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3333Re: Pi-FM APRS Tracker?

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  • fotograf_ale
    Aug 7, 2014
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    Dear friends,
    the attached file is the new release of the NBFM program, the narrow-band FM transmitter, for Raspbian distro.

    - name of program now ./nbfm (was ./nbfm.out Please modify your scripts).
    - text frame and colors
    - DMA used for higher frequency resolution (now <1kHz on 2m, was >10kHz)
    - compiler optimization
    - sub/audio tone frequency adjustment
    - added output power level control

    It would be very interesting if someone could give me reports on how it works on the field.
    For example, the packet generator would be an interesting item to test.

    As I've already said, the modulator is done using the DMA and it can't be so linear, but not only, because it also generates a lot of spurs.

    Even if it charges more the CPU and degrades the modulation quality, the advantage of using higher sampling rates is the reduction of the spurs (that are not the harmonics).
    Regarding this, please take a look at the following table, for 145MHz band (no subtone):

    Sampling rate     CPU load    Output Power
       11025             10%         -3.2dB
       22500             16%         -0.7dB
       44100             29%         -0.2dB
       88200             48%          0.0dB

    That is to say if I have a 250mW amplifier, I can achieve about 250mW at the nominal frequency when using 88kHz sampling.
    When, in the same conditions, I use 11kHz sampling, the output power is distributed more then a half to the spurs and less then a half to the nominal frequency (-3.2dB).
    So my advice would be to use 22kHz, when spurs create problems, because it's a good compromise between modulation quality and spurs.

    Up to 6m band (50MHz) the spurs are low so no problem to use 11kHz for sampling, and also the modulation is better because of the higher resolution.

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