News from the OsmoSDR frontier

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Christian Daniel -- maintech GmbH cd at
Wed Oct 17 16:55:42 UTC 2012

Hash: SHA1

Hello everybody,

since we found out that the Atmel SAM3U SSC interface is only capable
of transporting 500ksps worth of I/Q data, we heard a lot of
complaining because of the too narrow bandwith. Especially compared to
the rtl-sdr with about 2msps, our 500ksps was bit contrained. The
fact, that the OsmoSDR is much more sensitive to very small signals
and brings a 14bit ADC instead of 8bits did not really compensate for
the bandwidth loss.

Also the mismatch between E4000 output resistance and ADC input
brought some unwanted spectral effects.

Instead of just releasing stuff, we decided to fix both issues: I now
have a prototype with simple stack-on-board, that adds two op-amps to
decouple the E4000 from the ADC and lower the impedance and also this
board connecteds the available FPGA pins to the MCI (SD-Card)
interface of the Atmel SAM3U.

For this to get working, we had to rewrite major parts of the ARM
firmware and the FPGA VHDL code. Today this work is completed and I
can report success: The OsmoSDR now delivers up to 4msps at 14bits.
Also the strange peaks around Niquist/zero frequency are gone.

Find a screenshot of SDRangelove recording a DAB signal at 2msps
(SDRangelove needs some more love until it can cope with 4msps in
realtime - but we have verified that 4msps works without dropped
samples using the builtin FPGA test mode).

We will now integrate the changes into the OsmoSDR mainboard and
reroute the now free SSC pins to the pin headers instead of the MCI
interface. Also we will add the opamps.

For the already produced OsmoSDR boards, we will have more of the
prototype stack-on-top-boards.

We will keep you in the loop!

Best regards,

PS: Regarding the DAB screenshot:
- - The horizontal lines are correct - DAB uses a so called "pilot
symbol", which only has energy on a few carriers. That's why it looks
like a pause in the signal.
- - The vertical line is also correct - DAB does not use the middle
carrier to avoid I/Q offset problems on the transmitter side.

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