look like a case of 3x LO harmonic
mixing? This is actually one of the fundamental drawbacks of a direct
converting receiver. For good dynamic range, you want to drive the mixer hard by
a squarewave, but that introduces unwanted sensitivity around all odd LO
harmonics. So for a wideband tunable receiver, you would require a set of
switched octave filters.
I own a preselector-less "FiFi-SDR"
IQ-receiver, which is also suffering heavily from that effect, especially at LF
due to mediumwave broadcast stations. Instead of adding the filter
bank, I am actually considering to convert it to a high-IF
superhet with another mixer in the frontend.
Best 73,
Markus (DF6NM)
Sent: Saturday, April 30, 2011 1:16 PM
Subject: LF: Re: CW beacon "BRK" on 137.00khz
???????
Dear Gerhard, LF Group,
There is an NDB "BRK"
listed near Vienna on 408kHz, which is 3 x 136kHz.
Allowing for +/- a
little frequency for the AM sidebands this could be what
you are hearing -
the active whip would pick this up, but a tuned TX antenna
would attenuate
it considerably. Perhaps a bit more preselection required?
Cheers, Jim
Moritz
73 de M0BMU
----- Original Message -----
From: "Gerhard
Hickl" <[email protected]>
> I'm
working on a new DC-CW receiver and while testing, I found a
> CW-Signal
on 137.00 sending "BRK" every 10 seconds.
>
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