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Thread: Teensy Convolution SDR (Software Defined Radio)

  1. #126
    Senior Member DD4WH's Avatar
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    From my experience, if you want to look behind the theory in the Teensy Convolution SDR, I would recommend reading three sources at the beginning (Wolfgang: you probably already know most of these):

    https://github.com/DD4WH/Teensy-Conv...ks-&-Resources

    First: Youngblood (2002) part 2

    Second: Whiteley (2011)

    Third: Borgerding (2006)

    With these three, you are already quite well prepared for further challenges.

    73 Frank DD4WH
    Last edited by DD4WH; 02-08-2019 at 10:41 AM.

  2. #127
    Senior Member DD4WH's Avatar
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    I uploaded a new version of the Teensy Convolution SDR code into the github:

    * EEPROM_LOAD message copied into header of the file
    * no more hanging when there is no SD card inserted
    * added many nice optimizations by Bob Larkin --> thank you, Bob !!!

    All the best,

    Frank DD4WH

  3. #128
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    Good news:
    Now the next step (living display, control of the SI5351) is done!
    Thank you very much, Frank, for your hints. (Inside the 10300 lines I wasn't able to find the comment after line 1734.)
    Now it's time to mount and solder the 5 remaining buttons, and the I / Q input. (Before I do so, I will test my QSD with a PC program, HDSDR.)
    Thanks a lot, also for the literature notes!
    73, Wolfgang

  4. #129
    Senior Member DD4WH's Avatar
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    updated links in the Ressources - WIKI (some were old and did not work)

  5. #130
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    More good news:
    Now the receiver is running. In the moment I cannot hear anything, but the spectrum seems to be correct, and the right audio output connection I hope to find soon.
    Very glad about this nice receiver!
    Because of using a touch screen my next goal is to make it work.
    73, WolfgangClick image for larger version. 

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  6. #131
    Senior Member DD4WH's Avatar
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    I added some info on how to use the menu buttons, encoders and encoder push buttons in the Teensy Convolution SDR here:

    https://github.com/DD4WH/Teensy-Conv...nSDR/wiki/Menu

    Have fun!

    Frank

  7. #132
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    Now my receiver works, very well!
    Connected to my 80m dipole, I can hear ham radio stations in good audio quality, better as with my IC706.
    A mistake from me: false LSB/USB assignment .Changed I / Q connectors -> all ok.

    The lack of attenuator and LPF was no problem until now.

    Thank you, Frank, for the new informations (Wiki "Menu")!
    Wolfgang

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  8. #133
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    When testing the device, I noticed a few things that needed improvement:

    - There are two menus through which you can move with two up / down buttons.
    - Menu 1 contains 19 points, Menu 2 even 34 points.

    So sometimes you have to press the keys quite often,
    but above all, without a list you can not keep track of the whole thing.
    The device has a graphics screen!

    Here I will start to improve the usability.

  9. #134
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    Hello Frank,
    I have the following problem with FM reception :
    my strongest transmitter sends at 99.7 Mc.
    I tune to 33,233 Mc, but the audio is very distorted, only in AM mode to understand.
    What do I wrong?

    Thanks, Wolfgang

  10. #135
    Senior Member DD4WH's Avatar
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    Hi Wolfgang,

    for wideband FM reception, the SDR is in 3x undersampling mode, so your math is approximately right, if you divide the real receive frequency by 3 :-)

    However, in all demodulation modes -except for WFM- the SDR uses a digital IF of 1/4 * sample rate.

    Lets say you chose a sample rate of 234k, which in reality is exactly a sample rate of 234375Hz

    That is why the display would show ( 99700000 - 3/4 * 234375 ) / 3 = 33174739.6Hz, which is exactly what is displayed when you do the following:

    * adjust sample rate to 234ksps
    * Set demodulation mode to WFM
    * tune to 99.70MHz
    --> the frequency display automatically does the math for you, it shows the real receive frequency :-).
    * listen to fine audio
    * set demodulation mode to something else: the display shows 33.174.739Hz (and you know why it shows this ackward figure)
    * set demodulation mode to WFM (the display shows: 99.700.000Hz) and continue to listen :-)

    For wideband FM reception, you need a very large sample rate, because an FM signal has +-75kHz frequency deviation. According to Wikipedia, this corresponds to a modulation index of 5, which leads to a needed bandwidth of 180kHz for MONO FM reception. So for nice MONO audio, you need at least 192ksps sample rate.
    For STEREO you would better have a bit wider bandwidth, so I choose 234ksps. According to Wikipedia, you would need 270kHz of bandwidth (Carson formula: Bandwidth = 2 * frequency deviation * base band bandwidth [60kHz]) for all components of the FM signal: the L+R signal, the 19kHz pilot carrier, the L-R signal centred at 38kHz and the RDS data signal centred at 57kHz (RDS decoding is not -yet- implemented). Dont try WFM with the standard sample rate of 96ksps, it will sound extremely distorted, because the frequency deviation is +-75kHz and the signal will be aliased with such a small sample rate and unwanted signals will fold into your FM audio baseband.

    All the best 73s

    Frank DD4WH
    Last edited by DD4WH; 02-15-2019 at 08:50 PM. Reason: EDIT: learned more about frequency deviation in FM radio

  11. #136
    Senior Member DD4WH's Avatar
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    Sorry, I made a mistake: the Carson formula for the approximate bandwidth of an FM-modulated signal is the following:

    Bandwidth = 2 * (frequency deviation + baseband bandwidth of the audio signal)

    For an FM radio signal you have 15kHz of MONO audio (L+R), the pilot tone carrier at 19kHz, the L-R DSB signal centred at 38kHz and the RDS signal at 57kHz, that corresponds to a baseband bandwidth of approximately 60kHz. The frequency deviation of an FM radio signal is 75kHz. This holds for Europe, in other parts of the world there are additional digital data services and other frequency deviations, so the band width could be even higher.

    Europe -->
    Bandwidth = 2 * (75kHz + 60kHz) = 270kHz

    So we should use the sample rate of 281ksps for optimum wideband FM reception. However, the Teensy 3.6. comes to its limits at this high speed. The FM demodulation has to be done at this high sample rate and it uses the processor intense atan2f function and a complex multiply. [maybe the T4 will help here and allow for high speed stereo demodulation and simultaneous RDS decoding ;-)]

    All the best,

    Frank

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