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Hook an antenna to your Propeller, and listen to the radio! (New shortwave prog - Page 9 — Parallax Forums

Hook an antenna to your Propeller, and listen to the radio! (New shortwave prog



  • I think it's more because recent RF attention centers in the VHF and UHF bands where high tuning capacitance is unnecessary.

  • Martin_H wrote: »
    Is that because direct digital synthesis made them unneeded?

    I would imagine several other factors at play here.

    Integrated VCO's are easiest at high frequencies, and can be divided down to produce lower phase-noise
    outputs without needing discrete components like external varactors. (Although harmonic
    filtering is needed if not going into a mixer).

    LW and MW are not being used for much new stuff, everything's up in the higher bands

    Wide-range varactors need large supply voltages like 20 or 30V, which is cumbersome
    these days when the highest supply rail is 12V, 5V or even 3.7V!

    Also you can switch in capacitances and just need a small varactor for the smallest step between caps,
    which probably is more linear anyway.
  • I couldn't think of a subtle way to ask this on the P2 forum, but will the Prop 2 be a much better Radio than the P1? Especially with the PLLs or however it may generate high frequencies... Will there NOT be similar jitter issues?

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2017-11-02 19:38
    To the best of my recollection, all P2 outputs will be synchronized to the system clock. That means no asynchronous PLL-driven outputs like the P1 has. Typically, this would result in more phase jitter, given equal system clock rates. But the P2 will have a faster system clock, so the jitter will not be as bad as it is from the P1's NCO output. How it will compare to the P1's PLL output remains to be seen.

    OTOH, if the P2 implements clock outputs with a specified period (i.e. programmable counter modulus), rather than a specified frequency like the P1 has, outputs will be jitter-free, albeit with restricted frequency choices.

    Perhaps "someone" with more intimate knowledge of the P2 could "chip" in here? :)

  • Phil,

    Neat project!! ... and thanks for the comment "He Who Understands Inductors and Knows What to Do with Them"

    It's funny, that while I was on vacation on the coast of Lake Michigan, I was working in Parallel apparently with what you were doing, but you beat me to it. lol
    I knew what needed to be done, but did not get my head around a solution in code... Nice work!

    Without going into a huge amount of detail... since the 330uH is by itself and not in parallel with an external capacitor to form an LC tank circuit (excluding the inductors self-capacitance) we will look at the impedance based on the frequency.

    For the frequency range of 550kHz to 1600kHz the impedance of the 330uH inductor varies from 1.14K to 3.32K


    The Length of the Antenna also makes a difference and contributes an LC component to the impedance value above to form an RLC tank.

    You can get much better results and use a much smaller antenna if you use an RF transformer (two closely coupled coils or a single tapped coil) with a Capacitor to match the resonant frequency to something like 1100kHz (<-- approximate middle of the AM dial) and use that in place of your 330uH coil. The impedance of the antenna should also be matched to the RF transformer.

    Edit: Actually a 63pF capacitor in parallel with the 330uF inductor would be "tuned" to about 1100kHz but there wouldn't be a good way to couple your antenna to it unless you could wind a few (2-5) turns of wire around the outside of the inductor... one end tied to ground, and the other tied to your antenna.· At first I would try a variable capacitor that is within·63pF +/- 10 pF at least, but you may find a fixed value that works well also.· In this case we would not necessarily want a high 'Q' value for the coil, and or the cap, so a·5 to 10·Ohm resistor in series with the capacitor might also benefit.· What happens with a high 'Q' is that it becomes·a narrow band tuner, and you may loose other stations.· To be able to tune the other channels digitally, we want a little bit of padding here, so a wider band·tuner is more desirable.

    I built your circuit, and used a 500uH coil instead of a 330uH (1.73K to 5.03K impedance) ... I also used a 10Meg resistor for the ADC instead of the 470K resistor. This helps
    to provide less loading on the antenna from the feedback I/O pin, allowing weaker signals to be picked up.

    With 22 gage solid wire, I used a 6-foot piece of it for my antenna. The attached wav file is the result from a radio station a little more than 9 miles from my home.

    Other useful references:

    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 8/15/2008 5:02:55 PM GMT

    Thats what I was thinking. A resonant circuit in front of the digital stage will have a large response to the desired frequency band and reject all the stuff outside it. That stuff outside it just serves to increase your noise. This is a pretty cool trick. It reminds me of my childhood. We had one of those little electronics kits with the springs. My brother and I looked up how the old crystal sets were made in an old encyclopedia and figured out how to make one with the parts on the board. So much cooler than the single transistor radio that was in the pamphlet.

  • Thank you, Beau! And thanks for the detailed analysis of, and improvements to, the RF circuitry. I'm rather envious of your near proximity to an AM radio station, the closest strong station here being some 40 miles away. Between that and your RF improvements, the S/N of your recording is many decibels above what I was able to obtain!

    One thing I'd like to come up with is an easy-to-fabricate coil (or transformer) with your recommended values, made solely from hardware store and/or Radio Shack materials. I have a real aversion to winding coils that likely harkens back to childhood fumblings with fine enameled wire and empty toilet paper spools. I'm sure I'm not alone in this, so making this part easier will make the project more approachable for those who are more comfortable with digital design and programming.


    'Still some PropSTICK Kit bare PCBs left!

    Ive founbd many things that ive avoided over the years are not nearly as difficult as Id thought once Id just taken the time to sit down,think through the best way to proceed and practice. I too share your aversion to winding coils but one day I realized I just need to get the hang of it so I just practiced at it until I got it down.
  • potatoheadpotatohead Posts: 10,259
    edited 2017-11-13 17:38
    Phil, that is right. The P2 uses NCOs for frequency generation.

    Clock target is still 160mhz. Could end up running at 200.
  • potatohead wrote:
    The P2 uses NCOs for frequency generation.
    But is the modulus programmable? Or fixed at 2^32 as is done in the P1? If the former, is the frequency still programmable? Combining the two would provide the most flexibility.

  • Hello Everybody,
    Would be possible to do a FM instead AM demodulation on the Propeller ?
    The idea is to put a mixer with an oscillator in input to do a frequency changing and to use others frequencies using the FM this time.

    Your thoughts ?
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