Odd radio transmitter using square wave carrier...

Discussion in 'Wireless & RF Design' started by Externet, Aug 15, 2016.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Hi.
    Could a plain 10 MHz monolithic crystal oscillator drive a LT1010 amplifier for short range (under 50m) AM transmitter ?
    What undesirable effects a non-smoothed to sinewave RF carrier produces, and implications on AM ?

    Yes, needs to be around 10MHz; as compact as possible.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I used a 1MHz crystal oscillator with no amplifier and got a few meters of range. Range depended on the length of the antenna. It worked fairly well and didn't really cause as much interference on other wavelengths as you might expect.

    So my opinion is yes, you can get to 50 meters with modest amplification.
     
  3. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Thanks.
    Me too. Had one glued to a pet tortoise :rolleyes: to know if was nearby the yard. But had no modulation... A plain AM radio receiver did the detection.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A square wave will produce a lot of odd harmonics:

    Spectrum_square_oscillation.jpg
    This example is for 1000 Hz.

    So with a 10 Mhz square wave, you will find signals at 30, 50 , 70, 90 etc. Mhz.

    See this page of the wiki for more info:
    https://en.wikipedia.org/wiki/Square_wave

    Bertus
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    That might explain why I didn't see any problem in the AM radio band. I didn't look anywhere else. With a range of 10 feet, I wasn't too worried about it.
     
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    A (theoretically) infinite series of odd harmonics decreasing in 'half-power' (i.e. 3db) 'steps' (Recall your analytic geometry?). -- Of course inasmuch as 'zero transition time' is a physical impossibility the series won't actually approach infinity - though I daresay it'll give it a laudable try;) --- As a practical matter merely follow the Xmtr with a low-pass filter, verify that the signal is at lest 45 db 'down' on 30Mhz (or below your region's regulatory 'incidental radiation' limits) and call it good!:cool:

    Best regards
    HP:)
     
    Last edited: Aug 16, 2016
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  7. #12

    Expert

    Nov 30, 2010
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    Yes, it seems a tiny, simple LC tank would work wonders at smoothing out the higher frequency trash, but nobody will know a few hundred feet away.:p
     
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  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Ah! But you underestimate The Man! (Fox Charlie Charlie?) -- Whom, like the protagonist in E.A. Poe's 'Diddling...', has but two objects in mind - His pocket and yours!:rolleyes::mad:

    Best regards
    HP:)
     
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  9. DickCappels

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    Aug 21, 2008
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  10. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Yes square waves produce a bunch of odd harmonics but lucky any antenna that is good for x MHz will not be nearly as good as for 3x MHz and even worse for 5xMHz, etc.

    Ramping wavefronts do produce infinite harmonics however they are much more reduced than with a perfect square wavefront, being proportional to the inverse square of the harmonic number rather than with just the inverse. Theoretically there is no end but other things are going to quickly kill the higher harmonics like skin effect and again antenna length as well as an impedance mismatch.

    Yeah any inductor capacitor bandpass filter will quickly kill harmonics and boost the output by squashing the energy into a more narrow band. Probably a hand wound inductor would be just fine.

    The FCC is stating a limit of 100mw peak power into the final stage for some frequencies, but at 10MHz they state it in terms of field strength at a distance so you'd have to be able to measure that i guess if you want to test for basic compliance.
    Someone might want to mention how they would go about measuring this quantity.
     
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  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Friendly Candy Counter
     
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  12. bertus

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    Apr 5, 2008
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  13. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Ah! Yes then too there's

    "Freddie CupCake"
    And
    "Friendly Cousin Charlie" --- Unintentionally apropos in that, in my experience, greetings from 'friendly cousins' are generally of The Extended Hand 'variety':rolleyes:

    Exactly! -- Painlessly reduced annoyance/improved performance!:)

    Indeed! - Such seems to be the policy with 'Part 15 operation' on all the domestic broadcast bands -- Incl (CIP) WWV's 'clear channels':cool:

    IMNSHO, multiple samples about the periphery via an SA (FDO) is the best approach -- Though I suppose a (NIST certified) field-strength indicator may suffice granting sacrifice of ERP to 'margin of safety' is acceptable...

    Best regards
    HP:)
     
  14. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi there HP,

    Would you happen to know what that translates into going to watts at the transmitter?

    For 100uV/m at 10 meters and maybe at 3 meters, into watts at the transmitter. I dont work with radio very much and i am getting numbers that are just too unreasonable.
    So the question is, given a perfect measurement at 10 meters of 100uV/m, what is the transmitter power.
     
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  15. Aleph(0)

    Member

    Mar 14, 2015
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    MrAl I can't speak for HP but I'm saying there's no answer for that w/o knowing antenna's radiation pattern! So you need to know antenna gain for lobe of field you're measuring:cool:!
     
  16. Aleph(0)

    Member

    Mar 14, 2015
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    Now I'm asking what's the deal with square wave carriers:confused:? Cuz in other threads too ppl are wanting to use digital ICs as radio transmitters and when they're told abt problems cuz of harmonics other ppl say it doesn't matter for super low pwr. I say maybe not but according to old saying _courtesy (which here can mean clean signal) costs nothing_:)! Cuz everyone into electronics has some enameled wire and low value caps lying around to make lpf:cool:!
     
    Last edited: Aug 18, 2016
  17. Aleph(0)

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    Mar 14, 2015
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    Ha ha! You mean like friendly cousin Ann:mad::rolleyes:? HP I won't start on blaming you for being an enabler cuz amateur psychologist is as bad as a moocher:eek:!
     
  18. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Yes thanks, assume a gain of 1 with radiation pattern omnidirectional, so assume it radiates equally in all directions.

    The deal with the square wave carrier is that it is easy to generate with today's digital logic. Using a crystal we can get very tight frequency tolerance too with just a crystal square wave oscillator.
    Long time ago in a land far, far, away, i did the same thing at around 90MHz to transmit FM to a car radio. Because of the square wave harmonics i did not need an oscillator that ran at 90MHz, just a lower frequency and used one of the harmonics as the carrier.
    I got about 1/2 mile out but the transmitter was on the second floor of a building with a dipole antenna. I could listen to whatever music i wanted to broadcast on the way to work each day.
    I have done very little with radio after that though, some silly AM stuff that's about it.
     
    Last edited: Aug 18, 2016
  19. Aleph(0)

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    Mar 14, 2015
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    MrAl unity gain means isotropic pattern (whose parameters can be calculated from just square law relation) But that's not like pattern for any kind of antenna:confused:! Omnidirectional antennas like vertical monopoles try to have toroidal pattern but proximity of that darn 8,000 mile across rock messes with it:D! Closest thing I can think of to isotropic radiator is star which is just big a$$ pan-spectrum noise source:D
     
  20. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Yes sorry i realized that some time after posting that an isotropic would be different than an actual omni. So assume omni, or do whatever you want to come up with some workable solution to an estimate of the power required to produce 100uV/m at 10 meters distance from the vertical antenna. If you would like to do 3 meters distance too that would be good as i think those two are standard measuring distances or something.
    Alternately, if i have a 100mw transmitter with omni antenna then what is the field strength in uV/m at 10 meters.
     
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