Hello,
This mail is sent to all the E-mail addresses I have
found in the source code of tuner_e4k.c and librtlsdr.c
I have made a couple of modifications to the software and
I now want to ask whether you can make changes to the
standard package that you supply so Linrad users will
not have to download modified files from my site to
get proper operation of Linrad in the future.
The most important modification is the gain setting.
The tuner code requires knowledge of the gain table
or searcing for legal gain values by use of the returned
error code. I think that this is impractical and I changed it
so the routine will now always set the nearest possible gain
value and return it to the caller.
The routine e4k_set_enh_gain(...) does not affect the gain
of my hardware so I allow the gain setting function to use it
with always the same value because I have no idea what this
function is intended to do....
The default IF gain is too high. I reduced it by 6dB for
a 6 dB better dynamic range.
With the tuner_e4k.c currently incorporated in Linrad
there is an AGC action that degrades the dynamic range by
about 10 dB. I do not know anything about the e4000 internals
but in case this remaining AGC can be disabled and the gain
set 10 dB lower in the particular gain step controlled by the
AGC, the dynamic range would improve from 66 dB to 76 dB
with respect to the usual MDS (noise in 500 Hz bandwidth.)
That means going from outside the scale in the QST "Key
Measurements Summary" in their product testing to a mediocre
result 6 dB above the limit of the range they have for SSB radios.
The rtlsdr would be superior to hand held units like TH-D72A
or VX-8GR in adjacent channel rejection (QST July 2011 and
September 2011) if the AGC could be disabled.
The details are here:
http://www.sm5bsz.com/linuxdsp/hware/rtlsdr/rtlsdr.htm
The Extio_RTLSDR.dll file provides 6 dB worse dynamic range
because of the high IF gain setting. It also has a poor
noise figure.
--------------------------------------------------------------
To be able to use the callback in Linrad I have had to include
various structures in the Linrad c code. I need to have this
code in Linrad:
i = libusb_handle_events_timeout(dev_rtlsdr->ctx, &tv1);
Therefore I need the structure rtlsdr_dev which is not included
in rtl-sdr.h. That structure requires other structures and it
would be a good thing if those things as well as something
like this:
#define MAX_RTL2832_SAMP_RATE 3200000
#define MIN_RTL2832_SAMP_RATE 900001
were included in rtl-sdr.h
I attach the Linrad interface routine to rtlsdr in case any of
you are interested to see how I use the library.
I realize the chip is far more clever and that I do not use it
well. There is a substantial DC offset that seems to be possible to
auto-remove. The AGC I want to get rid of might be something
related to:
/* disable auto mixer gain */
e4k_reg_set_mask(e4k, E4K_REG_AGC7, E4K_AGC7_MIX_GAIN_AUTO, 0);
Hard to know without any documentation on the chip....
73
Leif / SM5BSZ
Hi all,
I've been watching with interest Steve's commits to the direct_sampling branch
of the rtl-sdr repository. I haven't seen much mentioned about it though, so
I'm just wondering whether anyone knows how it's progressing, and whether it's
at the point yet where I should crack open one of these devices and try it out?
Thanks,
Adam.
Hi list,
I've stumbled upon the rtl-sdr and am quite excited. Have already
ordered a dongle, but it is going to take a while until it arrives.
Since I'd like to play around with it already, is there a capture file
available somewhere for download to test out the functionality (working
with GnuRadio etc)? I.e. someone with a dongle doing
./rtl_sdr /tmp/capture.bin -s 1.8e6 -f 392e6
(or whatever frequency there is something audible on, preferrably
something that can easily be identified like FM radio) and publishing
the capture.bin file?
This might also be of interest to people who consider to play around
with SDR, but do not know if they should buy a dongle.
Would be very cool if there was something available or somebody on the
list could provide one.
Best regards,
Joe
Hi,
The v2 prototype I still had a couple of issues so I'll describe them
here and the corresponding fixes:
- Bad TVS Diode: There is a diode that's supposed to protect against
ESD near the antenna connector. Turns out that the component supplier
swapped reels IIRC and so that's not a TVS diode that was mounted. My
board had the rework to fix this already done. I don't know if any
board shipped with the wrong part. It's the small ~ 0603 sized
component near the antenna connector and it's supposed to be greenish.
The result of this is a ~ 10-15 dB attenuation of the input signal.
- Missing LNA bias inductor: The v2 has a LNA at the input but it's
missing it's bias inductor which means it's powered off. So instead of
boosting the signal by 18 dB, it actually attenuates it quite a lot.
Solution is simple: Solder a 0603 bias inductor. Schematics call for a
470nH inductor and make sure to choose a good RF rated one and not
random junk.
- FPGA 1.2V LDO Oscillation: The output capacitor of the LP3965
regulator generating the 1.2V for the FPGA core has too low an ESR (a
ceramic cap is mounted). This regulator needs a capacitor with an ESR
greater than 0.5 ohm and lower than 5 ohm for it to be stable. Without
this, the LDO is unstable and has high spectral content at 55 kHz and
harmonics. This noise is present at the output but also leaks to the
input voltage of the LDO which is the 3.3V rail that powers the LNA
... This is causing unwanted images of the signal at f +- 55kHz f+-
110 kHz ... The solution is simple: replace the cap by a tantalum one
within the right ESR range. ( I used a TR3A106K016C1700 from Farnell
). This is before the fix: http://i.imgur.com/rRDDQ.png and this is
after : http://i.imgur.com/VlOW7.png
- Impedance mismatch between E4K output and ADC input: Ideally we'd
like to imagine that the E4K IQ output have very low impedance and
that the ADC input have very high impedance. Turns out that neither is
true: The E4K has a ~ 500 ohm output impedance and the ADC is a track
and hold type and the capacitance of the input changes when you start
sampling, causing a small current flow. The combination of the 2
creates a noise at the sampling frequency on the IQ lines ... The
effect of this is currently unknown and is being investigated ... it
might be possible to lower the noise floor a bit and get less DC
offset with this. But it certainly doesn't have as much impact as the
3 erratas above for sure.
See for components location. http://i.imgur.com/M4Lvt.jpg
Cheers,
Sylvain
