June 2012 - osmocom-sdr - lists.osmocom.org

Linrad with rtlsdr with and the e4000 tuner.
by Leif Asbrink 05 Jul '12

05 Jul '12

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

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Super Efficient SMD Schottky Barrier from Comchip Technology Corporation
by Jay Salsburg 30 Jun '12

30 Jun '12

http://beta.globalspec.com/featuredProducts/detail?id=372 <http://beta.globalspec.com/featuredProducts/detail?id=372&campid=736&exhibi tId=209851&uid=%2D249525238&uh=e8a921&md=120628&mh=221961> &campid=736&exhibitId=209851&uid=%2D249525238&uh=e8a921&md=120628&mh=221961

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RTL direct sampling
by Adam Nielsen 24 Jun '12

24 Jun '12

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.

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Caputed RTL-SDR file for playing around?
by Johannes Bauer 22 Jun '12

22 Jun '12

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

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v2 Prototype issues and fixes
by Sylvain Munaut 21 Jun '12

21 Jun '12

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

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