James, Markus!
Many thanks for your replies! Yep, "BRK" at 408kHz seems to be the
source of the signal. Thank you Jim for that hint.
BRK is about 50km from my QTH and quite strong here.
In front of my new RX, I'm using a LF filter which has a 3db bandwidth
of about 3kHz centred on 137.00kHz. Because I do not see the signal with
my "big" resonant TX-Antenna but only on the rather broad MiniWhip, I
think the mixing occurs there. A narrower filter would not improve the
situation.
Anyway I'm aware of the problem now and this helps a lot in improving
the design.
73 es tnx
Gerhard
Am Samstag, den 30.04.2011, 17:24 +0200 schrieb Markus Vester:
> 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)
>
>
> From: James Moritz
> Sent: Saturday, April 30, 2011 1:16 PM
> To: [email protected]
> 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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