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Thread: Teensy Convolution SDR (Software Defined Radio)

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    Senior Member DD4WH's Avatar
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    Oct 2015
    Central Europe

    Teensy Convolution SDR (Software Defined Radio)

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    Audio sample of AM reception of Jil FM from Algeria on medium wave 531kHz received here in Central Europe yesterday night with the Teensy Convolution SDR: HERE
    The sound is a bit treble-boosted, because it was recorded with a very wide filter and the recording with the smartphone also boosted the treble and attenuated the bass a bit.

    I am quite happy now with the reception and audio quality of the very first prototype of the Teensy Convolution SDR.

    The source code of this prototype alpha version is HERE

    - a Software defined Radio for LongWave/MediumWave/ShortWave & wideband FM Stereo
    - it needs a quite simple hardware quadrature sampling detector (QSD) like the Softrock or similar [I use this very simple QSD provided as a PCB by Joris kthkit[at]xs4all[dot]nl
    - unlike other standalone-SDRs that use IQ-input from a QSD, the main filtering and demodulation is done in the frequency domain by Fast Convolution, this enables much steeper filters than the usual phasing approach

    Specifications at the moment:

    EDITED 2017_05_03 for new features:

    * Teensy 3.6 with audio board working in floating point
    * reception from about 12kHz to 30Mhz (in undersampling mode 50MHz to 151MHz)
    * decimation-by-8
    * FFT and inverse FFT with 1024 points using the new CMSIS DSP lib complex FFT function
    * filtering with 513 tap FIR filter (in decimated 12ksps) --> comparably steep filters are normally available in commercial receivers from 2000$ upwards ;-)
    * filter coefficients are calculated by the Teensy itself and the filter bandwidth can be chosen arbitrarily by the user
    * demodulation of real synchronous AM with selectable sideband and "stereo" option, 9 "different" AM demodulators, LSB, USB, pseudo-Stereo-USB, pseuso-Stereo-LSB (maybe more to come . . .)
    * demodulation of wideband FM Stereo transmissions
    * display up to 192kHz of frequency spectrum
    * Zoom into the spectrum for greater frequency resolution detail: Zoom FFT enables us to have up to 10Hz resolution in the spectrum display
    * superb audio quality
    * needs about 46% of the processor ressources at 96ksps sample rate and SAM demodulation, about 48% at 192ksps and AM demodulation, SAM at 192ksps needs 78% because of the atan2f calculations
    * autotune on demand --> press a button and the carrier of an AM station is caught with an accuracy < 1Hz
    * automatic gain control AGC borrowed from the excellent wdsp library by Warren Pratt
    * dynamic frequency graticules on the x-axis of the spectrum display
    * S-meter
    * display of signal strength in dBm or dBm/Hz
    * small frequency display showing the SAM carrier frequency
    * real time clock working and on display
    * optional manual IQ amplitude and phase imbalance correction
    * automatic IQ amplitude and phase imbalance correction to account for imbalances in your hardware IQ production
    * menu system to adjust many parameters
    * save and load settings to/from EEPROM
    * play MP3 or AAC files in Hifi stereo quality from the SD card
    * reception of wideband FM broadcast stations by 1/5 undersampling with the same hardware --> MONO, but HiFi quality when using 192ksps sample rate
    * automatic test for the "twinpeak syndrome" - a fault in the mirror rejection that occurs sometimes at startup. This is now reliably detected by the automatic IQ phase correction algorithm and the codec restarted to cure the problem
    * AGC now has an optical indicator for the AGC threshold and many more AGC parameters can be adjusted
    * Codec gain has now its own automatic AGC
    * Spectrum display now also has an "AGC" that prevents empty screen for small signals
    * manual notch filter with adjustable notch frequency and notch bandwidth

    What is the SDR still lacking? --> wishlist ;-)

    - at higher sample rates, tuning and filter bandwidth setting is disturbed by clicking noises. Audio should be set muted temporarily in such situations
    - automatic realtime-clock adjustment by tuning into one of the time-signal-stations, eg. DCF-77
    - Noise reduction in the frequency domain: I am very interested in implementing one of those sophisticated noise reduction algorithms that work in the frequency domain, eg. this one: HERE
    - morse code decoder
    - PSK31 decoder
    - record and play IQ recordings on/from SD card

    This thread is meant to discuss questions and developments of this and other SDRs built with the Teensy.
    Last edited by DD4WH; 05-03-2017 at 06:23 AM.

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