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Thread: Teensy Convolution SDR (Software Defined Radio)

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    Senior Member DD4WH's Avatar
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    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

    What is the TEENSY CONVOLUTION SDR?
    - 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 pa0rwe.nl/wp-content/uploads/2015/07/sdr_schematic.gif
    - 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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