RTL-SDR ghost signal at center frequency & hangs in GR spectrum view

Jay Salsburg jsalsburg at bellsouth.net
Mon Apr 16 04:58:59 UTC 2012

Without direct examination, I would say, the peak shown in [1] is energy
from the Local Oscillator being directly converted. Many receivers that use
direct conversion suffer from the oscillator coming through the mixer. If
you were to build a Direct Conversion Tuner from scratch with the option of
suppressing the "birdie" at the conversion frequency, the mixer would have
to designed as a Double-Balanced Demodulator, that suppresses the carrier
wave frequency.

It appears you are using GNU RADIO. Not being completely familiar with its
operation, I can guess that this software can be configured to suppress the
CW energy before the I/Q stream is decoded.

It is advised not to connect the Tuner's RF input directly to an antenna
without a coupling capacitor, there will is a DC component and may damage
the LNA in the Tuner chip. Coupling Capacitors on the Differential I/Q
outputs are to isolate DC paths between the Tuner and the Converter Chip,
also advisable not to directly connect them for the same reason. 

-----Original Message-----
From: osmocom-sdr-bounces at lists.osmocom.org
[mailto:osmocom-sdr-bounces at lists.osmocom.org] On Behalf Of Benedikt Heinz
Sent: Sunday, April 15, 2012 7:02 AM
To: osmocom-sdr at lists.osmocom.org
Subject: RTL-SDR ghost signal at center frequency & hangs in GR spectrum

Hi everyone,

can someone explain to me, what the origin of the ghost signal close to
center frequency is?
It can be seen at [1] and [2] - both are EzTV668 with no antenna connected.
It seems to me, that the higher the frequency, the more one can see the
"burst-like" nature of this signal. Is this some mixer artefact from the E4k
The best workaround is probably tuning a bit off the actual target frequency
and using frequency xlate?

Also, in [3] (german) the blind spot at 0Hz due to the coupling capacitors
between E4k & RTL is mentioned.
Is it possible to replace the capacitors with 0-Ohm resistors to work around
this issue, or will this cause other unwanted side-effects?
Or does this make no sense at all since one should always tune a bit off and
use xlate due to the ghost signal?

I guess it should also be possible (w/ some hot glue & wires) to insert some
jumpers (like OsmoSDR) instead, so one can use the ADCs directly for LF/MF
with some extra analog stuff?

Regarding the OsmoSDR source - I also gave the RTL-source from gr-baz a try
and noticed, that with averaging enabled in the spectrum view in GR, the
top-block GUI usually won't react to user input any longer when using
OsmoSDR, but this doesn't happen with the gr-baz RTL source, although I used
the same sample rate in both cases. Does anyone understand why it doesn't
hang with the gr-baz source?

Thanks for your answers!



[1] http://hackdaworld.org/~hunz/rtl-sdr/wo_ant_2m_osmo.png
[2] http://hackdaworld.org/~hunz/rtl-sdr/wo_ant_70cm_osmo.png
[3] http://www.mikrocontroller.net/topic/253371#2610540

Benedikt Heinz
hunz at jabber.berlin.ccc.de

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